ESS Beam Commissioning Considerations

J. Galambos, July 23, 2012

Beam commissioning tasks for the European Spallation Source (ESS) are outlined here, from the beam physics perspective. First, in Section 1 preparation activities leading up to the start of beam commissioning are described with rough manpower estimates. This also includes preparations of high-level software applications. Sections 2-5 describe specific beam commissioning activities with a general description of the task, beam characteristics typically used for the task, the beam diagnostic requirements, analysis software that will be required, and descriptions of calculations and simulations that need to be done in the design stage. Section 2 describes general commissioning tasks that are needed through out the accelerator such as beam steering, Section 3 covers the RFQ and MEBT, Section 4 covers RF scans, and Section 5 covers the HEBT. Specific beam instrumentation requirements can follow from the prescriptions described here.

1 General Commissioning Considerations

Commissioning is defined here as the first operation of a beam-line with beam. Before the commissioning, preparatory activities need to be performed, and these activities involving the beam physics team are discussed. Rough level of effort needed for these preparatory activities as well as for commissioning are given.

1.1 Preparatory activities

1.1.1 Beam calculations

Many of the commissioning activities described below require beam dynamics calculations performed a-priori. Some of these calculations will impact device design, and others are needed for the understanding beam performance during the commissioning activities. For instance the beam stopping distance as a function of the beam energies expected during a DTL cavity phase scan are needed for the design of an energy-degrader/Faraday cup beam diagnostic. Also, the allowable beam charge that can drift through unpowered superconducting cavities, without exciting them enough to perturb the trailing end of the tuning macro-pulse should be evaluated. Many of these calculations are called out in the specific commissioning activities described below. (~ 1 year effort)

1.1.2 Field maps

Magnets will be measured before installation, producing field vs. current maps and field quality (multi-pole error levels) information. The field vs. current will be critical for commissioning and the error components useful for beam simulations later. Beam physics is the primary customer of this information and will likely be responsible for uploading measurement information to a database, developing tools for retrievable and use in applications, codes, etc. (~ 3-6 month effort). A more significant effort is developing a control room system that interfaces between the physics units (field levels) and engineering units (power supply current). This requires an understanding of cross-cutting systems such as the magnet / power supply relationships (i.e. data base population and retrieval methods), and magnet and power supply current limits. A real-time software system should be created that convert inputs and outputs of power supply currents (operator actions) of fields (physicist actions) and keeps everything consistent. This is a 1-year activity. It should be led by someone with good software skills, as it involves real-time programming that needs to be robust, but will require close interaction between controls, physics, electrical and magnet systems. At SNS this activity was split between physics and controls, with the real-time software implemented on VME hardware by controls. But to do it over again I would put this effort in the high level physics applications so that the primary customers has a more direct ability to fix problems and implement new features.

1.1.3 High Level Applications

High-level applications here are considered to be software that involves beam physics, or software that collectively manipulates values across many components or systems, i.e. not normally provided by controls.

General rules of thumb for application development are:- Best to test a new algorithm first with some kind of script. If the algorithm

looks useful, then deploy a GUI interfaced application. Initially it may take a week to a month to get a reasonably complicated control room script going, but after practice these can be thrown together in as little as 1 day.

- The first GUI application a physicist develops may take 3-6 months. Next time it takes 1-3 months to develop a GUI application with minimal error exception handling. 2-3 months more field use to make it useful / robust, and adding features goes on forever.

- Only about half of applications that are started turn our to be useful

A critical need is for some sort of simple beam model (e.g. envelope) to be running in the control room, configurable by the machine state (e.g. magnet values, RF amplitudes, …). This will be used in many of the commissioning tasks described below. Note XAL comes with such a model, but using this requires configuring its lattice setup appropriately first (but presumably this would be necessary for any model). At SNS this is done from a database similar to BLED, but populating the

database required significant effort (~ 1 year). In part this was because the database schema was evolving during the population, and it relied on table information from other groups (e.g. power supplies) that were not populated so physics ended up doing this. But this may happen at ESS as well.

A minimal list of applications is shown in Table 1.1 with comments on whether any modifications would be needed for ESS, if ESS adopts the XAL framework. Note that XAL does not rely on any naming convention.

Table 1.1. A partial list of high-level applications, needed for commissioning. The SNS “XAL” version is listed, with an estimate of possible time needed to convert to ESS specifics.

Purpose Description XAL app ESS ModificationSave/compare/restore Used to snapshot the state of the

accelerator for fast recoverySCORE 3 months (need to configure the

ESS signals)Trajectory correction Save trajectories, restore to them,

flatten to axis, etc.Orbit Correction 1 month

Emittance measurement at the LEBT and/or MEBT

Analyze data from a slit/collector measurement

Emittance analysis 4 months (dependent on ESS profile measurement specifics)

Energy Measurement Display real time beam energy measurement from TOF, jitter, etc.

Energy Meter None

beam arrival time shap-shot

Snapshot of BPM phase along the linac, compare to golden save, etc.

Fingerprint None

Local transverse bump at an arbitrary point

Specify an arbitrary number of correctors (> 2), specify desired bump position and angle at a point

Knobs None

Beam Loss viewing View, compare to snapshot, limits, …

Loss Viewer 4 months

Magnet cycling program

For hysteresis Magnet cycling 1 month

General purpose beam model

Beam model with nice GUI setable from machine

Online model 6 months (but SNS is also planning on generalizing this)

Trajectory difference Apply transverse kick and compare measured downstream position change to model prediction

Orbit difference None

DTL phase scan Scan RF phase, for phase and amplitude setup

PASTA 4 months

Beam profile viewer PTA 6 months (present version is very dependent on SNS profile measurement specifics – start over)

Longitudinal kick response

Perturb an RF cavity and compare downstream beam arrival time

RF shaker 1 month

Scale cavities Increment a collection of like signals (e.g. RF phases) by a prescribed amount

Saddam 1 month

General purpose scan application

Scan 1 quantity (or 2 for parametrics) and measure an arbitrary number of other quantities (used for MEBT RF setup, DTL energy degrader)

Scan1d, scan2d 4 month

SCL cavity setup Scan the SCL cavities for RF setup and scale around fail(ing) cavities

SCL RF tuneup 6 months (should be redone – but SNS is working on this)

Transverse matching Use profile measurements to adjust quadrupoles for transverse matching

wireanalysis 6 months – needs re-writing, but SNS is starting)

Snapshot for model setup

A tool (service) that other applications can use to log the machine setup, that can be used later to configure a model

pvlogger none

1.1.4 Software Training

In addition to the time estimated for application modifications, the beam commissioners will all need to gain some familiarity with the control room software tools before beginning beam commissioning. At a minimum each beam commissioner should spend some time running beam-tuning applications that are testable with a virtual accelerator [] (e.g. trajectory correction). This will identify some software issues, and also help gain a familiarity with the software. Commissioners will need to have some familiarity with navigating the engineering support screens for systems such as magnets, RF, timing, etc. to be able to diagnose problems. Also all commissioners should have a familiarity with EPICS tools for examining signals from a terminal window etc., which is a useful method of last resort. At least some of the beam commissioners (those with some software skill) should gain a familiarity of interacting with the other system signals using their favorite script. Many of the engineering support screens and signals will not be available until the last minute, but to the extent possible, it will be useful to spend time at consoles interacting with these systems and interacting with the system owners, even before beam is available. Each engineering system owner should give training-tutorial lessons to the commissioner/operators prior to beam commissioning about the general operational principles, the specific control sequences, settings etc., allowable reset functions, and when to call for help.

1.1.5 Global coordinates, interface with engineering systems

The beam lattice will define where beam-line components must be installed. This has large implications, e.g. on tunnel length, penetration locations, civil construction elevation changes etc. Although quite simple, interfacing of this information with the conventional facility groups must be done carefully as it has significant impact. In addition, a coordinate system, polarity definition, etc. will have to be defined for the site/ project. The positions of all the beam-line components (magnets, instruments etc.) in this coordinate system will have to be entered into a database used by physics and alignment groups (among others). Also, coordination with the alignment group, magnet measurement group, beam instrumentation group and others to ensure the proper usage of this information will be needed. ~1 year effort.

1.2 Polarity checks

A final quadrupole magnet polarity check is useful prior to beam commissioning. This involves in-tunnel check of the quadrupole and dipole field polarity with a device that measures the field direction (e.g. a Hall probe). The magnets can be powered at low levels. Some special training to allow tunnel access with powered magnets is likely (these will be low voltage power supplies, but none-the-less

special training is likely needed). The corrector elements need not be tested in this manner, they will be tested with beam later It is possible to transport beam with a corrector polarity problem, but not a main quadrupole polarity problem.

1.3 Commissioning shift strategies

For beam commissioning shifts, it is useful to have at least 2 physicists on shift at a time. Having more than one physicist per shift allows discussion of problems, and facilitates cross training. Of course there will be staff from other groups needed continually through the commissioning period, both planned and on call. For extended commissioning stretches of 1-2 months of 24/7 commissioning, a pool of at least 12-15 beam commissioners is needed. If there are three 8-hour shifts /day, this corresponds to 3-5 shifts per week per commissioner, about half of which will be at night. In addition to the commissioning shifts, beam physics time is needed for data analysis, software debugging, interacting with other groups, etc. outside of shifts. Extending this level of effort beyond 1-2 months is difficult without a larger pool of commissioners. The commissioning 12-15 person “core pool” can include dedicated staff from other groups (e.g. controls or instrumentation) if this is possible. However, the other groups will also be busy fixing their systems and involved in the commissioning as mentioned above.

The 24/7 approach described above is commonly used. It is difficult to startup all the equipment needed for beam commissioning, and once equipment is running it is most efficient to keep the beam on rather than turn things off at night and attempt to re-establish beam conditions the next day. This approach requires having call in support from all groups (RF, instrumentation, controls, electrical, vacuum, …) at all hours, and does suffer the inefficient delays of support staff coming in after normal hours, and often requiring more support from other groups.

A different approach was taken at J-PARC for some beam commissioning [1]. They had 12 hour commissioning shifts during the day and a 12-hour break at night. During the day commissioning activities, a full contingent of support group staff was available, which made commissioning more efficient. Early on with minimal equipment (e.g. the front-end commissioning) it may be feasible to turn off equipment to a safe state for the nighttime break and not staff the control room. However later on with many superconducting cavities powered and the cryogenic system running, it is too time consuming to power down the equipment at night and restart in the morning. Much of the equipment would need to be left on, with at least one “operator” on staff 24/7. This is occasionally done at SNS during beam-study / start up periods, typically with beam running at a minimal 1 pulse/minute to ensure all the equipment is working properly.

2 General Commissioning Tasks

Certain commissioning tasks and tools are used repeatedly throughout a linac and transport lines. These tools are described here. Typically a single general-purpose software application can be used throughout the accelerator.

2.1 BPM + corrector polarity check

Prior to correcting the trajectory, the polarity of the dipole corrector elements and position measurement elements should be verified, using a trajectory difference technique. The method is simple: apply an upstream kick to the beam with a dipole corrector and compare the measured change in the beam trajectory with a model predicted change. While it is difficult to precisely know the initial absolute position and angle of the beam at the start of a beam-line, predicting the change of the trajectory is independent of the beam initial conditions (for a linear transport system), which facilities comparison to a model prediction. Figure 1-1 shows an example application of this technique. The model predicted change in position should match the BPM measured position changes.

- If all (or almost all) the BPM changes are opposite the predicted changes for all correctors, there is s systematic error between correctors and BPMs. Use a profile measurement to determine which system is correct and add a -1 sign to the other.

- If only one BPM change is opposite the predicted change, this BPM likely has a polarity issue (e.g. swapped cable).

- If all (or almost all) BPM changes are opposite the predicted position change for only one corrector, the corrector likely has a polarity issue (e.g. swapped cable).

- If the BPM predicted changes agree with the measured changes for all the correctors, either both systems are correct, or both systems have a polarity issue. To determine if the latter situation is the case, steer the beam at a wire scanner profile measurement station to determine which way the beam is really going, as an independent check.

Repeat this technique for all dipole correctors (horizontal + vertical).

Beam Parameters: Short pulse (5-10 s), low current (10-20 mA), manual beam trigger.

Beam Measurements: Beam position averaged over the macro-pulse

Figure 1-1, A trajectory difference example. The blue line is a model predicted trajectory response along a beam-line to a dipole kick. Red dots are the measured change in BPM position, from the kick.

2.2 Trajectory Correction

After checking BPM and corrector polarities, the trajectory can be corrected with a model based application. If the polarities have not been checked and either fixed or otherwise accounted for, an empirical response based transfer matrix approach can be used to center the beam at the BPM locations.

The correction application features should include selectable BPMs to use, selectable correctors to use, account for the corrector power supply limits, be able to save trajectories, and correct to these saved orbits later.

Beam Parameters- A short beam is adequate for this technique. 5-10 s is what SNS uses for

initial tune-up, but the beam position averaged over a user selectable portion of the nominal 2.86 ms macro-pulse to examine and/or correct the trajectory should be available.

- 1 Hz minimum repetition rate for practical purposes.

- The beam current is not critical, 10-50 mA should be sufficient. Low current is better if an empirical response matrix is being created, as this can be a lossy exercise.

Beam destination- a beam stop downstream of the last BPM used in the correction.

Beam measurements:- beam positions and amplitudes averaged over the macro-pulse, and

appropriately time-stamped for correlation within the same macro-pulse.- beam current averaged over the macro-pulse (to use as a filter for

determining good pulses for data-acquisition in the high level application) appropriately time-stamped for correlation with the BPM beam-position measurement.

2.3 Transverse Matching

Transverse matching in a linac or transport line is used to adjust Twiss parameters to some desired value. Typically matching done at lattice transitions, where the beam is most sensitive to miss-match. The process involves two main steps:

Determining the Twiss parameters at an upstream location (point “a”) Determining the modification of quadrupole values to produce the desired

Twiss parameters at point “b”, downstream of point “a”.

The quadrupoles being adjusted should be between point “a” and point “b”. Typically the desired Twiss parameters at the point “b” are the design Twiss parameters.

The most straight-forward method to determine the Twiss parameters at upstream point “a” is to have a suite of at least 3 profile measurements downstream of point “a” in each plane (there are three Twiss parameters being solved for: emittance, and ). Four profile measurements are preferable for redundancy, as one measurement station may not be functional, and for better statistics in fitting the initial Twiss parameters. The profile measurements should be made as close to each other as possible (e.g. consecutive quadrupoles in FODO or consecutive doublets) to minimize error in the model beam size reconstruction. In principle this is a simple exercise (linear lattice), but space charge and intervening RF focusing in accelerating sections can complicate the profile reconstruction, and shorter sections of modeling have less uncertainty. The Twiss parameters at point “a” are adjusted, so that the model reconstruction of the beam sizes at the profile measurement stations is best reproduced. The machine state (i.e. magnet and RF field levels) during the measurement is used in the model setup. Typically an envelope model is used for this purpose. Figure 2-1 shows an example of this technique, for a miss-matched beam.

Figure 2-1. Example solution of initial Twiss parameters upstream of the start of the SNS HEBT transport line to best match measured beam sizes. Dots= measure beam size, lines = model beam sizes, red= horizontal, blue= vertical.

A further refinement of the initial Twiss reconstruction at point “a” is to repeat the process described above for different quadrupole settings (5-10% perturbations) downstream of point “a”, but upstream of at least the last measured profile. The initial Twiss solution at point “a” should remain the same. An average of the solutions arrived at can be used to represent the Twiss parameters at point “a”. (see Figure 2-2 shows an example of this technique.

Initial Twiss parameters can also be constructed using a single profile measurement station, by using multiple quadrupole settings between the Twiss solution point (“a”) and the profile measurement point. In this case it is best to have a beam waist at the measurement point somewhere in the range of quadrupole values used, to have an un-ambiguous model reconstruction.

Figure 2-2. Example of multiple solutions of the initial Twiss parameters at the start of the SNS MEBT (RFQ output), using profile measurements in the MEBT taken with different MEBT quadrupole settings. RFQ output Twiss parameters: design (large circles) and measured (squares), for vertical (top) and horizontal (bottom). Note the measured vertical Twiss is different from the design expectation.

Finally, after the initial Twiss parameters are known at some point “a”, downstream magnet strengths are adjusted to provide a beam with desired Twiss parameters (e.g. design and ). This process is also typically performed with the same envelope model with appropriate space charge and acceleration effects included. It is possible to adjust and , but not the emittance. Thus at least two quadrupoles per plane being adjusted should be included in this solution process. Also it is important to include magnet limitations, and sometimes it is necessary to allow for more than 2 quadrupoles/plane if a magnet is being run near an operational limit.

Finally, the beam sizes are measured downstream of the adjusted quadrupoles, to verify the matching has produced the desired result. Figure 2-3 shows an example of measured beam profiles (and a model comparison) after the matching exercise, at the beginning of the SNS HEBT transport line (same case as Fig. 2-1).

Fig. 2-3 Example of measured profiles after quadrupole adjustments to produce a matched beam at the start of the SNS HEBT (FODO lattice). Dots= measure beam size, lines = model beam sizes, red= horizontal, blue= vertical.

Beam Parameters- A short beam is often required for this exercise at low energy, for wire

survivability. But in general, one should use a beam pulse long enough to avoid start-up transients (e.g. source startup and LLRF stabilization). At SNS we avoid using the first 10-20 s of the macro-pulse if possible. As long a beam pulse as possible is desirable to understand the stability of the profiles, but at least 50 s should be targeted.

- 2 Hz minimum repetition rate for practical purposes. - The beam current is important for this measurement as space charge

influences the beam profiles, and beam current measurement up to the operational value (50 mA) should be requested. .

Beam destination- a beam stop downstream of the last profile measurement used in the

matching.Beam measurements:

- beam profile RMS sizes are the primary product for this exercise. There is a use to have the beam profile throughout the macro-pulse available to the beam commissioners at a higher level. Commissioners should be able to select the time slice within the macro-pulse over which the profile measurement data is supplied. Also, the methodology for calculating the beam size should be available to the commissioner. For example, the use of a Gaussian fit, or an RMS calculation (including the ability to prescribe the noise floor for the RMS fit). Also the ability to deselect bad data points from a profile is useful (e.g. when some accelerator component did not work properly during a scan).

Calculations- A beam envelope model with space charge and RF focusing included is

needed. Some solver/optimization scheme to find the initial Twiss parameter s using the model is needed, and well as for finding the quadrupole settings for matching. The model should be configurable from either the live machine

and from a snapshot of the machine settings that is available later, so that data analysis can also be done “off-line”.

2.4 Beam Loss Fault Studies

As the beam energy reaches ~ 100 MeV (e.g. after the start of the spoke cavity section), loss monitors will start to detect ionizing radiation from beam loss and verification that the beam loss machine protection system is properly functioning is an important commissioning activity. This generally consists of: 1) ensuring that a stable short beam pulse is available (e.g. 5 s), 2) conceiving of a way to spill beam along the beam-line being commissioned (e.g. reducing quadrupole focusing strength at a point, or creating a large local bump), 3) verifying the loss monitor fast protection system does trip the beam under the proper conditions. This process is usually done with a manual beam trigger as it entails significant beam loss.

Comparing upstream and downstream current measurements can provide a rough estimate of the total quantity of beam lost, which can be used to estimate the amount of beam lost near the BLM(s). The BLM fast-protection trip threshold should be set to protect the machine from a specified fractional beam loss at 5 MW. Using the estimated beam loss for the local spill, the BLM trip threshold can be adjusted. Even a localized 5 s beam spill can cause a large BLM response, and care should be taken to ensure the BLM loss signal is not saturated during this exercise.

Beam calculations:- A way to selectively lose beam along the beam-line being commissioned

should be anticipated in advance. It may not be possible to principally lose beam at individual loss monitors, but loss coverage methods along the beam line should be anticipated. Reducing transverse focusing strength, insertion of an intercepting device near by a BLM, and use of local bumps are ways to do this, but may require a temporary by-pass of other machine-protection settings.

Beam Measurements:- Beam current measurements upstream and downstream of the local beam

spill to aid in estimating the local beam loss quantity (integrated charge)- Beam loss monitor (integrated loss during a macro-pulse) for a single pulse

Beam parameters:- A short pulse 5-50 s, manual trigger macro-pulse. Start with a short (say 5-

10 s) beam and attempt a local loss, if sufficient beam loss response can be detected. Use longer pulses as needed.

3 RFQ / MEBT Commissioning.

The primary tasks for MEBT beam commissioning are setting the RF cavities to bunch the beam, and adjusting quadrupoles to match the beam into the Drift Tube Linac (DTL). Additional tasks include flattening the beam trajectory, and characterizing the beam phase space distribution with an emittance device.

3.1 RFQ setup

The goal is to set the RFQ amplitude to the proper value. The high-power RF may provide a rough estimate of the RF field level for a given klystron power, but this needs to be checked with a beam measurement. A well-tuned beam out of the LEBT should be first ensured. A beam stop in the MEBT is useful for the RFQ amplitude setting. The MEBT transverse focusing elements can be set to their nominal values, and the re-buncher cavities (not yet set) can be turned off. The RFQ field amplitude is varied, and the current transmitted through the RFQ (measured at the entrance to the MEBT) is measured as a function of applied RFQ field. Comparing this transmission with a calculated expectation provides information on the set-point for the RFQ field amplitude. An example of this measurement is shown in Figure 3-1.

One can check the sensitivity of this measurement to various beam pulse lengths (up to the limit on the beam stop), and the results should be independent of beam current with good RF control, as the ESS RFQ should be intensity independent up to 50 mA. Also the upstream and downstream of the RFQ beam current waveforms can be compared to investigate the quality of the RF amplitude control during the macro-pulse.

Calculations: The RFQ transmission vs. applied field, the beam stop power limitations.Measurements: Beam current at the entrance to the RFQ and at the entrance to the MEBTBeam parameters: Short pulse (5-50 s), current (10-50 mA), 1 Hz rep rate (minimum). Beam Measurements: Beam current upstream and downstream of the RFQ. Current averaged over the macro-pulse and current waveforms during the macro-pulse.

Figure 3-1. RFQ transmission vs. RF power. Measurements (red) and model fit (blue). The 100% gradient corresponds to 690 kW, and full current is 23.6 mA .

3.2 RF buncher cavity setup

This task involves scanning each buncher cavity and observing the change in beam arrival time at a downstream detector. The beam arrival time will be measured by the Beam Position Monitors (BPMs). The phase scan is repeated for multiple cavity amplitudes, and there will be two RF phase points at which the downstream arrival time is independent of the cavity amplitude (i.e. non-accelerating). One point is complete bunching, and the other point is complete de-bunching.

It is important to have the beam unaffected by other cavities during this measurement, downstream of the cavity being set. If it is necessary for the beam to drift through another cavity (from the one being scanned), before it reaches the phase measurement device, the additional cavity must be turned off to not affect the drifting beam. Also, the phase measurement should be close enough to each cavity so that it can detect the beam before the beam becomes de-bunched so much that the arrival time is difficult to measure. Typically at low energy this is a few meters, so it should not be an issue in the MEBT. An example measurement using this technique at SNS is shown in Fig. 3-2.

Fig. 3-2. Example beam arrival time measurement in the SNS MEBT, vs. RF phase, at several RF amplitude settings.

Beam Parameters- A short beam is adequate for this measurement. 5-10 s is what SNS uses. - 1 Hz minimum repetition rate is useful for practical purposes. - The beam current is not critical, as the MEBT cavities are low Q, and the short

beam pulse will likely not excite them enough to perturb the beam. The minimum current for which the phase detectors work well is suggested (perhaps 10-20 mA).

Beam destination- a beam stop downstream of the BPM used for setting the last cavity.

Beam measurements:- beam phase (arrival time) and amplitude averaged over the macro-pulse,

and appropriately time-stamped- beam current averaged over the macro-pulse (to use as a filter for

determining good pulses for data-acquisition in the high level application) appropriately time-stamped for correlation with the BPM beam-phase.

Calculations: - Calculate the error in the cavity phase and amplitude settings arising from

different resolution in the BPM phase measurements. This will give the requirement for the BPM phase accuracy. At SNS we attempt to set the MEBT cavity to 1 deg. and amplitude to 1%, which ended up with a requirement for BPM phase measurements of +1 deg. However, in practice there seems to be

a repeatability error of 2-3 degrees when using this procedure, which is not well understood.

- Calculate the distance a 3 MeV beam (50 mA at 350 MHz bunches) de-bunches below the level at which the arrival time can be accuratlely measured (coordinate with diagnostics). Make sure there is a phase detector for each cavity within this distance.

- Calculate the slope of the beam arrival time vs. cavity phase at downstream BPMs for each cavity. These values will be used to set the cavity amplitude during commissioning. Also the sign convention used will determine which slope sign solution to use

3.3 Differential Current Protection Check

Typical beam loss monitors will not detect beam loss from the low energy (3 MeV) beam in the MEBT, and a differential current protection system will likely be used. This system monitors upstream and downstream MEBT beam current levels, and stops beam if the downstream current falls below a threshold. The commissioning activity involves determining a threshold that is safe yet robust to noise (avoids false trips). Possible ways to destroy beam transmission are to turn off a quadrupole, or insert an interceptive device if available.

Beam Parameters- A beam pulse long enough to measure the protection system is required. 50

s should be provided, which is sufficiently longer than the 5-10 beam shut-off time

- 1 Hz minimum repetition rate for practical purposes. - The beam current should be high enough to avoid noise issues, e.g. 30-50 mA

Beam destination- a beam stop downstream of the last BPM.

Beam measurements:- beam current up stream and downstream in the MEBT.

4 Linac RF Setup

Each warm linac (DTL) tank RF phase and amplitude must be set to the design values. The amplitude can be set roughly with RF measurements alone, but not to the level of ~ 1% needed for proper setup. Furthermore the RF phase in the structure relative to the beam can only be set using a beam measurement. Techniques for doing this fall into two classes: Energy-degrader / Faraday cup method [2,3], and time-of-flight techniques, which are discussed below.

For time-of-flight, only the phase scan signature matching method [4,5, 6, 7] is described here. This method is more general than the earlier “Delta-T” method [8,9], which assumes a linear response to the beam, and in practice requires the starting

RF phase and amplitude to be reasonably close to the solution to converge. A concern in applying these TOF schemes is maintaining the integrity of the bunch as it drifts between the RF structure and the phase detectors. A pitfall is beam de-bunching for low energy breams (i.e. the first DTL tank), which can limit the range of useful RF phase variation. Also any intervening RF structures between the cavity being tuned and the detectors must be turned off. In the case of intervening superconducting cavities, excitation of these cavities by the drifting beam itself can impact the measured arrival time, and care must be taken to use a low enough intensity beam, or somehow detune the cavities so that they do not affect the drifting beam.

4.1 Energy Degrader

With the “energy degrader” approach an intercepting material (degrader) of known thickness is inserted in the beam downstream from the cavity, followed by a charge measuring device (e.g. Faraday Cup), as shown schematically in Fig. 4-1. The degrader thickness is chosen to stop incoming beam with energy slightly below the cavity design output. Thus only when the output beam is close to being fully accelerated is charge detected. The cavity phase is scanned and the phase width of an “acceptance” can be determined by examining the width of the detected beam signal in the Faraday cup. This process is repeated for several amplitudes, as indicated in Fig. 4-2(a). The width of the acceptance is plotted vs. RF amplitude, as shown in Fig. 4-2(b), and the calibration between the RF drive and actual cavity voltage can be determined by matching this curve with an expectation of this trend based on model predictions for the given degrader thickness. The cavity phase setting is determined relative to the acceptance boundaries at the nominal amplitude setting, again by comparison with model predicted expectations for the given energy degrader thickness. Finally we note that the beam bunch width can also be determined by the width of the rise and fall of the curves in the phase scans shown in Fig. 4-2(a).

Beam Parameters- The chopper can be used to eliminate the source startup transients. It is

useful to do RF phase setups initially with a very low intensity beam, to avoid any issues associated with beam loading, as the LLRF may not be well tuned initially. A 5-10 s beam of low current (10 mA) should be safe to avoid beam-loading issues. Inserting an aperture restriction can facilitate achieving the low intensity beam. The measurement can be repeated later with a longer pulse higher intensity beam to ensure the cavity settings are not affected by beam loading (i.e. to ensure the LLRF compensation is properly working)

- 1 Hz minimum repetition rate for practical purposes.

Beam destination- The FC used for the measurement.

Beam measurements:

- Beam charge transmission to the Faraday cup (FC). The FC macro-pulse waveforms should be available for selection of the time slice used for analysis by the commissioner, as the LLRF may not have adequate control of the beam during the macro-pulse startup (especially in early commissioning)

- An upstream BCM measurement that can be used to filter bad beam shots, for retaking data (good time stamp synchronization needed with the FC signals)

- Also, a waveform of the measured RF field in the tank during the macro-pulse with beam used for the FC measurement is useful. One can check to verify that the cavity field is indeed unaffected by the presence of the beam.

Calculations:- Beam transmission through the chosen energy degrader as a function of DTL

tank RF amplitude and synchronous phase.

Figure 4-1. Schematic of the energy degrader method experimental setup.

a)

b)Figure 4-2. An example energy degrader application for a Drift Tube Linac tank in the SNS linac. a) the measured beam charge detected downstream of the degrader vs. RF phase for several amplitudes. b) the width of the detected acceptance windows in the scans, vs. amplitude setting of the RF, measured vs. theory.

Cavity Phase (deg)

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Normalized RF Amplitude

Phas

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g)

4.2 RF Scan / Signature Matching

DTL tanks typically consist of 10’s of accelerating cells (gaps), each of which “kicks” the beam. The change in beam beta and the longitudinal phase advance through the DTL tank can be significant. If the RF field amplitude and/or phase are not set at the design values, the beam will undergo longitudinal phase space oscillations about the design expectation through the linac. The output beam energy, and arrival time at downstream BPMs varies in a complicated, typically unique manner for off-normal cavity setups, hence the signature-matching name.

The first step in setting the RF is to find the rough region of RF phase and amplitude to set the klystron. The RF group will have a rough idea of the amplitude, based on the measured RF power and design shunt impedance. The klystron phase rough setting region can be found by watching the cavity field macro-pulse waveform (zoomed to the region where the commissioning beam appears) as the klystron phase is varied, with LLRF feedback and feed-forward turned off. When the synchronous phase is fully de-accelerating the field waveform will exhibit maximum increase when the beam arrives. When the synchronous phase is fully accelerating the field waveform will exhibit maximum decrease when the beam arrives.

The cavity will be scanned in the vicinity determined by the rough estimate described above, by +10-20 degrees. The phase difference (difference in arrival time) will be measured with a pair of downstream BPM (with sufficient calibration for the measurement - e.g. cable length matching, common RF reference, … to ensure ~ 1 degree phase relative accuracy), as depicted schematically in Figure 4-3.

Figure 4-3. Schematic setup for beam based Time-of-Flight measurements.

At low energy, the range of the scan over-which the beam remains sufficiently bunched to produce measureable signals downstream at the BPMs may only be a few 10’s of degrees. Make sure any downstream cavities between the scanned tank and the phase detectors are turned off. The first scan is performed with the DTL tank being scanned also turned off. This is used to get a reference measurement of the un-accelerated beam TOF, which provides additional information. This additional step may not be practical for the first tank due to de-bunching. Then the scans are repeated for 2 or more cavity amplitudes near the expected DTL tank design setting. The difference between the measured tank-on and tank-off TOFs and the model predicted tank-on and tank-off TOFs are compared. Input to the model is varied to best match the measured and modeled curves. The unknown model inputs that are varied are the input beam energy, the cavity amplitudes used at each scan, and the offset between the klystron RF and the beam synchronous phase. An example result from the technique is shown in Figure 4-4. The klystron amplitude calibration provide the amplitude set-point and the RF phase offset provides the RF phase setting. An additional output from this technique is knowledge of the input (and output) beam energy. The fitting results are typically quite sensitive to beam energy (i.e. within 100-300 keV for DTL examples).

Figure 4-4. Example of the phase scan signature-matching technique for a) the fourth Drift Tune Linac tank at SNS. The dots are measured TOF differences (red and blue are 2 amplitudes separated by about 2%. The lines are model predictions, after solving for the beam input energy, the klystron amplitude calibration, and the klystron RF phase offset from the beam synchronous phase.

Beam parameters, measurements- same as for the energy degrader measurement- 1 Hz minimum repetition rate for practical purposes. Faster is better, up to

the point that the LLRF can be used to change the RF phase reliably over the control system and have the new phase point stabilize.

Beam destination:- Nearest downstream Faraday cup (or other beam stop) after the last BPM

being used.

Beam Calculations- A single particle beam acceleration model through the DTL tanks, to the last

BPM used for measurements, for arbitrary phase and amplitude at the entrance to the tank.

4.2.1 SCL Cavity Phase Scans

Acceleration in the superconducting cavity structures typically involves a relatively small change in beta and minimal longitudinal phase advance per cavity. As such, the acceleration can be well approximated by a simple single kick-model (at SNS a single kick/cell model is used, but approximating the entire cavity as a single kick would likely work equally well for setting cavity phases to within 1% amplitude , 1-degree phase). At SNS, all cavities are run at their maximum safe operational gradient, to maximize beam energy and power. Only the cavity phase is set by this beam measurement, but the cavity amplitude is found using this technique and could be adjusted. The cavity amplitudes predicted by the SNS RF group, using cavity field probe and RF power measurements was within ~ 10% of the beam based measured values.

The general technique used here is similar to that shown schematically in Figure 4-3. In this case, the scans are performed throughout the full 360 degrees, as the beam energy should be high enough for the bunch to be well enough bunched throughout the scan. Also it is important that any cavities between the one being set (i.e. scanned) and the last BPM measurement device are unpowered to not affect the drifting beam. Also a low enough intensity beam should be employed, so that it does not excite intervening cavities enough to measurably change the beam arrival time at the BPMs being used. At SNS a 5 s, ~10 mA beam is used to ensure this condition. Inserting an upstream aperture limitation can reduce beam intensity. By providing a variable aperture size (or varying pulse length) one can measure the change in arrival time of the drifting beam with beam intensity to verify the condition of drifting beam being unaffected by cavity excitation is satisfied.

The actual scan and data analysis for this technique is quite simple. An example downstream BPM phase difference for a 360-degree phase scan is shown in

Figure 4-5. The response over a full 2 RF phase scan is nearly sinusoidal, as expected from an ideal RF kick. The line represents the measurement data, and the dots are the model predictions after adjusting for the input beam energy, klystron amplitude calibration and klystron phase offset. Setting all the cavities in this manner gives a measurement of the beam energy (to within ~ 1 MeV at 1 GeV), which can subsequently be used to setup transport to the target.

There will be many SCL cavities to set, and this process can be lossy as well. At SNS we disable all steering correctors, as the beam energy / rigidity changes throughout the SCL cavity setting process, and continually correcting the trajectory throughout this process is not worth the benefit. In any case, initially the beam will be debunched before reaching the end of the linac, and position measurements will not available. An intermediate beam stop(s) between the DTL and linac dump will be useful destinations. Also, the downstream quadrupole focusing needs to be increased as the beam energy is increased. Initially SNS planned to update quadrupole strengths as the beam energy increased by ~100 MeV increments from the 186 MeV warm linac output energy. In practice now, SNS only employs one quadrupole strength update through the entire cavity setting process to full energy (~ 925 MeV).

Figure 4-5 Typical TOF response for a superconducting cavity phase scan.

Beam parameters: - 5-10 s, ~10 mA .

Beam Measurements:- Beam current measurement upstream, to filter bad pulses from the

measurements.

Beam destination:- Nearest beam stop or beam dump downstream of the last BPM phase

detection used.

Beam Calculations- A single particle beam acceleration model through the SCL cavities, to the last

BPM used for measurements, for arbitrary phase and amplitude at the entrance to the tank.

5 HEBT / Target Interface Commissioning

The primary beam-commissioning task in the HEBT and Target approach will be qualifying the beam properties on Target. Although the initial power on target will be low, some sort of beam property qualification will be needed even for initial low power operation. Specifics will be detailed as the Target design evolves, but some general concepts can be planned from the start. As with any transport line, trajectory difference, trajectory correction, and transverse matching will be performed initially (as discussed in previous sections).

The final beam energy on target will not be known until the final SCL cavity setup. A tool to scale the HEBT lattice settings to this energy should be available (while variable energy beam can fairly readily be transported down a straight linac, the HEBT line with bends will need more care).

The primary quantities needed to qualify the beam on target are the beam center position and the peak beam intensity. The most direct measurement of these quantities is by an imaging system of, for example, a phosphor screen on the target surface. An alternative is to measure the beam profile upstream of the target (e.g. with a harp) and propagate the profile downstream with a model.

5.1 Imaging system target qualification

The beam centering is straightforward with an imaging system. The location of the peak intensity relative to appropriate fiducials (which are also visible on the imaging system) directly specifies the beam center. However the peak intensity is

more difficult to specify, as the imaging system response to different intensities is difficult to predict a-priori. This response should be measured by producing a small beam of known intensity (safe enough for the target to handle in a single pulse), and rastering this beam across the target face measuring the imaging system response. This will create a response map that can be used to construct a profile distribution. However, the response can change with time (as the phosphor and optical element damage buildup occurs), so it should be repeated. Other than providing a rastering capability for the beam and the beam intensity for this procedure, this task is primarily a beam instrumentation development effort.

5.2 Profile extrapolation technique

Another technique is to measured beam position and sizes downstream in the HEBT and use a model to extrapolate these to the proton beam window and target. Beam positions measured at the final few BPMs can be easily extrapolated through the remaining HEBT lattice to the proton beam window and target. But good knowledge of the window and target centers in the same coordinate system used by beam-line components is critical.

Given the use of non-linear elements to broaden the beam profile, a measurement of the profile downstream from these elements and upstream of the window and target is essential (e.g. say at a harp). Additional profile measurement stations are also required to have a good knowledge of the beam Twiss parameters entering the final approach to the window, which is used in extrapolating the measured profile to the window and target (see discussion of solving for upstream Twiss parameters in the section on matching above). The beam profiles measured at the harp can be scaled by the model predicted change in RMS size from this matching exercise, and an allowance for beam scattering in the window on the beam size should be provided. The total charge delivered should be measured by a reliable BCM. With the peak charge and the extrapolated beam profile the peak charge density on the target can be calculated (also account for target rotation).

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