recent high efficiency rf source developments at slac national accelerator laboratory
DESCRIPTION
Recent High Efficiency RF Source Developments at SLAC National Accelerator Laboratory. Presented by Jeff Neilson on behalf of Electrodynamics Dept members: Mark Kemp, Aaron Jensen, Erik Jongewaard and Sami Tantawi , Chief Scientist for RFARED division - PowerPoint PPT PresentationTRANSCRIPT
Recent High Efficiency RF Source Developments at SLAC National Accelerator LaboratoryPresented by Jeff Neilson on behalf of Electrodynamics Dept members:Mark Kemp, Aaron Jensen, Erik Jongewaard and Sami Tantawi, Chief Scientist for RFARED division
Work supported by the Department of Energy
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Overview
• RF and Accelerator Research and Engineering Division at SLAC
• Energy Recovery for Pulsed RF Sources
• Scalable High Efficiency Klystron
HPRF capability at SLAC is Highly Vertically Integrated
• World’s only integrated capability to conceive, design, build test
− Very high peak power sources (up to 150MW) and components (up to 500MW)
− Associated modulators for sources− High gradient (170 MeV/m) normal conducting rf accelerator
structures
• This capability all under one roof allows prototype and test in a tight, rapid development cycle
• Unique capability to provide multivendor source of RF vacuum devices through licensing to industry
SLAC High Power RF Research & Engineering Has Spanned 0.3 -100 GHz and up to 150 MW Peak Power
5045 KlystronS-Band
•2.856 GHz, 65 MW
•> 800 produced since 1983 •MTBF > 90,000
hrs
B Factory Klystron476 MHz
1.2 MW CW
XL4 & XL5X-Band
11.4 GHz, 50 MW@SLACCERN
Sinc. TriestePSIBNLLLNL
XPX-Band
11.4 GHz, 50 MWPPM focused
W-Band Sheet Beam Klystron
95 GHz
Span wide range• 0.3 - 100 GHz• 1.2 MW CW – 150 MW Peak
Unique infrastructure geared toward fabrication and testing of specialized high power vacuum RF electron beam devices
B Factory klystron (MW-class cw) completing bake-out
• Precision in-process machining• 8 Ultra-high vacuum bake-out
stations• 12 Hydrogen Braze/Retort
Furnaces and 3 vacuum furnaces• 5 vacuum cathode processing
stations• Sputter and evaporative coating
chambers
Large Test Capability
• 13 instrumented 150 MW pulse power modulators
• Two MW-class CW test stands
• Two shielded test bunkers
Infrastructure is unique. Although originally sized for a higher klystron production rate, now utilized to build a much larger variety of high power RF devices and structures
5045 klystron in final test
High Power RF, Accelerator, and Pulsed Power Electronics Capability “Under One Roof” Enables Integrated System Design
Klystron modulators• SLAC inductive adder topology is
generally replacing line-type topologies
• SLAC Marx topologies coming into use for long pulse SCRF applications
Ultra-fast beam kicker drivers
• Solid state nanosecond-switching pulse generators for transmission line beam deflectors Inductive adder modulator next to a SLAC
6575 standard modulator. 3x volume reduction.
RF Source Development Renaissance at SLAC
After nearly 10 years of continuous attrition of personnel devoted to RF source technology, recent changes underway:• Upper management decision to maintain as one of SLAC core
competencies• All departments reorganized with new management• New funding being allocated for rf source R&D• Encouraged to actively seek outside funding sources • Two new hires in Electrodynamics department• New THz initiative – seeking to fill new staff scientist position to
lead program
Seeking to apply HPRF source and accelerator expertise to a broader set of problems, beyond DOE Office of Science
Spent Beam Energy Recovery for Pulsed RF Sources
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Motivation for Energy Recovery in Pulsed RF Sources
• Growing attention placed upon energy usage at laboratories• Potential for providing a time-phased way to improve an
existing, aging facility• Future, high-rep rate (>kHz) applications place very tough
demands on modulator
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Depressed Collector Technology
• Used to improve the effective efficiency of vacuum tubes• CW depressed collector technology is mature
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CW Spent Beam Energy Recovery Methods not Suitable for Pulsed Systems
Typical CW biasing methodology
Ringing on cathode from parasitic elements results in phase jitter – Need better solution
Parasitic elements
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The SLAC Pulsed Depressed Collector
• The collector stages self-bias as electrons impact the stage surfaces
• The time-varying potentials of the stages are determined by the spent beam characteristics and the collector electrical impedance
• Recovers energy for the next pulseClaims:
1)This is the first demonstration of a pulsed depressed collector (using a single power supply) in a high power vacuum device
2)This is the first method to recover energy from the spent beam during the rise and fall time of the pulse
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Self-Biasing Concept
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Multiple Benefits of SLAC Self-Biasing Approach
• Recovers energy during rise, fall, and flat-top
• Modulators would have a relaxed requirement on rise and fall times
• Existing systems can be retrofit
• No extra power supplies. Cost no longer proportional to number of stages• Concept is not limited to a particular klystron or modulator topology
• Bias tuning can be accomplished through adjustment of the storage capacitance. Can be done “automatically” if desired
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Case for Economic Justification : A 5045 depressed collector
5045
Klystron RF Efficiency 45%System Efficiency (no recovery) 25%Pac,no recovery 107 kWSystem Efficiency (with recovery) 33%Pac, with recovery 82 kW
Average power consumption can potentially be reduced by over 25%
Implementation expense recover in ~10 years
>$800k/year electricity savings if implemented on 80 LCLS stations.
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Pulsed Depressed Collector Experiment
Purpose
• Show that self-biasing concept works
• Confirm models
• Answer major concerns brought up in internal discussions
Approach
• Get results as quickly as possible
• Not about optimizing for best efficiency
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Subbooster Klystron Depressed Collector Fabrication
ExistingSubbooster
Klystron
AfterBakeout
stage 1stage 2
location ofold collector
spentbeam
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SLAC Klystron Test Lab Experimental Setup
SubboosterKlystron
DepressedCollector
Energyrecovered
Energy dumpedbetween pulses
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Example Result of Self-Biasing Collector Stage
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Klystron Test Lab Experimental Setup
• Five separate circuit element parameters are available to tune
N:1
Zc,load
ZL,leakage
ZL,magZc,primaryCollector
Stage
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Comparison to PIC Simulations
•The stage potentials vary over time and depend on the biasing impedances and the collected currents•This problem is solved iteratively with a PIC and SPICE code
2D Magic PIC Simulation
SPICE Circuit Simulation
Collected Stage Currents
ResultingStage Potentials
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Comparison to PIC Simulations
Biasing Impedance/Stage Voltage Maximum
22 nF/15kV
44nF/11kV
66nF/9kV
Measured Collector Efficiency
18% 19% 17%
Simulated Collector Efficiency
20% 19% 18%
•To the left, simulated results are compared for two different circuit nodes
•A good match is obtained over a fairly large range of different biasing conditions
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Application to Long Pulse Systems
• Modulator rise time less of an issue for long pulse (ms and longer) but potential interest for retrofit of existing systems for energy recovery during flat top
• Unfortunately transformer approach does not scale well as loss and cost go up with pulse length
Solution - An “Inverse” Marx Energy Recovery Modulator
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An “Inverse” Marx Energy Recovery Modulator
•Capacitors charge in series, and discharge in parallel
•A transformerless, solid-state topology
•Using resonant recovery, can passively recover energy back to the modulator
In-between pulses
During pulse
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Repeat Test Using Inverse Marx Approach
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Comparison of Simulation to Experiment
• Good match between PIC/SPICE model and experiment• Additionally, we can pre-charge biasing capacitors to produce a more-square pulse
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Next-Generation Modulator Systems Both Provide and Recover Energy
Next-generation modulator development completes holistic approach to RF system design
Traditional Modulator
(DC to Pulse Converter)
AC Power RF Power SourceRF Out
SLAC Energy Recovery Modulator
(Pulse to DC Converter)
SpentBeam
Energy
Recovered RF Power
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Summary and Next Steps
Summary• Basic concept has been experimentally demonstrated• Good matches with circuit and PIC simulations
Next step
• Implement in a challenging application:- Scale up to the SLAC workhorse klystron, the 5045
(three orders of magnitude greater peak power than “subbooster” klystron)
- Apply to ultra-short pulse, high repetition rate klystrons
Scalable High Efficiency Klystron
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Unique Modular MBK Combining Scheme As Method to Produce a Scalable Design
Goal is to generate a design which can be scaled to higher power levels using modular design• Each module to use
low perveance beam for high efficiency
• Reduced NRE costs for new designs
• Economy of scale• Graceful power
degradationProposed scalable design where power
scales as (2N)2
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X- Band Multi-Beam KlystronDesign Specifications
Parameter Design GoalBeam Voltage (kV) 60Frequency (GHz) 11.424Output Power (MW) 5Beamlets 16Beam Focusing Periodic Permanent Magnet (PPM)Efficiency (%) 60+Cathode Loading (A/cm2) < 10
X-Band Multi-Beam Klystron
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Gun Design
X-Band Multi-Beam Klystron
Semi-automated gun design using R. Vaughan’s gun synthesis approach
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RF Cavity and PPM Design
X-Band Multi-Beam Klystron
RF Cavity PPM Period
Magnet IronCavity
Drift Tube
To minimize size and the number of beamlets, the PPM iron will be plated with copper to act as a cavity wall.
RF Cavity Goes Here
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65%+ Efficiency Predicted
X-Band Multi-Beam Klystron
One-Dimensional AJDISK RF Design
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Rapid 2D Transport Simulation
X-Band Multi-Beam Klystron
Pseudo Port Model (Using MAGIC2D)1D Simulation voltages are set at the port boundary. Simulation Time: ~1minute
I1/I0 (consistent with 1D simulation)
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Initial PPM Field Profile Based on Charge Density
X-Band Multi-Beam Klystron
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2D PPM Transport98%+ Transmission
X-Band Multi-Beam Klystron
Normalized Axial PPM Field
Beam Transport Using Port Model
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Port Model Agrees with Cavity Model
X-Band Multi-Beam Klystron
Port Model: Black ParticlesCavity Model: Red Particles
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Two Energy Product PPM Stack
X-Band Multi-Beam Klystron
Simple Script for Quickly Building the PPM Stack
New PPM stack with optimized pole piece geometry to reduce the number of energy products required
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Beam Transport Including Collector
X-Band Multi-Beam Klystron
MAGIC Port Model
Collector
PPM Stack
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Magnet Modification for the Input and Output Cavities
X-Band Multi-Beam Klystron
Magnet Geometry for the Input and Output Cavities
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3D PPM Stack
Iron Magnet
Cutout for input and output waveguide
X-Band Multi-Beam Klystron
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Magnet Cutouts Have Minimal Effect on Beam Transport
X-Band Multi-Beam Klystron
3D DC Transport
X-Band MBK 3D Layout
Collector
Gun
Input and Output Waveguide
The cathode to collector tip distance is ~30 cm(the maximum diameter is ~6.5 cm)
PPM StackThe outer diameter of the iron pole piece is used to shunt the field to match the current density as the beam bunches.
X-Band Multi-Beam Klystron 45
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Summary
• Simulations confirm the scalable MBK module will meet desired specifications
• Mechanical design and drafting are underway
• Modeling of the combining scheme is being finalized