Design and Status of the VISA II ExperimentG. Andonian, R. Agustsson, A.Murokh, C.Pellegrini,

S.Reiche, J.B.Rosenzweig, G.Travish, UCLAI. Ben-Zvi, V. Yakimenko, BNLL.Palumbo, C. Vicario, INFN

Abstract VISA II is a follow-up venture to the succesful Visible to Infrared SASE Amplifier (VISA) experiment. This is a collaborative effort headed by UCLA and the ATF (Accelerator Test Facility) at BNL. There are two main experiments associated with the VISA II program. The first is the study of strong bunch compression, via a chicane compressor, and deep saturation investigations. The next undertaking is the study of the physics of a chirped-beam SASE-FEL, where a linear energy chirp is imparted on the beam and injected into the undulator. The VISA II program encompasses a number of hardware upgrades at the ATF, including the installation of the chicane compressor and addition of sextupole magnets to mitigate second order effects.

More information can be found athttp://pbpl.physics.ucla.edu/visa

PBPLParticle Beam Physics Lab

L inac Sections

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Transport

20° DispersiveSection

Matching L ine

Undulator

Golay cell

SA SE Diagnostics

F -L ineB eamline-3 H -line

Schematic of the V I SA beamline at the A T F (not to scale).

Compressed Bunch Overview

Chirped Beam Overview

SASE-FEL Experimental Program

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b) Power Profile at Saturation

Summary of Simulations

Summary of VISA I Results

Beamline Description

a) Saturation is evident at about 3m, well before the end of the undulator, allowing for deep saturation studies.b)Slippage plays an important role in the compressed bunch case. Current is represented by the dashed curve.c)The FEL Spectrum is peaked at about 685 nm.

The follwoing plots were generated by GENESIS 1.3, a 3-D FEL Code

Linearization of Transport

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Longitudinal Density Profile

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Longitudinal Density Profile

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a) Initial Trace Space after linac.

b) Final Trace Space after dispersive section (without sextupoles).

c) Final Trace Space after dispersive section (with sextupoles).

The addition of sextupoles to the dispersive line (F-line) will yield control over second order momentum effects. This allows for a better understanding when running VISA II in the chirped mode.It was discovered from the VISA I runs, that the natural compression in the dipsersive line was dominated by second order effects. These effects will be minimized and yield control of beam properties at the undulator entrance.

The following plots were generated using ELEGANT. Summary of Simulations

Electron and Photon DiagnosticsLaser DiagnosticsRF Diagnosticse-Beam DiagnosticsTemporal Struc.ChargeEmittance Energy DiagnosticsAdvanced DiagnosticsTemporal Struc.Bunch length (compression)

Spectrometer, Joulemeters, CCDsCouplers, Phase DetectorsYAG and phospor Screens, BPMsCTR MeasurementsFaraday Cup Quad ScanningDipole Spectrometers

FROGInterferometer, E-O Technique

The successful VISA I experiment was designed to investigate physical properties of SASE-FEL. A novel bunch compression mechanism was developed during the VISA I experiment. This scheme utilized second order momentum error effects in the dispersive line. VISA I also was successful in benchmarking the numerical codes used for start-to-end (cathode to undulator) beam simulation.

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Theoretical Studies on the spectral response of an initially chirped electron beam. Simulations show that total energy at saturation is not affected by chirping up to 4% (no gain degradation). It is necessary to go to 2% chirp to overcome the intrinsic frequency width of the FEL amplification in order to achieve a correlation between frequency and time. An ideal case for FROG measurement.

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VISA I Performance Parameters

EnergyEnergy SpreadEmittanceChargePeak CurrentGain LengthWavelength

70.7 MeV0.17 %3.3 µm140 pC250 A

17.9 cm842 nm

PeriodNumber of PeriodsParameter (K)Length

18 mm2201.264 m

Comp.Beam Undulator

71.2 MeV0.10 %2.1 µm250 pC

55 A29.7 cm831 nm

Non-Comp.Beam

Saturation Curve of VISA I.Measured data and simulation.

3-D Rendered Image of chicane compressor

Chicane Compressor

The chicane compressor will be installed along the H-Line (initial transport). The dipole magnets were designed and fabricated at UCLA. The bend radius is 1.2 m, and the nominal field is .1964T. The effective magnetic length is 44.7 cm.Once installed and operational, the chicane is expected to compress the beam down to a bunch length of 30 µm. Such a short bunch yields a current on the order of 1 kA. The design of the vacuum port allows for studies of CSR. There are two locations for diagnostics at the midpoint of the chicane, due to a novel diagnostic assembly.

Current Profile

Radiation Power