Injector SetupMiniphase Stephen Benson JLab Description of injector
Injector Setup/Mini-phase Stephen Benson, JLab Description of injector setup sources of drift Mini-phase procedure for injector Checking the rest of the machine.
Gun Setup Beam parameters that need to be set in the injector: Drive laser parameters: Set pulse length using autocorrelator, set transverse profile using Brewster window camera, and set position on cathode using cathode camera. Set power using Unser monitor or 1 G dump Faraday cup. Gun voltage. Set open loop. Magnets. 14 correctors and 8 focusing elements. Center in some focusing elements. Dipole bus set for 9 Me. V energy. RF gradients and phases. Spectrometer phasing and gradient setup at first, then use 1 st cavity focusing, second cavity gradient, and energy spread from injector. Don’t know how to set the laser vs. buncher phase or buncher gradient. Match to linac to set quads. Use spreadsheet model and iterate. Once the injector is setup, only change the phases and cavity 3 gradient and maintain laser state.
Reminder of Injector Geometry SRF cavities (2) Solenoids (2) Electron gun (1) Viewers (3) Quads (4)
Injector / Drive Laser Diagnostics all focusing elements are placed upstream of the pick off plate distance from the pick off to the cathode = distance from pick off to reference screen the reference screen used for D. L. profile and position measurements
Injector / Drive Laser Diagnostics Typical auto-correlator signal JLab FEL D. L. (observed in the control room permanently) Drive Laser transverse profile dynamic range ~500: 10 -bit frame grabber, 60 d. B SNR CCD
Drive Laser “ghost” pulses If “ghost” pulses get large they can effect the beam images and throw you. Problem: this does not work for CW beam Beam current measurements with a linear circuit current measurements with A Log-amp circuit (60 d. B)
Magnetic Elements There are 23 magnetic elements in the injector. The MBH 0 F 06 H is almost always zero, as is the MQZ 0 F 02. Not shown are the injector string. Note the strengths of the magnets before the unit. Use 0 F 01 to set position in the buncher, 0 F 01 A to center in 2 nd solenoid, 0 F 02 to “center” in the unit 0 F 03 to center in MQJ 0 F 05, 0 F 05 to center in MPJ 0 F 06, 0 F 06 V to center vertically at ITV 0 F 06 and 0 F 06 A to center in the ITV 0 F 01 hole. Never change magnets during a miniphase!
Injector Phase Monitor Multiplexer Laser Cathode Buncher Vector Voltmeter
Phasing the Injector • • Once beam is steered up before the cryounit, zero the buncher GASK signal using the drive laser phase. Obtain reasonable spots on ITV 0 F 04 and ITV 0 F 06 using laser/buncher gang phase and cavity 3 phase. Use injector phasing script to set laser/buncher gang phase and cavity 3 phase. Check centering in the unit and last two quads. Set cavity 3 gradient to center the beam at ITV 0 F 06. Check Happek. If low, reoptimize using laser phase and buncher gradient. If you have to change these, go back to the beginning and iterate. Record miniphase images and phases.
Injector / Transverse beam profile ITV 0 F 04 image is sensitive to cavity 4 phase (size) and cavity 3 phase (x-position) ITV 0 F 06 image is sensitive to cryounit gradients, cavity 3 phase (x-position)cavity 4 phase, vertical size, and laser phase (horizontal size) Phases drift more and faster than gradients. Also get small drift in vertical steering.
Pulsed and CW beam measurements / transition Interferogram (autocorrelation) Most affordable way to measure ps and sub-ps bunches Works with pulsed beam (tune up) and CW beam, essentially at any average current Used at JLab FEL to ensure that the bunch length does not change for pulsed or CW and when the average current is increased Ultimately needs to be setup in vacuum (or N 2 purge) due to atmosphere absorption of THz Phase information is lost – no direct bunch profile reconstruction A detector measuring total CTR (CSR) power – a bunch length monitor Power spectrum
Conclusion Setup of injector is moderately complicated but straightforward. It can take several hours to a couple days and can change every corrector and RF parameter in the injector. One the injector is setup, images are taken of ITV 0 F 04 and ITV 0 F 06. If these are reproduced in practice, the beam is also reproduced. Only phases are changed. Linac phasing establishes beam 10 degrees off crest in the linac. Vernier cavity is used to adjust the longitudinal match. Verified using Happek scan Path Length corrector does not reproduce. It must be checked each time the machine is started up. Steering of the second pass at 5 F 10 might be possible if losses are found in the linac. Note: The Path length and 5 F 10 correctors are the only allowed correctors. Not quads can be changed in a miniphase.
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