Injector Setup/Mini-phase

Description of injector setup sources of drift Mini-phase procedure for injector Checking the rest of the machine.

Stephen Benson, JLab

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 1G 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 MeV energy.

RF gradients and phases. Spectrometer phasing and gradient setup at first, then use 1st 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

Viewers (3)

Solenoids (2)

Quads (4)

SRF cavities (2) Electron gun (1)

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 profiledynamic range ~500:10-bit frame grabber, 60 dB SNR CCD

Drive Laser “ghost” pulses If “ghost” pulses get large they can effect the beam images and throw you.

Beam current measurementswith a linear circuit

current measurements withA Log-amp circuit (60 dB)

Problem: this does not work for CW beam

Magnetic Elements

There are 23 magnetic elements in the injector. The MBH0F06H is almost always zero, as is the MQZ0F02. Not shown are the injector string. Note the strengths of the magnets before the unit.Use 0F01 to set position in the buncher, 0F01A to center in 2nd solenoid, 0F02 to “center” in the unit 0F03 to center in MQJ0F05, 0F05 to center in MPJ0F06, 0F06V to center vertically at ITV0F06 and 0F06A to center in the ITV0F01 hole.Never change magnets during a miniphase!

Injector Phase Monitor

Multiplexer VectorVoltmeter

Cathode Buncher

Laser

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 ITV0F04 and ITV0F06 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 ITV0F06. • 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

ITV0F04 image is sensitive to cavity 4 phase (size) and cavity 3 phase (x-position) ITV0F06 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

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 N2 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

Interferogram(autocorrelation)

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 ITV0F04 and ITV0F06. 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 5F10 might be possible if losses are found in the linac.

Note: The Path length and 5F10 correctors are the only allowed correctors. Not quads can be changed in a miniphase.