CLARA Gun Cavity OptimisationNVEC

05/06/2014

P. GoudketG. Burt, L. Cowie, J. McKenzie, B. Militsyn

CLARACompact Linear Advanced Research Accelerator

• Beam Energy ~250MeV• SASE Saturation length <15m• Seed with Ti:Sa 800nm, lase up to 8th harmonic• Seeding with HHG at 100nm also possible• Single spike SASE, electron bunch length ~50fs FWHM and charge <20pC • Seeding, peak current ~400A, flat top ~300fs and charge <200pC

Photocathode Guns• Allows the production of

extremely short bunches dependent on laser pulse length.

• High RF gradients and solenoidal fields allow for emittance preservation.

• Removable photocathode inserts allow for higher quantum efficiency through the use of metal photocathode surfaces.

Laser Pulse

Electron Bunch

Phot

ocat

hode

Solenoid

Bucking coil

RF TM010 p-mode

Basic parametersUnits Operating mode

Repetition Rate 100 Hz 400 Hz

Frequency GHz 2.9985

RF peak power max MW <10

RF average power max (estimate)

kW 10

Gun gradient MV/m 120 100/80

Bunch charge pC 20-250 250

Operating mode Pulsed/Train pulsed Pulsed

RF feedback Required

Choice of number of cells

Cavity design

100 Hz 400 Hz

100 MV/m 120 MV/m 100 MV/m 120 MV/m

Pulsed power, MW

Average power, kW

Pulsed power, MW

Average power, kW

Pulsed power, MW

Average power, kW

Pulsed power, MW

Average power, kW

1.5 cell 5.7 1.7 8.2 2.4 5.7 6.8 8.2 9.8

2.5 cell 10.0 3.0 14.4 4.3 10.0 12.0 14.4 17.3

In order to remain below the 10 kW average power limit, and the 10 MW peak power limit set by the klystron, the only option that allows peak fields of 120 MV/m to be reached is a 1.5 cell gun.

Target power 7MW peak due to losses in transmission system.

a

b

r

blend

a = minor radiusb = major radiusr = iris radiusblend = blending radiusC1 length = length of 1st cell including iris

C1 length

Changing ellipticityRatio is , at this point the maximum field on the cavity wall is no longer on the cell to cell iris. This is the chosen ellipticity. Major radius = 14 mm

(MHz)

Ellipticity

Ellipticity

Ellipticity is . Minor radius is held at 8mm (min cooling channel dimensions) and major radius is changed.

Maximum surface H changes by less than 1% over the whole range.

R/Q changes by ~2% over the whole range.

Q increases with higher ellipticity.

Mode separation is ~22 MHz

Mode separation

• Mode separation is the only thing that gets worse at lower iris radius

• Choose a lowest acceptable mode separation and go with that iris radius

• No lower than 20 MHz• Iris radius = 13.4 mm

15 MHz/m was required at LCLS to reduce beating between modes

Stored Energy 6.23 J

Maximum Surface H 220693 A/mRatio of cathode E field to maximum surface E field 1.06

E field flatness 0.9999

Operating Mode Frequency 2998.49 MHz

Zero Mode Frequency 2978.41 MHzQ0 16101.7

Coupling (k) 6.86E-3

Mode Separation 20.08 MHz

Integrated Axis field 6.69 MV

R/Q (no Transit Time factor) 190.29

R (no Transit Time factor) 3.06E+06

Superfish optimisation: final results

E

H

Probe aperture optimisation• In order to minimise the peak H-field, a large probe aperture was chosen• This has the effect of changing the cell frequency, necessitating a retune of

that cell• The peak magnetic field in the cell is now 2.54x105 A/m.

– The calculated temperature increase for that location is 25K• The baseline H-field in that location is 1.94x105 A/m.

– The calculated temperature increase for that location is 14K

Cathode plug: 10 mm diameterCathode plug (10 mm diameter)

3rd generation plug

Cathode plug optimisation

Additional optimisation was performed on the cathode plug profile. An elliptical profile was adopted to maximise the (E field at centre)/(max E field on cathode) ratio.

0 0.5 1 1.5 2 2.5 30

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Electric peak field ratio

Ellipse radius along z directionRa

tio

0.6 ratio chosen

Field flatness with cathode and probe coupler.

Peak field distribution - Electric

Peak field distribution - Magnetic

coupZ0 = distance between C2 iris and coupler tip

External Q

CST gives the Q0 as being 14970.The target Qe should be 15000.CoupZ0 = 11.5 mm

Some adjustability should built-in in order to allow for adjusting the Qe to the effectively measured Q0.

Minimising coupler parasitic mode transmission

Tuneable alternative: H feedShort

RF in RF in

• Shorts are now used which can be adjusted and used for pumping.

• Tee sections provide a reflective element to provide matching.

Mechanical Design

• Up to 10kW average RF power will need to be handled.

• Thermal simulations have been extensively performed on all gun components in order to ensure power handling capability.

• Solutions to integrate the gun with the solenoids, photocathode transfer system and other components are mostly complete.

Conclusion

• The RF design is approaching its final phase and mechanical design is already under way.

• Planned manufacture later this year.

• Planned installation from early 2015.