CLARA Gun Cavity Optimisation NVEC 05/06/2014 P. Goudket G. Burt, L. Cowie, J. McKenzie, B. Militsyn

CLARA Compact Linear Advanced Research Accelerator Beam Energy ~250MeV SASE Saturation length <15m Seed with Ti:Sa 800nm, lase up to 8 th 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 Photocathode Solenoid Bucking coil RF TM010  -mode

Basic parameters UnitsOperating mode Repetition Rate100 Hz400 Hz FrequencyGHz RF peak power maxMW<10 RF average power max (estimate) kW10 Gun gradientMV/m120100/80 Bunch chargepC Operating modePulsed/Train pulsedPulsed RF feedbackRequired

Choice of number of cells Cavity design 100 Hz400 Hz 100 MV/m120 MV/m100 MV/m120 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 cell 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 radius b = major radius r = iris radius blend = blending radius C1 length = length of 1 st cell including iris C1 length

Changing ellipticity (MHz ) 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 Energy6.23 J Maximum Surface H A/m Ratio of cathode E field to maximum surface E field1.06 E field flatness Operating Mode Frequency MHz Zero Mode Frequency MHz Q0Q Coupling (k)6.86E-3 Mode Separation20.08 MHz Integrated Axis field6.69 MV R/Q (no Transit Time factor) R (no Transit Time factor)3.06E+06 Superfish optimisation: final results

EHEH

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 2.54x10 5 A/m. – The calculated temperature increase for that location is 25K The baseline H-field in that location is 1.94x10 5 A/m. – The calculated temperature increase for that location is 14K

Cathode plug: 10 mm diameter Cathode plug (10 mm diameter) 3 rd 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. 0.6 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 Q 0 as being The target Q e should be CoupZ0 = 11.5 mm Some adjustability should built- in in order to allow for adjusting the Q e to the effectively measured Q 0.

Minimising coupler parasitic mode transmission

Tuneable alternative: H feed Short 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.