LCLS X-Band RF System
R. Akre June 17, 2003
The LCLS beam is run off crest on the RF in order to set up a particle position verses energy correlation. The correlation is used to compress the bunch when it is run through a chicane. The RF waveform is a sine shape and the correlation set up by running the bunch off crest has a large second order component, it’s not very linear. The X-band RF station, with higher frequency, has a much larger curvature in RF voltage verses time. The beam is run on the decelerating crest in order to remove most of the second order, nonlinear, part of the correlation. The required 22MV of peak accelerating gradient will require a klystron power output of 24MW.
More information can be found in the CDR Chapter 7, by Paul Emma.
Modulator – Pulse Transformer
The 5045 klystron runs at 350kV, 400 amps, and 3.5uS flat top. The pulse transformer has a 15:1 turn ratio which requires the modulator output to be 23kV at 6kA. If we assume 2uS rise fall times, figure 1, the average power is 92kW.
The XL4 klystron runs at 450kV and 350 amps at 0.1uS flat top. By using a pulse transformer with a 19:1 turn ratio, the beam voltage can reach 450kV with a 23.5kV, 6.7kA input. Assuming 2uS rise fall times the average power is 40kW. The higher turn ratio pulse transformer will increase this value, but tuning of the modulator can reduce this value.
RF Control
In a similar manner to the feedback controlled S-band stations, the X-band RF system will be run from the existing modulator and control system, figure 2. This allows all the features of the existing system to be used. The existing system will be used to adjust the phase and amplitude and is expected to be stable to within 10 picoseconds in phase and 2% in amplitude. An external feedback will measure phase and amplitude, receive information from BPMs and Bunch Length Monitors, process the information, and adjust phase and amplitude to meet LCLS specifications.
Phase shifting will be done at the S-band level and then multiplied up to X-band. The existing controls in the IPA chassis will be used. The phase shift setting and read back will need to be scaled by a factor of 4 (SCP Software or hardware). Amplitude control will need to be done with an X-band attenuator after the multiplier at the mW power level. The power will then be amplified to the kilowatt level by a TWT amplifier and used to drive the klystron. Veneer controls at the TWT input for amplitude and phase will be used by the feedback system.
Figure 1. A typical beam voltage plot of a linac modulator. The 1.5uS rise fall times will be power dissipated in the collector.
HIGH POWER PLUMMING
The X-band klystron will be mounted in the position of klystron modulator 21-2 which is next to penetration 21-3 Figure 3. The system will use WR100 waveguide out of the klystron to a mode converter to WC293. The WC293 will run from the gallery above straight down to the tunnel below. Once in the tunnel a mode converter will change back to WR100, which will be used to route the RF to the structure. The structure is located 3 feet over and 5 feet upstream of the penetration. There will be three high power Bethe hole couplers in the system, one at the klystron output, one at the accelerator input, and one at the accelerator output. The coupler at the klystron output will be used to feed into the existing control system for 21-2 klystron. The accelerator input and output couplers will be routed to the new RF feedback system in the temperature compensated hut.
Figure 3. Non-artists, cut and paste, conception of a XL4 Klystron in Gallery Powering 53cm Structure in Tunnel.
CONCERNS
The X-band klystrons have been used for many years at NLCTA to run accelerating structures similar to the one proposed for LCLS. Three areas of concern are stability, reliability, and structure damage from beam interception.
NLCTA does not require the phase and amplitude stability listed in the LCLS specifications. Tests will need to be done to determine if the system will meet LCLS requirements without any major changes. There is no reason to believe the XL4 phase and amplitude sensitivity to the modulator (beam voltage) stability will be much more than a 5045 klystron.
NLCTA has not run at 120Hz and the reliability of the tubes at 120Hz needs to be evaluated. NLCTA has run at 60Hz with 1.6uS flat top pulses. LCLS requires 120Hz with 0.1uS flat top pulses. The average power to the tube could be significantly reduced with upgrades to the modulator, reducing the rise and fall times. If the tube is run below 30MW average power will no longer be a concern.
The 7mm diameter aperture of the standard NLC type X-band structures will be the limiting aperture in the main linac. Interception of high power End Station beams could be damaging to the structure. Wakefields from the structure and/or protecting collimators needs to be evaluated. Removal of the structure, either automatic or manual, during high power End Station running is an option.
LCLS Injector Klystron Power Levels
Device Operating Point Required Loss + Notes
Power Overhead
Gun (20-6) 30MW (7MV) 44MW -1.7dB 1,7
L0 20-7,8 46MW (75MV) 61MW -1.2dB 1
X-band 21-2 17MW (22MV) 24MW -1.5dB 2,3,4,5
Deflector 3MW 48MW -12dB 1,6
Notes:
1. 150ft of WR284 S-band rectangular guide has 1dB of loss.
2. 10m of WR100 X-band guide has 1dB of loss.
3. 10m of WC293 X-band guide has 0.04dB of loss.
4. Assume 0.6m X-band structure has shunt impedance of 30MW
5. X-band klystron XL-4 50MW at 410kV 315A
6. The output of the D accelerator structure in 20-5 will be used as
the RF Source for the RF Deflector.
7. Phase stability of Gun RF from 225ft of WR284 (0.014°S/(ft-°F))
Water 0.1°F => 0.3°S Requires phase correction.