LCLS LLRF System October 10-13, 2005 LLRF05 B. Hong, R. Akre, A. Hill, D. Kotturi, H. Schwarz SLAC, Stanford, Menlo Park, CA 94025, USA Work supported under DOE Contract DE-AC03-76SF00515

Outline  Overview of LCLS Injector RF System  LCLS RF Stability Requirements  Low Noise RF Distribution System  RF Control and Monitoring System

LCLS INJECTOR / LINAC  The LCLS linac consists of 4 sections. The injector L0 (2 S-Band Klystrons) will reside in an off axis tunnel at the end of sector 20. L1 will have 1 S-band and 1 X-band klystron to feed their linacs, respectively. L2 (28 klystrons) and L3 (48 klystrons) are made up of the current main SLAC linac sectors 21 to 30.

The site layout of the LCLS linac

LCLS RF Stability Requirements  Phase and Amplitude requirements by the physics design: LCLS specifications require RF stability of 0.1%rms in amplitude and 100fSrms in a 850nS fill time S-Band structure, but the tightest tolerance is 125fSrms in a 100nS fill time X-Band structure. See Table on next page.  Bandwidths of the X-band System Beam seen due to structure fill time ~ 10MHz Structure RF Bandwidth ~ 40MHz XL4 Klystron > 100MHz  Bandwidths of the S-Band System Beam seen due to structure fill time ~ 1.2MHz Structure RF Bandwidth ~ 16MHz 5045 Klystron ~ 10MHz  The bandwidth for the system will be determined based on noise levels of the as built RF system and rise times of interest, somewhere between 1.2MHz and 10MHz.

ParameterSymbol|ΔI/I0| < 12% |  ΔE/E0  | < 0.1% Unit mean L0 rf phase (2 klystrons) 0 S-band deg mean L1 rf phase (1 klystron) 1 S-band deg mean LX rf phase (1 klystron) xx 0.5 X-band deg mean L2 rf phase (28 klystrons) 2 S-band deg mean L3 rf phase (48 klystrons) 3 S-band deg mean L0 rf voltage (1-2 klystrons)ΔV0/V00.10 % mean L1 rf voltage (1 klystron)ΔV1/V10.10 % mean LX rf voltage (1 klystron)ΔVx/Vx0.25 % mean L2 rf voltage (28 klystrons)ΔV2/V20.10 % mean L3 rf voltage (48 klystrons)ΔV3/V % BC1 chicaneΔB1/B10.01 % BC2 chicaneΔB2/B20.05 % Gun timing jitterΔt0Δt00.8 psec Initial bunch chargeΔQ/Q0ΔQ/Q % RMS tolerance budget for <12% rms peak-current jitter (column 3) or <0.1% rms final e− energy jitter (column 4). The tighter tolerance is in BOLD, underlined text and both criteria, |DI/I0| < 12% and |  DE/E0  | < 0.1%, are satisfied with the tighter tolerance applied. All tolerances are rms levels and the voltage and phase tolerances per klystron for L2 and L3 are  Nk larger, assuming uncorrelated errors, where Nk is the number of klystrons per linac.

Phase Noise of SLAC Master Oscillator Noise Floor -120dBc/38Hz = -136dBc/Hz = 120fS rms Jitter in 5MHz BW Old OscillatorNew Oscillator New Oscillators Have a noise floor of 476MHz Noise Floor -143dBc/38Hz = -158dBc/Hz < 16fS rms Jitter in 10MHz BW

LINAC Front End RF and Timing System PEP PHASE SHIFT ON MAIN DRIVE LINEMDL RF with TIMING Pulse – Sync to DR Master Oscillator is located 1.3 miles from LCLS Injector 1.3 Miles to LCLS Injector LCLS must be compatible with the existing linac operation including PEP timing shifts

RF Distribution System

Linac Phase Reference System  Main Drive Line - 3 1/8 Rigid Coax Anchored to Concrete Floor Every Sector  Must not introduce noise over 2 miles. Diurnal Temperature Variations 200fS rms/Sector  Phase Reference Line - Each Sector Independent 1/2 “ Heliax  332ft/Sector. Diurnal Temperature Variations 640fS rms errors The PAD measures phase noise between the reference RF and the high power system. The beam sees 3.5uS of RF from SLED.

Noise Levels  RF Reference Noise Floor  Oscillator -148dBc/Hz SSB at 2856MHz  RF -138dBc/Hz SSB at 2856MHz  Integrated Noise  -138dBc/Hz at 10MHz = -65dBc = 32fS rms  SNR = 65dB for phase noise  Added noise from MIXER (LO noise same as RF)  SNR of 62dB  ADC noise levels  SNR of 70dB – 14bit ADS5500 at 119MSPS 8.5MHz

RF Control and Monitor Points Summary  RF Gun1 Klystron 3 RF monitors  Beam Phase Cavity 1 IQ modulator 1 RF monitor  L0-AAccelerator 1 Klystron 2 RF monitors  L0-B Accelerator 1 Klystron 2 RF monitors  L0-T Transverse Accelerator1 Klystron 2 RF monitors  L1-S Station 21-1 B, C, and D accelerators 1 Klystron 4 RF monitors  L1-X X-Band accelerator X-Band  S25-Tcav1 Klystron 2 RF monitors  S24-1, 2, & 3 Feedback3 Klystrons  S29 and S30 Feedback2 IQ modulators 476MHz  Totals 2856MHz10 modulators 16 monitors

RF Monitor  LO MHz : RF 2856MHz  IF 25.5MHz (8.5MHz x 3 in sync with timing fiducial)  Double-Balanced Mixer  Mixer IF to Low Pass Filter and Amp  Amp output to ADC (119MSPS or 102MSPS)

SLAC Linac RF – New Control The new control system will tie in to the IPA Chassis with 1kW of drive power available. Reference will be from the existing phase reference line or the injector new RF reference I and Q will be controlled with a 16bit DAC running at 119MHz. Waveforms to the DAC will be set in an FPGA through a microcontroller running EPICS on RTEMS. Existing System

RF Sub-Systems  RF Gun  Beam Phase Cavity  L0-AAccelerator  L0-B Accelerator  L0-T Transverse Accelerator  L1-S Station 21-1 B, C, and D accelerators  L1-X X-Band accelerator  S25-Tcav  S24-1, 2, & 3 Feedback  S29 and S30 Feedback  The RF Monitor unit will be capable of measuring phase and amplitude for 2 RF channels.  The RF Control unit will be of two versions. The fast version will digitize a 1k sample of data to drive an IQ modulator. The slow version will be capable of setting an I and Q value on a pulse to pulse basis.

RF Gun The RF Gun will be driven from klystron The RF monitors on the gun will be used in feedback loops to control the operating frequency along with providing phase and amplitude information to the longitudinal feedback.

Beam Phase Cavity The Beam phase cavity will be used to measure the bunch position relative to the RF. The measurement will be done at 120Hz and provide information to the feedback system. The cavity is located between L0-A and L0-B accelerator structures.

L0-A - First accelerator in the off axis injector L0-B has the similar control scheme.

Injector Transverse Deflector Cavity Station 20-5 and Station 24-8 will be connected to the new control system and used to drive the L0 and Sector 25 transverse deflector cavities, respectively. The cavity is used to impart on the beam a deflection vs. longitudinal position correlation. The beam is deflected to a screen where bunch length can be measured.

L1-S The L1 linac consist of klystron 21-1 powering 3 accelerator structures in the main linac.

X-Band 2nd order correction An X-band accelerator structure will be located at station 21-2, just before BC1. The beam will be run on the decelerating crest to remove the second order curvature in the bunch vs longitudinal position correlation before compression in BC1.

S24-1, 2, and 3 (Three Units) Two klystrons in sector 24 will be used to correct the phase and amplitude of the RF as seen by the beam in LCLS L2 before the beam enters BC2. The klystron phase and amplitude will be adjusted based on bunch length and energy measurements. A third klystron will be capable of use by the feedback system if one of the other two should fail.

S29 and S30 Phase Control (Two Units) The phase of sectors 29 and 30 will be adjusted in opposite directions to change the average amplitude of the RF as seen by the beam as it passes through the sectors without effecting the average phase as seen by the beam.