Operational Experience with LCLS RF systems

LCLS RF systems RF Gun 23MW 5.75MV, ~115MV/M Laser 30° from crest Standing Wave structure OTR screens (7) YAG screens (7) Wire scanners (7) L0A 35MW, 57MV, ~20MV/m 2° nominal (usually on crest) Dipole magnets (8) Beam stoppers (2) S-band RF acc. sections (5) L0B 60MW, 72MV, ~24MV/m 10° nominal (usually on crest) L1S 147MV, 20MV/m 25° nominal Transverse Cavity 2MW, 1.4MV ~4MV/m 180° transverse : diagnostic 250MeV 50% 25% 25% 2-km point in 3-km SLAC linac L1S 135MeV BC1 L1X X-band ~20MW, 20MV, 33MV/M 160° nominal OTR

General RF System Design

RF Gun

RF Gun System Specifics Standing wave structure – field probes to measure RF Contains vacuum sensitive cathode – want to avoid arcs: Hardware RF power interlock. Hi Q system, needs precise temperature control to stay on resonance. Gun operates at high peak field – 115MV/M.

Gun RF Performance Amplitude Jitter 0.04% Phase Jitter .053° Spec 0.1% Phase Jitter .053° Spec 0.1° (but same read back as feedback) Good Reliability

RF Gun Issues Minimal problems Field probes overheat at high rate, limited to 30 Hz. Re-designed probes to be tested on Gun 2. Software RF feedback can cause problems with interaction with modulator hardware trips. Software tries to maximize power during hardware RF power ramp.

L0A RF System Standard S-band RF system Noise problems (modulator Thyratron?) 0.2% amplitude Spec 0.14% 0.16° Spec 0.14° System operates with good reliability

L0B RF System Standard S-band RF system Good Jitter .024% amplitude Spec 0.14% .043° Phase Spec 0.14°

L0B RF System Overall Excellent jitter (amplitude and phase) performance. Shows how well the S-band Modulator / Klystron can work! Had some processing problems early in the run (L0B is our highest gradient S-band structure), but seems OK now. L0B Amplitude normally run in feedback for energy stability. RF long term stability < 1% p-p.

Transverse Cavity RF system Standard S-band RF system Operates 90° off crest. Jitter OK, but fliers .017% amplitude 0.15° phase Regularly used as a diagnostic

L1-S RF System

Uses SLED cavity to drive 3 structures L1S System Specifics Uses SLED cavity to drive 3 structures Operates 25° off crest to provide acceleration and energy chirp for compression 147MV, 135MeV energy gain Phase noise important, off crest operation converts phase noise to bunch length and arrival time error.

L1S System Operation Amplitude noise: 0.043% Phase noise 0.10° Spec is 0.14% Phase noise 0.10° Spec is 0.14° Long term stability from beam feedback (BC1 energy)

L1X RF System Uses 60cm X-band structure XL-4 Klystron Originally developed as part of Next Linear Collider Project. Operates at ~33MV/M NLC gradient ~70MV/M XL-4 Klystron Operating at 20MW Design 50MW X-band operates 160° off crest (net deceleration) to compensate for curvature of energy vs. time from S-band.

System reliability has been good L1X RF System Amplitude noise 0.62% Spec is 0.25% Phase noise 1.2° Spec is 0.5° System reliability has been good

L1X Transverse Kicks L1X produces an unexpectedly strong transverse kick on the beam. Kick is ~50 degrees out of phase with energy gain. Adjusted timing of RF to eliminate power in couplers during beam passage – did not reduce kick. Not yet understood Emittance vs position Transverse kick Vs. phase Energy vs. phase

Noise and Stability Issues Noise measurements shown were performed with the same sensors that are used in feedback. Not really a fair measurement. Beam based energy measurements also used in feedback Just beginning serious measurements of noise and stability: Initial indications are that overall phase noise is < 1°, but doesn’t yet meet the 0.1° requirement. Low noise on individual stations indicates that the high power components are capable of meeting the spec.

Study noise / stability Near Term Plans Study noise / stability Multiple measurements available: Beam phase cavities, beam energy, bunch length monitors etc. Lack of a single “gold standard” for measuring phase complicates things Study X-band deflection, wakefields, emittance growth. May remove the cavity during the down-time (Sept 07) for RF measurements.

RF operates with good reliability. Overall RF Status RF operates with good reliability. All systems running at design gradient. Gun currently at 5.75MV, not 6, but believe we can turn up at any time. Phase noise needs some work, but high power equipment seems OK Need to investigate transverse effects in L1X.