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Operational Experience with LCLS RF systems

This article describes the operational experience and performance of the LCLS RF systems, including the RF gun, S-band and X-band systems, and the transverse cavity. It discusses issues, such as jitter and noise, and outlines plans for future studies.

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Operational Experience with LCLS RF systems

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  1. Operational Experience with LCLS RF systems

  2. 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

  3. General RF System Design

  4. RF Gun

  5. 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.

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

  7. 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.

  8. 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

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

  10. 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.

  11. 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

  12. L1-S RF System

  13. 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.

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

  15. L1X RF System • Uses 60cm X-band structure • 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.

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

  17. 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

  18. 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.

  19. 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.

  20. 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.

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