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LCLS-II Beamloss

LCLS-II Beamloss. Update 12/8/11 J. Welch. Overview. Dumps. Beamline sections. Assumed Parameters. ¼ of LCLS-I Charge. *From CDR. Open Issues. How much beam loss might there be in the dump line given high energy extraction from deep taper and self-seeding? Need reasonable MCB scenarios

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LCLS-II Beamloss

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  1. LCLS-II Beamloss Update 12/8/11 J. Welch

  2. Overview

  3. Dumps

  4. Beamline sections

  5. Assumed Parameters ¼ of LCLS-I Charge *From CDR

  6. Open Issues • How much beam loss might there be in the dump line given high energy extraction from deep taper and self-seeding? • Need reasonable MCB scenarios • Extra loss across BC1 or BC2 for higher dark current or profiling with collimator? • presently set at 2% of beam, (~50W BC2) • Should chicane collimators be able to intercept nearly all the beam? • Losses from pulsed kicker are not well understood.

  7. Dark Current Assumptions Dark current observed from LCLS-I Gun: typically 2 pC to 20 pC, with maximum of 42 pC ( 0.2 – 2.4 nA, max 5 nA) [F. Zhou 2011] Dark current from an accelerator section is estimated to be 19 nA based on Ross (1991). It is assumed not to propagate beyond one section. Getting new measurements of dark current in Linac sections.

  8. Beamloss Requirements • LCLS-II is designed for one bunch at 0.25 nC • Performance Requirements are set to be consistent with 10 bunches per pulse, 120 Hz, 0.25 nC. • MCB has additional conservative factor of 2 above beamloss requirement.

  9. MCB Definition: RSS 2011 • 2.3.3 Maximum Credible Beam The concept of a maximum credible beam is used to determine the required shielding and protection devices. It defines the highest beam power that the accelerator can deliver to a point assuming that the BCS devices that limit beam power have failed. In estimating the maximum credible beam, conservative, but reasonable, assumptions should be used. Assumptions will be reviewed by the RSO and RSC. For example, it is not necessary to assume that BCS device failure will coincide with a significant effort to retune the accelerator to deliver higher beam power than was approved for the facility. • MCB is not maximum conceivable beam • Need feedback on interpretation

  10. MCB scenario • We are commissioning multi-bunch on TD11 and all BCS fails. Beam goes to high energy and high power. • multi-bunch commissioning only makes sense for source currents such that the final beam poweris less than design capability of dump - 5 kW. Scaling 13.5 GeV back to 250 MeV, the 5 kW beam would have a power of about 100 W at TD11. • make MCB at TD11 2 x 100 W = 200 W to make it more conservative. • similarly MCB at main dumps is 10 kW (not 100 kW in LCLS-I)

  11. LCLS-II MCB • Decline to use the explosive emission scenario • never observed at LCLS • gun is not designed to run with explosive emission • Judge that simultaneous continuous operation with explosive emission from the cathode and simultaneous BCS failure is not credible. • Accidents scenarios • Iris error and feedback off could generate higher bunch charge. • typo/software bug in the charge setpoint • . . . • Judge not reasonable to assume such accidents are coincident with both multibunch commissioning and failure of BCS. • max charge off cathode ~ 2 nC due to space charge – much less than 10 x 0.5 nC limit assumed in MCB • New Cathode • even if QE is 10% we would still only want 0.25 nC bunches. • New multibunch optics would be needed to generate higher currents for operation.

  12. LCLS-I/LCLS-II MCB • LCLS-I is based on current from continuous explosive emission from the cathode • LCLS-II is based on current limited to 2x performance requirement.

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