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Methodology for Tracking Downstream End of CC-1 with ASTRA and Dark Current Measurements

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This document outlines the methodology for tracking the downstream end of CC-1 using ASTRA, along with dark current measurements derived from a molybdenum cathode. It discusses the use of PI parameters relevant for a bunch charge of 3.2 nC and bunch compression techniques. The analysis includes various acceleration settings for the electron gun (Egun) and CC-1, dark current distributions, and the wakefield calculations processed through ABCI and verified with MAFIA. Key insights on emittance growth and beam alignment criteria are also provided.

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Methodology for Tracking Downstream End of CC-1 with ASTRA and Dark Current Measurements

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  1. Methodology • Tracking to downstream end of CC-1 with ASTRA • Using PI parameters (possibly) suited for 3.2nC/bunch and bunch compression • Egun @ 40MV/m, -12°; CC-1 @ 60MV/m, -15° (wrt max. acceleration) • Also, egun @40MV/m, -8°; CC-1 @ 20MV/m, -20° (wrt max. acceleration) • Dark current originating uniformly on 16mm dia. Mo cathode plug • Also, dark current originating uniformly from 5mm dia cathode • Using gaussian approximation to “Han” time distribution (see next slide) • I(t) = A*sin2.5(wt)*exp(-B/sin(wt)); B=5 • ~ A*exp(-.5*t2/s2) • Using apertures from R. Andrew’s drawings • Using ABCI to calculate collimator wakefield • Verified with MAFIA by R Wanzenberg

  2. PITZ Dark Current Measurements (Han, et al, EPAC04) Measurements made at screen 1 about 78cm from photocathode old new new “6 times worse than previous rf gun”

  3. FLASH/PITZ Dark Current Measurements Summary (Han, et al, 2006) PITZ 40 – 42 MV/m; dark current measured 78cm from photocathode FLASH

  4. A0 Dark Current Measurements (Hartung, et al, PAC01) Gun @ 35MV/m; dark current measured 45 cm d.s. of gun

  5. Dark Current Time Distribution @ Cathode RF phase (deg)

  6. Dark Current Intensity vs. Distance from Cathode (table) • Egun @ 40MV/m, -8°; CC-1 @ 20MV/m -20° (wrt max. acceleration) • Dark current originating uniformly on 5mm dia. cathode • 10k electrons at photocathode

  7. Dark Current Intensity vs. Distance from Cathode (table) • Egun @ 40MV/m, -12°; CC-1 @ 60MV/m -15° (wrt max. acceleration) • Dark current originating uniformly on 16mm dia. Mo cathode plug • 2K electrons at photocathode

  8. Dark Current Intensity vs. Distance from Cathode (graph) • Egun @ 40MV/m, -12°; CC-1 @ 60MV/m -15° (wrt max. acceleration) • Dark current originating uniformly on 16mm dia. Mo cathode plug

  9. Transverse Wakefield from Collimator (from ABCI) Note: these calculations agree with MAFIA calculations done by R. Wanzenberg gaussian bunch, sz = 1.8mm 1.0 cm dia: peak wake = 750 V/pC/m 1.2 cm dia: peak wake = 520 V/pC/m 1.4 cm dia: peak wake = 370 V/pC/m 1.6 cm dia: peak wake = 270 V/pC/m 1.8 cm dia: peak wake = 200 V/pc/m 2.0 cm dia: peak wake = 160 V/pC/m

  10. Beam Alignment Criteria at the Collimator Limit transverse jitter of downstream beam to < .1sy Dqcoll<20mrad. Using average wake field ~ ½ peak wake field; 3.2 nC; Pbeam=4.7 MeV/c 2 cm dia. collimator: Dyjitter < 380 mm 1 cm dia. collimator: Dyjitter < 80 mm Limit emittance blowup downstream beam to < 10%. De/e = 2*Ds/s < Dx/s  Dqcoll less stringent than previous criterion  see next 2 slides

  11. Simulation for Offset Dark Current Collimator 20K particles; 1 cm dia. collimator; max. kick = 750 V/pC/m * .001m * 3200 pC / 4.5 MeV = .5 mrad

  12. Emittance Growth

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