CesrTA Wiggler RFA Measurements

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Presentation transcript:

CesrTA Wiggler RFA Measurements Joe Calvey LEPP, Cornell University 6/25/09 1 1

Introduction We have 6 wigglers installed in our L0 straight region Currently, two are instrumented with RFAs One is bare copper, the other is coated with TiN Each wiggler has three RFAs Center of a wiggler pole (approximately dipole)‏ Longitudinal field Intermediate (half way between pole and field null)‏ Each RFA has 1 grid and 12 collectors across the top of the beam pipe Wiggler RFA Data: CTA09

Wiggler “off” Data Probably some residual field 45 bunch train of positrons .75 mA/bunch 4ns spacing 5 GeV Cu wiggler Response in all detectors is roughly comparable Pole Center Longitudinal Field Intermediate Region 3 3 3 3

Wigglers on, 1x45x.5mA e+ 4ns spacing, 5 GeV Cu wiggler Pole Center No signal in longitudinal field detector Modest multipacting in other two RFAs Consistent with a low SEY for copper Pole Center Longitudinal Field Intermediate Region 4 4 4 4

43 bunches, .75mA/bunch, 56ns We did runs with large bunch spacing to investigate the primary photoelectron distribution Plots show current in the center pole detector in the Cu and TiN wigglers We actually expect a higher primary photon flux in the Cu wiggler Cu: 4 photons/m/particle vs TiN: 2.1 photons/m/particle But direct radiation is irrelevant: only scattered photons can generate primaries in the detector region The radiation pattern in L0 is complicated We need a better model of reflectivity, which includes photons scattered from upstream Photon flux is higher at 2 GeV (next slide) Cu Wiggler, 5GeV TiN Wiggler, 5GeV 5 5 5

43 bunches, .8mA/bunch, 56ns 2 GeV vs 5 GeV Cu Wiggler, 2 GeV TiN Wiggler, 2 GeV Cu Wiggler, 5GeV TiN Wiggler, 5GeV 6 6 6

1x45x.75 mA e+, 4 ns spacing Outer collectors scale linearly with bunch current Strange structure emerges in the center collector at higher bunch currents Peaks appear at different (nonzero) retarding voltages More on this in a later talk This effect is suppressed somewhat in the TiN wiggler Cu Wiggler, 5GeV TiN Wiggler, 5GeV 7 7 7

Summary Low beam current data seems to be consistent with a low SEY for processed copper We need to confirm with simulations and comparisons with adjacent drift RFAs At higher beam currents, a complicated structure emerges in the center collectors Probably an effect of the structure of the RFA Long bunch spacing data indicates that we need a better photon model Many of the reflected photons may be coming from upstream We are very interested in any suggestions for experiments we can do to test these tentative conclusions Wiggler RFA Data: CTA09