Characterization of the Fast Ion Instability at CesrTA David Rubin Cornell University

March 6, 2007 ILCDR07 2 Characterization of the Fast Ion Instability at CesrTA Signatures of fast ion effect -Tune shift that increases with bunch number n b  Q y ~ (p N 0 n b )/[  y (  y +  x )]  sb ~ N b 3/2  z 1/2 /[  y (  y +  x )] 3/2 -Multi-bunch instability growth rate that depends strongly on bunch number  mb ~ N b 3/2 n b 2 (L sep ) 1/2 /[  y (  y +  x )] 3/2 -The nonlinearity of ion focusing channel will lead to bunch dependent dilution of vertical emittance -Coupling of ion frequencies to the electron beam revealed in spectrum of beam frequencies -Single bunch instability growth rate that scales strongly with beam size

March 6, 2007 ILCDR07 3 Characterization of the Fast Ion Instability at CesrTA Linear theory indicates instability growth times ~ 1 turn for typical damping ring parameters Simulations suggest growth times are much longer A. Wolski’s introductory talk Effects consistent with the Fast Ion Instability have been observed in several storage rings (ALS, PLS, ATF)… …but we cannot predict with confidence the impact of ion effects on the performance of the ILC electron damping ring. – There is a lack of quantitative data with which we can benchmark the simulation codes in the appropriate (low emittance) regime: further studies will be very important.

March 6, 2007 ILCDR07 4 Cesr TA Cesr TA instrumentation provides for measurement of all effects - Bunch by bunch/ turn by turn BPM system  dependence of tune on bunch number - Streak camera  measure of bunch dependence of instability threshold - Bunch by bunch beam size monitor  dependence of beam size on bunch number along train - Analysis of vertical spectrum  characteristic ion frequencies - Clearing electrodes in wigglers may contribute to our understanding of ions Flexibility of Cesr TA optics allows for survey of all dependencies - beam size - bunch charge and spacing - train length - beam energy - particle species (electrons and positrons) - residual gas pressure

March 6, 2007 ILCDR07 5 Experimental Reach ParameterValue E2.0 GeV N wiggler 12 B max 2.1 T xx 2.25 nm QxQx QyQy 9.63 QzQz  E /E 8.6 x  x,y 47 ms  z (with V RF =15MV) 6.8 mm cc 6.4 x  Touschek (N b =2x10 10 &  y =5pm ) 7 minutes Baseline Lattice

March 6, 2007 ILCDR07 6 Cesr TA Cesr TA 2GeV low emittance optics T wigglers (16m) -reduce damping time by X10 to 50ms -in region with zero dispersion reduce emittance by ~X10  x ~ 2nm   x ~300  m,  y ~10  m 45 bunch train - 4ns spacing  L ~9mm 2 e10 particles/bunch Electrons and positrons Pressure ~ 1nT

March 6, 2007 ILCDR07 7 Cesr TA beam parameters 45 bunch train - 4ns spacing  L ~9mm 2 e10 particles/bunch Electrons or positrons Pressure ~ 1nT Linear FII theory   mb ~ 1.2 turns  Q y ~ 0.04 Cesr TA Fast Ion Effects

March 6, 2007 ILCDR07 8 Electron cloud and ions? Train of 15 bunches - 14ns spacing 0.75mA  1.2E10 particles Witness bunches at 14ns intervals (  f=1kHz   Q v =0.025) Tune shift consistent with e-cloud Positrons e-cloud Electrons e-cloud and ions?

March 6, 2007 ILCDR07 9

March 6, 2007 ILCDR07 10 e+ Beam Size vs Bunch Current 0.5 mA 0.75 mA 1 mA vertical bunch-by-bunch visible light beam size, 45 bunch train, positrons  x =130nm Notice advancing onset of beam size blow up as a function of bunch current 0.25 mA 0.35 mA

March 6, 2007 ILCDR07 11 Fast Ion Instability? - 45 bunch train - Electrons - 14ns spacing - 1.2e10/bunch QhQh Instability? Linear theory predicts 100 turn growth rate for 45 th bunch QvQv Vertical beam size

March 6, 2007 ILCDR07 12 GaAs Detector for X-ray Imaging Signal (ADC Counts) Position (  m) Fast enough for single bunch resolution First bunch-by-bunch beam size data in CHESS conditions  = 142 +/- 7  m Different symbols represent different bunches Pinhole camera setup at B1 hutch NEW: GaAs arrays from Hamamatsu 1x512 linear array 25  m pitch 1 st sample has just arrived

March 6, 2007 ILCDR07 13 CesrTA Beamsize Monitor Concept Simple optics for x-ray beam size monitor with ~10  m resolution –High transmission –2 keV operation (works for both 2 GeV and 5 GeV) –Hundreds (2 GeV) to thousands (5 GeV) of photons per bunch passage Explore other detector possibilities (eg, InSb arrays) Collaboration with CHESS colleagues for optics and device development as well as integration with existing Xray lines p q 25um Be detector zone plate Multilayer W/C mirrors;

March 6, 2007 ILCDR07 14 Bunch Spacing - Feedback With colliding beams CESR operates with 9 trains of bunches, 5 bunches/train and 14ns spacing (142 MHz clock) Separate system for vertical/horizontal/longitudinal, electrons and positrons In order to accommodate 4ns spacing we will change the clock frequency to 125MHz (16ns) And install 4 parallel systems with 4ns displacement Combine error signals in modulator to drive single kicker Transverse strip line kicker has 8ns pulse width - compatible with 4ns spacing Longitudinal kicker cavity frequency and Q are incompatible and need to be replaced

March 6, 2007 ILCDR07 15 Bunch Spacing Meanwhile, we are testing a simple analog system so that we can begin to explore behavior of trains with 4ns spacing kicker pickup e + bunch

March 6, 2007 ILCDR07 16 Cesr TA –Flexible optics - range of emittance, 2  200 nm - positrons and electrons  complementary measurements –Flexible bunch spacings suitable for damping ring tests –Flexible energy range from 1.5 to 5.5 GeV –Instrumentation that provides for measurement of all dependencies –Beam parameters very similar to ILC damping ring. Fast ion effects in CesrTA anticipated to be good indicator of fast ion effects in damping ring.