Observations and Predictions at CesrTA, and outlook for ILC G. Dugan, on behalf of M. Billing, K. Butler, J. Crittenden, M. Forster, D. Kreinick, R. Meller, M. Palmer, G. Ramirez, M. Rendina, N. Rider, K. Sonnad, H. Williams, CLASSE, Cornell University, Ithaca, NY J. Chu, CMU, Pittsburgh, PA R. Campbell, R. Holtzapple, M. Randazzo, California Polytechnic State University, San Luis Obispo, CA J. Flanagan, K.Ohmi, KEK, Tsukuba, Ibaraki, Japan M. Furman, M. Venturini, LBNL, Berkeley, CA M.Pivi, SLAC, Menlo Park, CA, US Work supported by the US National Science Foundation (PHY , PHY , and PHY ), US Department of Energy (DE-FC02-08ER41538), and the Japan/US Cooperation Program June 6, 2012ECLOUD'12 1 6/6/12

Electron cloud studies at CesrTA  Electron clouds can adversely affect the performance of accelerators, and are of particular concern for the design of future low emittance damping rings.  Studies of the impact of electron clouds on the dynamics of bunch trains in Cesr have been a major focus of the Cesr Test Accelerator (CesrTA) program.  In this presentation, we report measurements along bunch trains of  coherent tune shifts,  coherent instability signals,  (coherent damping rates), and  emittance growth.  The measurements were made for a variety of bunch currents, train configurations, beam energies and transverse emittances, similar to the design values for the ILC damping rings.  The measurements will be compared with simulations which model the effects of electron clouds on beam dynamics, to extract simulation model parameters and to quantify the validity of the simulation codes. June 6, 2012 ECLOUD'12 2

Coherent tune measurements  A large variety of bunch-by-bunch coherent tune measurements have been made, using one or more gated BPM’s, in which a whole train of bunches is coherently excited, or in which individual bunches are excited.  These data cover a wide range of beam and machine conditions.  The change in tune along the train due to the buildup of the electron cloud has been compared with predictions based on the electron cloud simulation codes (POSINST and ECLOUD).  Quite good agreement has been found between the measurements and the computed tune shifts. The details have been reported in previous papers and conferences.  The agreement constrains many of the model parameters used in the buildup codes and gives confidence that the codes do in fact predict accurately the average density of the electron cloud measured in CesrTA. 2.1 GeV positrons, 0.5 mA/bunch Black: data Blue, red, green: from POSINST simulations, varying total SEY by +/-10% June 6, 2012ECLOUD'123 Vertical Horizontal

polar angle Since synchrotron radiation photons generate the photoelectrons which seed the cloud, the model predictions depend sensitively on the details of the radiation environment in the vacuum chamber. To better characterize this environment, a new simulation program, SYNRAD3D, has been developed. This program predicts the distribution and energy of absorbed synchrotron radiation photons around the ring, including specular and diffuse scattering in three dimensions, for a realistic vacuum chamber geometry. The output from this program can be used as input to the cloud buildup codes, thereby eliminating the need for any additional free parameters to model the scattered photons. Photon reflectivity simulations (1) SYNRAD3D predictions for distributions of absorbed photons on the CesrTA vacuum chamber wall for drift and dipole regions, at 5.3 GeV. June 6, 2012ECLOUD'124 Direct radiation chamber wall

Measured tune shifts (black points) vs. bunch number, for a train of mA/bunch 5.3 GeV positron bunches with 14 ns spacing, followed by witness bunches. Red points are computed (using POSINST) based on direct radiation and a uniform background (free parameter) of scattered photons. Blue points are computed using results from SYNRAD3D (with no free parameters for the radiation) as input to POSINST. June 6, 2012ECLOUD'125 Photon reflectivity simulations (2)

Using a high-sensitivity, filtered and gated BPM, and a spectrum analyzer, bunch-by-bunch frequency spectra have been collected for a variety of machine and beam conditions, to detect signals of single-bunch instabilities which develop along trains of positron bunches. Under conditions in which the beam is transversely self-excited via the electron cloud, these frequency spectra exhibit the vertical m = +/- 1 head-tail (HT) lines, separated from the vertical betatron line by approximately the synchrotron frequency, for many of the bunches along the train. The amplitude of these lines typically (but not always) grows along the train. We attribute the presence of these lines to a vertical head-tail instability induced by the electron cloud. Bunch-by-bunch frequency spectra June 6, 2012ECLOUD'126

30 bunch train: bunch by bunch spectra Beam parameters: 2.1 GeV; H (V) emittance: 2.6 nm (20 pm); bunch length 10.8 mm; tunes (H, V, S): (14.57,9.6, 0.065) momentum compaction 6.8x10 -3 (H,V) chrom = (1.33,1.155) Avg current/bunch = 0.74 mA. L-FBK off; H-, V-FBK at 20% June 6, 2012ECLOUD'127

Detailed features of horizontal and vertical lines A lower frequency (~3 kHz) shoulder in the horizontal tune spectrum is attributable to the known dependence of horizontal tune on the multibunch mode. In many cases, there is bifurcation of the vertical tune spectrum, which starts to develop at the same bunch number as the head-tail lines, and is not well understood. Run 126 Run 166 June 6, 2012ECLOUD'128

The electron cloud density can be inferred from the tune shifts. Red: directly calculated from Black: From simulation Horizontal and vertical betatron tune shifts Horizontal and vertical betatron tunes shift along the train due to the buildup of the electron cloud. June 6, 2012ECLOUD'129

Vertical head-tail lines: correlation with cloud density at 2.1 GeV HT-line onset Cloud density at HT-line onset: ~8x10 11 /m 3 Cloud density inferred from tune shifts (directly calculated: red. From simulation: black). June 6, 2012ECLOUD'1210

Vertical head-tail lines: correlation with cloud density at 4 GeV HT-line onset Cloud density at HT-line onset: ~2x10 12 /m 3 Cloud density inferred from tune shifts (directly calculated) June 6, 2012ECLOUD'1211

Vertical head-tail lines: species dependence at 2.1 GeV Vertical head-tail line excitation for positrons and electrons compared. 30 bunch trains of 0.75 mA/bunch electrons (data set 154) and positrons (data set 166) with the same settings for the chromaticity, single bunch emittance, bunch length, and other beam parameters. June 6, 2012ECLOUD'1212

The amplitude of the HT lines depends strongly on the vertical chromaticity, the beam current and the number of bunches For a 45 bunch train, the HT lines have a maximum power around bunch~30; the line power is reduced for later bunches. There is a weak dependence of the onset and amplitude of the HT lines on the synchrotron tune, the single-bunch vertical emittance, and the vertical feedback. Under some conditions, the first bunch in the train also exhibits a head- tail line (usually m=-1 only). The presence of a ``precursor'' bunch a few hundred ns before the start of the train can eliminate the m=-1 signal in the first bunch. – One explanation is that there may be a significant ``trapped'' cloud density near the beam which lasts long after the bunch train has ended, and which is dispersed by the precursor bunch. Indications from RFA measurements and simulations indicate this ``trapped'' cloud may be in the quadrupoles and wigglers. Additional observations from the frequency spectra June 6, 2012ECLOUD'1213

Precursor bunch effect: 2 GeV, 0.75 mA/bunch 30 bunch train Red: with no precursor Blue: with 0.75 mA “precursor” bunch placed 182 ns before bunch 1 (1960 ns after bunch 30) Bunch 1 spectrum Vertical m=0 Vertical HT m=-1 Precursor bunch eliminates: Lower head-tail line at f v kHz. (Sync freq 20.7 kHz). Structure on upper edge of vertical line Small line at kHz (f v kHz) June 6, 2012ECLOUD'1214

Using an x-ray beam size monitor (XBSM), bunch-by-bunch beam position and size measurements have been made on a turn by- turn basis for positron beams. From the beam size measurements, the evolution of the beam emittance along trains of bunches has been measured. Bunch-by-bunch beam size measurements June 6, 2012ECLOUD'1215

Beam size evolution along the train for different bunch currents 45 bunch train of positrons, 2.1 GeV, 14 ns spacing Note enhancement of bunch 1 size 0.5 mA/bunch 1 mA/bunch 1.3 mA/bunch For fixed bunch current, beam size growth along the train is not very sensitive to the chromaticity, the bunch spacing, the initial beam size or the feedback gain. June 6, 2012ECLOUD'1216

Comparison of HT line power and beam size growth under identical conditions 30 bunch train of positrons, 4 GeV, 1.1 mA/bunch, chromaticity (H,V) = (1.3, 1.4). May 23, 2012IPAC' GeV beam size plot overlaid on HT line power plot from slide 11

Comparisons with analytic estimates June 6, 2012ECLOUD'1218 Beam energy (GeV)24 Observed instability threshold (x10 12 /m 3 ) 0.82 Analytic estimate (x10 12 /m 3 ) Analytic estimate (in coasting beam approximation) [Jin, Ohmi]: [Jin, Ohmi]: H. Jin et al., “ Electron Cloud Effects in Cornell Electron Storage Ring Test Accelerator and International Linear Collider Damping Ring,” Jpn. J. Appl. Phys. 50, (Feb. 2011). sync tune

Comparisons with PEHTS simulations Fourier spectrum of dipole motion Simple lattice, CesrTA beam parameters, 2 GeV Evolution of beam size Realistic lattice (83 int. pts.), CesrTA beam parameters, 2 GeV June 6, 2012ECLOUD'1219 Beam energy (GeV)245 Observed instability threshold (x10 12 /m 3 ) 0.82 PEHTS predicted instability threshold (x10 12 /m 3 ) 1.25 [Jin, Ohmi]: H. Jin et al., “ Electron Cloud Effects in Cornell Electron Storage Ring Test Accelerator and International Linear Collider Damping Ring,” Jpn. J. Appl. Phys. 50, (Feb. 2011).

Comparisons with CMAD simulations Fourier spectrum of dipole motion CesrTA beam parameters, 2 GeV Evolution of emittance CesrTA beam parameters, 2 GeV June 6, 2012ECLOUD'1220 Beam energy (GeV)24 Observed instability threshold (x10 12 /m 3 ) 0.82 CMAD predicted instability threshold (x10 12 /m 3 ) 1.6

Sub-threshold (incoherent) emittance growth PEHTS simulations, below the coherent instability threshold 2 GeV beam energy Observed (XBSM) beam size growth below the instability threshold may be due to incoherent effects 0.5 mA/bunch Evolution of beam size below the instability threshold. Realistic lattice, CesrTA beam parameters, 2 GeV. -suggests 20% growth in equilibrium emittance at a cloud density of 0.8x10 12 /m 3 June 6, 2012ECLOUD'1221

June 6, 2012ECLOUD'1222 Outloook for ILC Vacuum chamber design using SYNRAD3D EC mitigation specifications EC buildup simulations to estimate ringwide average cloud density Comparison with analytic estimate of instability threshold

DR Vacuum System Conceptual Design June 6, 2012ECLOUD'1223 ILCDR antechamber design: version 1 and 2 Version 1: flat ends Version 2 slanted ends Version 1: 10 mm gaps Version 2: 20 mm gaps Same profile in quads and drifts in wiggler cells Arc quads and drifts Fully- absorbing photon stops

SYNRAD3D predictions for photon absorption sites June 6, 2012ECLOUD'1224 Red: no antechambers Blue: version 1 Cyan: version 2 Chamber surface and scattering parameters taken to be the same as for CesrTA.

EC Working Group Baseline Mitigation Plan Mitigation Evaluation conducted at satellite meeting of ECLOUD`10 (October 13, 2010, Cornell University) June 6, 2012ECLOUD'1225 SuperKEKB Dipole Chamber Extrusion DR Wiggler chamber concept with thermal spray clearing electrode – 1 VC for each wiggler pair. Y. Suetsugu Conway/Li

26ECLOUD'12 EC Suppression by Wiggler Electrode: Crittenden Wang EC density from buildup simulations Trapping in quadrupoles : Density is average over 20 sigma around the beam, just before the pinch, in units of /m 3. Length is in meters. Dipoles have no grooves. Photons from SYNRAD3D; Peak SEY = 0.94 (TiN) Numbers from “INVESTIGATION INTO ELECTRON CLOUD EFFECTS IN THE ILC DAMPING RING DESIGN”, J. Crittenden et. al, TUPPR063, IPAC’12 Solenoids produce 100% suppression of cloud near the beam

June 6, 2012ECLOUD'1227 Beam energy (GeV)245 CesrTA observed instability threshold (x10 12 /m 3 ) 0.82 CesrTA threshold density, analytic estimate (x10 12 /m 3 ) ILCDR threshold density, analytic estimate (x10 12 /m 3 ) 0.23 ILCDR estimated ringwide average density, from simulations (x10 12 /m 3 ) ~0.035 Comparison with instability thresholds Analytic estimate (in coasting beam approximation) (Jin,Ohmi): PEHTS simulation (6.4 km DR) [Jin, Ohmi]: H. Jin et al., “ Electron Cloud Effects in Cornell Electron Storage Ring Test Accelerator and International Linear Collider Damping Ring,” Jpn. J. Appl. Phys. 50, (Feb. 2011).

Summary and Conclusions (1) The CesrTA research program has investigated the dynamics of trains of positron bunches in the presence of the electron cloud through measurements of bunch-by- bunch coherent tune shifts, frequency spectra, and beam size. Coherent tune shifts have been compared with the predictions of cloud buildup models (augmented with a new code to characterize the photoelectrons) in order to validate the buildup models and determine their parameters. Frequency spectra have been used to determine the conditions under which signals for electron-cloud-induced head-tail instabilities develop. Drive-damp measurement techniques are being developed to characterize the stability of bunches in the train before the onset of the head-tail instability. An X-ray beam size monitor has been used to determine the conditions under which beam size growth occurs, and to correlate these observations with the frequency spectral measurements. Simulation codes have been used to model the cloud-induced head-tail instability. The predicted features of the instability agree reasonably well with the measurements. The success of the cloud buildup and head-tail instability codes in modelling the observations gives confidence that these codes can be used to accurately predict the performance of future storage rings. June 6, 2012 ECLOUD'12 28

Summary and Conclusions (2) For the ILCDR, an antechamber design has been developed with the help of the SYNRAD3D photon simulation. EC buildup simulations have been done based on the ILCDR EC mitigation prescriptions. The expected ringwide average cloud density is around 0.035x10 12 /m 3, dominated by cloud in the quadrupoles and sextupoles. Based on simple analytic estimates and extrapolation from CesrTA, the instability threshold in the ILCDR should be 4-5 times higher than the expected ringwide average cloud density. Incoherent emittance growth for the ILCDR has yet to be fully studied, but is expected to be small because of the low ringwide average cloud density. June 6, 2012 ECLOUD'12 29

Backup slides June 6, 2012ECLOUD'1230

Measurements of bunch-by-bunch damping rates As the electron cloud builds up along a train, it will modify the coherent damping rates of each bunch, as well as producing tune shifts. Measurements of bunch-by- bunch damping rates provide additional information on the nature of the effective impedance of the cloud. Bunch-by-bunch damping rate measurements have been done for –betatron line: Drive a single bunch via the transverse feedback system’s external modulator. Observe the output from a button BPM, gated on the same bunch, using a spectrum analyzer in tuned-receiver mode set to the betatron line frequency. Measure the damping rate of the betatron line’s power after the drive is turned off. –m=+/-1 head tail lines: Same technique as for the betatron line, but the tuned receiver is set to the head-tail line frequency. In addition, a CW drive is applied to the RF cavity phase to provide the longitudinal excitation necessary to observe the head-tail line below the instability threshold. June 6, 2012ECLOUD'1231

Betatron line drive-damp measurements Amplitude vs time for one of the bunches Damping rate vs. bunch number for a 30 bunch train of positrons at 2.1 GeV, 0.72 mA/bunch. (Estimated single bunch damping rate: 200/s). June 6, 2012ECLOUD'1232

m=-1 head-tail line drive-damp measurements Amplitude vs time (in ms) for one of the bunches Damping rate vs. bunch number for a 30 bunch train of positrons at 2.1 GeV, 0.72 mA/bunch. (Estimated single bunch damping rate: 110/s). June 6, 2012ECLOUD'1233