New Results on the Electron Cloud at the Los Alamos PSR

Transcription

1 New Results on the Electron Cloud at the Los Alamos PSR Robert Macek, LANL, 4/15/02 Co-authors: A. Browman, D. Fitzgerald, R. McCrady, T. Spickermann, & T. S. Wang - LANL For more information see the website for the 8th ICFA Mini Workshop on Two-Stream Instabilities in Particle Accelerators and Storage Rings, Santa Fe, NM Feb 16-18, Also see the website for the International Workshop on Two-Stream Instabilities in Particle Accelerators and Storage Rings, KEK Tsukuba, Japan, Sept 11-14,

2 Outline Introduction- electrons surviving the gap a key issue for e-p at PSR New diagnostic electron sweeping detector Result from e-sweeper Sample signals Swept & prompt electrons as a function of intensity and other beam parameters Saturation of the swept electron signal Other interesting phenomena Bursts Recovery after clearing the gap Conclusions and plans for the future 2

3 Electron signals from RFA in straight section 4 2 Beam Pulse Electron Signals V rep = + 25V 1.5 Amplitude (V) 1 V rep = - 1V V rep = - 30V Beam Pulse 0.5 V rep = - 275V V rep = - 300V Time (ns) RFA signal has contributions from trailing edge multipactor and captured electrons released at end of beam pulse plus their secondaries Key issue is how many electrons survive the gap to be captured by the beam Signals averaged for 32 beam macropulses, ~ 8 µc/pulse beam intensity, device is labeled ED42Y, Transimpedance = 3.5 kω, opening ~1 cm 2 3 Bk95, p6-12

4 Motion of captured electrons Captured electrons are the ones that drive the e-p instability Oscillate against the beam during the entire passage of the beam pulse (~40 oscillation periods) Confined to the beam region for almost all of the beam pulse Released at the end of the beam pulse with energy that depends on initial conditions and pulse shape but can reach ~ 100 ev Electron radial position (mm) vs time (ns) Beam Pulse 8 µc/pulse Electrons initially at zero velocity in the gap before arrival of the beam pulse 4 Bk 95, p 38

5 Electron Cloud at end of gap from LBNL POSINST Simulations (M. Furman & M. Pivi) 5

6 Electron Sweeping diagnostic Designed by A. Browman to measure e-cloud surviving passage of the gap Short HV (~1kV) pulse is applied to electrode to sweep electrons into RFA Cross-section Collection Region Acceptance of New Detector-α= (Particles inside blue lines hit detector region-v=-100 volts) Detector (V=0) Pipe (V=0) Y (meters) Plate (V=-100) Accepted fractional area= X (meters) 6

7 Picture of installed electron sweeper RFA, ED42Y E-sweeper, ES41Y 7

8 Sample Electron Data from Electron Sweeper Signals have been timed correctly to the beam pulse Prompt electrons strike the wall peak at the end of the beam pulse. Contributions from: Trailing edge multipactor Captured electrons released at end of beam pulse Beam Pulse HV pulse Device basically acts a large area RFA until HV pulse applied ~10 ns transit time delay between HV pulse and swept electron signal is expected Swept electron signal Swept electron signal is narrow (~10 ns) with a tail that is not completely understood Prompt Electron Signal Bk 98, p µc/pulse, bunch length = 280 ns, 30 ns injection notch, signals averaged for 32 macropulses, repeller = - 25V, HV pulse = 500V 8 Bk 98, p 51

9 Swept Electrons in pipe vs time after end of beam pulse Early and typical results from electron sweeper for 5µC/pulse looking just after extraction Peak signal or integral have essentially the same shape curve Long exponential tail seen with ~170 ns decay time Still see electrons after ~1 µs Implies a high secondary yield (reflectivity) for low energy electrons (2-5 ev) Amp Vpeak Int τ = 170 ns δ eff d = exp c τ me c 2E Implies neutralization lower limit of ~1.5% based on swept electrons signal at the end of the ~100ns gap T(ns) 9 Bk 97, p 84-5

10 Prompt and Swept Electrons vs Beam Intensity Sig(V) E-sweeper ES42Y 10/07/ (fixed buncher voltage and accumulation time) Prompt Swept (3.2E-8)*Q (7E-5)*Q Q(µC) The saturation of swept electrons above 5 µc/pulse is not restricted to variations of beam intensity but includes other variables that affect the prompt signal such as: Variation in beam loss Bursts Changes in pulse shape Saturation explains several puzzles: Why instability threshold is unchanged by increases in losses or vacuum pressure Why the threshold intensity curves vs buncher voltage do not plateau at some intensity 10 Bk 98, p 132-3

11 Saturated Swept and Prompt e s vs local losses Averaged (16 macro pulses) ES41Y signals for three different bumps and local losses Swept e signals Prompt e signals Loss Signals (LM59) for the three bumps (0, +2, +4 mm in section 4) 11 Bk 98, p 142-3

12 Electron cloud varies with location in the ring Section 4 and 9 are rather similar Section 0 near the stripper foil has the most flux but intensity dependence of the prompt e s is much different, varies as the 1.5 to 2nd power of intensity and not as n 7 bursts are greatly reduced at this location Many more seed electrons from the processes at the stripper foil Lots of e s seen in dipoles and quads using biased collection plates but lacking the details obtained from RFA or e-sweeper Highest vacuum pressure rise associated with inductor region of section 5 Seems reasonable to assume that line density of e s surviving the gap in section 4 represent a lower limit on the average density around the ring 12

13 Do the swept e-data complete the picture? A roughly constant 1-2% neutralization was main missing ingredient Saturation of swept e- s explains why increasing the seed electrons by losses, high vacuum doesn t change the threshold Is the picture complete? Do the electron bursts need to be explained? How would they be expected to impact the instability? Interesting new and not well-explained data from other electron experiments Order of magnitude more prompt electrons generated by unstable motion Recovery after sweeping the gap Effect of added beam in the gap Influence of microwave-like longitudinal instability at low buncher voltage Behavior just below threshold Data given to Mike Blaskiewicz for analysis and comparison with his simulations 13

14 Electron burst phenomenon (110 turns) ED42Y ES41Y Local Loss monitor signal 14 Bk 98, p 53

15 Recovery after Clearing Gap of electrons Beam Pulse HV pulse E-sweeper signal 15 Bk 98, p 50-1

16 What can cause burst phenomena for prompt e s? Fluctuations in the seed electrons local losses don t seem to be strongly implicated Correlations of ED42Y and ED92Y suggest that it might be fluctuations on the beam but don t see anything obvious on wall current monitor signal above 4-5 µc/pulse Situation changes at lower intensity (i.e ~ 3 µc/pulse) See sudden increase in e s as buncher voltage is lowered Beam stable transversely during accumulation while below the standard threshold definition See a microwave-like longitudinal instability and some evidence for beam in gap 16

17 Correlations of bursts in sec 4 and 9 ES41Y ED92Y 17 Bk 99, p 102

18 Longitudinal instability appears at low buncher voltage At low intensity (2.8 µc/pulse) production beams and for low buncher voltage, we see a microwave-like longitudinal instability that is correlated with a large increase in prompt e s and bursts Signals below are for single accumulated macropulse (no averaging) 1.2 kv 2.7 kv 18 Bk 99, p 40-1

19 Large increase in e s when longitudinal structure appears Data below for low intensity, production beam (2.8 µc/pulse) Signals show strong bursts but the data below is for average over 128 repeated macropulses 2.7 kv 1.8 kv 1.2 kv Released at extraction 19 Bk 99, p 40-1

20 Summary, Conclusions & Future Plans Measurements of the electrons surviving the gap using the electron sweeping detector provide the ~ 1-2% neutralization needed to explain the e-p threshold at PSR in the coasting beam model The saturation behavior of the electrons surviving the gap resolve the longstanding puzzles of why increasing losses and vacuum pressure do not affect instability threshold intensity The fundamental cause(s) of the electron burst phenomenon is(are) unresolved at this time Future plans TiN coated electron sweeper will be installed in straight section 4 so that we can study prompt and swept electrons in a ~3 m section that is all TiN coated Last year s sweeper will be installed in extraction channel to study electrons from passage of a single bunch Prototype extraction line wirescanner and BPM with bias fields to repel the electron cloud will be tested 20