A compendium of e-cloud studies at PSR

1
A compendium of e-cloud studies at PSR

• Robert Macek, LANL, 3/15-18/04

2
Outline for this topic

• Introduction
• Electron cloud (EC) diagnostics used at PSR
• Studies of beam parameters affecting EC formation
• Beam intensity
• Longitudinal and transverse beam profiles
• Beam in gap
• High frequency longitudinal structure
• Coherent beam centroid motion
• Electron cloud dissipation
• Studies of sources of seed electrons
• Losses
• Vacuum
• Stripper foil
• Bursts, recovery from sweeping
• Studies of electron suppression
• Clearing fields
• TiN coatings
• Weak solenoids
• Beam Scrubbing

3
PSR Layout with EC Diagnostic
Circumference 90m Beam energy 798
MeV Revolution frequency 2.8 MHz Bunch length
250 ns (63 m) Accumulation time 750 ms
2000 turns
4
Sources of primary electrons at PSR
Source
es/stored p

• 400 keV convoy electrons from H- stripping
  2
• Secondaries from convoy es
  0.1 - 1
• Secondary emission from foil (0.02/traversal)
  0.6-1
• Knock-on electrons from foil
  0.2
• Thermionic emission from foil
  lt0.1
• Secondary emission from beam losses (0.1-100/
  lost proton)
  0.001- 1
• Residual gas ionization (2-4x10-8 torr)
  lt0.01
• gt2.7-5.2
  

Depends strongly on accumulation, store time and
injection setup
Very strong function of foil heating, usually
not present in production operations but can be
made large in beam experiments
5
Mechanism 2 trailing edge multipactor
Electron born at wall from say losses
Beam
Energy gain in one traversalis high enough for
multiplication
Energy gain is possible in wall-to-wall
traversals on trailing part of beam pulse
Bk87, p111
6
Mechanism 1 production by captured electrons
7
Electron motion in simple case

• Electron subject to cylindrically symmetric
  radial force and has no initial angular momentum
• Equation of motion is
• Assume Gaussian transverse beam profile with line
  density ?(zbct)
• Solve resulting equation of motion numerically
  

for
8
Amplification by trailing edge multipactor in a
simple model
9
Electron Cloud Diagnostics used at PSR
10
Outline of discussion on e-cloud diagnostics at
PSR

• Cloud parameters of interest
• Electron density distribution as function of
  time, and 6 phase space coordinates of electrons
• Source strengths of various sources of electrons
• Much can be learned from flux striking the wall,
  its time structure and energy spectrum
• Retarding Field Analyzer (RFA)
• Flux striking the wall
• Time structure
• Energy spectrum of electrons striking the wall
• Electron Sweeping Detector (ESD)
• RFA with pulsed electrode to sweep low energy es
  into the RFA
• Biased collection electrodes
• Foil current
• Vacuum pressure rise
• Fast ion gauge
• Ion pump current pulse

11
Retarding Field Analyzer

• Described in R. Rosenberg and K. Harkay, NIM A
  453 (2000) p507-513.
• LANL augmentation is fast electronics (80 MHz)
  on the collector output
• Minimal perturbation of beam/wall environment
• Use of repeller permits collecting a cumulative
  energy spectrum
• Measures electrons striking the wall, not
  electrons remaining in the pipe

12
RFA Electronics Block Diagram
13
Electron signals from RFA in straight section 4

• 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
mC/pulse beam intensity, device is labeled ED42Y,
Transimpedance 3.5 k?, opening 1 cm2
Bk95, p6-12
14
Electron energy cumulative spectrum (3D profile)
15
Comparison with simulations

• Code such as POSINST developed by Furman and used
  by him and Pivi to simulate the multipacting
  process and produce flux and energy distribution
  of electrons striking the wall in PSR
• Code and simulation ingredients
• 2D space charge field of a rigid proton beam with
  measured longitudinal beam profile and Gaussian
  transverse profile (?x ?y1cm)
• Electrons move in 3D under influence of 2D space
  charge field of protons and 3D field of electrons
• Detailed model of secondary emission at the wall
• Seed electrons were assumed to be from uniform
  losses at the wall
• 100 es per lost proton
• Proton loss rate 4x10-6 per stored proton per
  turn for an 8 mC/pulse beam

M. Furman and M Pivi, PAC01, also PRSTAB 034201 
16
Electron sweeping detector
17
Electron Sweeping Diagnostic (ESD)

• 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
Pulsed Electrode Network
18
Acceptance of ESD

• Acceptance mapped by calculating trajectories for
  thousands of initial positions on a grid in the
  pipe and selecting those that entered the arc of
  RFA box
• zero initial velocity (cold electrons)

Collection Region
Potentials and Trajectories
19
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
• 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 is narrow (10 ns) with a
  tail that is not completely understood

7.7 mC/pulse, bunch length 280 ns, 30 ns
injection notch, signals averaged for 32
macropulses, repeller - 25V, HV pulse 500V
Bk 98, p 51
20
Electron cloud dissipation (swept electrons in
pipe vs time after end of beam pulse )

• Early results from electron sweeper for 5mC/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 ms
• Implies a high secondary yield (reflectivity) for
  low energy electrons (2-5 eV)
• Implies neutralization lower limit of 1.5 based
  on swept electrons signal at the end of the
  100ns gap

Bk xx, p yy
21
Picture of installed RFA and electron sweeper
22
Biased collection electrodes
23
Biased collection electrodes

• We have tried a variety of biased electrodes over
  the last decade
• parallel and curve plates, BPM striplines, split
  cylinders etc in drift regions, quadrupoles and
  thin striplines in a dipole
• various combinations of bias fields
• Many puzzling features emerged during their use
• Peculiar voltage plateau curves
• Electrons seen in the plane transverse to a
  dipole field
• Complicated situation to interpret
• Signal is net charge collected i.e., incoming
  outgoing (secondary emission)
• Bias electric field dominates during the gap
  passage and can collect electrons during the gap
• Beam electric field dominates during bunch
  passage
• Electrodes in the pipe see large induced signals
  (100-200 V) from coupling to the beam that must
  be filtered out to see the electron cloud signal
• Biased electrodes change the beam-wall
  environment and can alter the multipacting
  process
• An detailed simulation of cloud formation in
  presence of bias electric fields could be
  compared with observations and might provide some
  insight
• They did provide our first evidence for a flood
  of electrons for high intensity beams
• RFA and electron sweeper are better devices for
  measuring the electron cloud

24
Biased Strips in a Dipole

• 4 copper strips on kaptan tube (0.010 thick)
• Leads brought out through the nearby quad to
  vacuum feed throughs
• Vertical and horizontal connected in pairs
• One set biased
• The other grounded
• Signal is heavily filtered to suppress large
  induced AC signals from beam (up to 100 V)
• Signal is integrated by the filter

25
Sample signal from strips in dipole
26
Biased Strips in Dipole at 7.6 mC/pulse
27
Biased Strips in Dipole at 3.8 mC
28
Electron Signal from Biased BPM Striplines in Quad
29
Foil current
30
Foil current

• Net current from foil is due to secondary
  emission
• Under conditions of more foil heating also see
  thermionic emission current

5 mC/pulse Production beam
cm42
foil
8 mC/pulse beam with 200 ms store
cm42
foil
31
Vacuum pressure rise as an e-cloud diagnostic
32
Pressure rise for high intensity stable beam (May
2000)22
Electron Signal from RFA (ED42Y)
Fast ion gauge signal (installed in a RFA port
ED42X)
Pressure change from 4x10-9 Torr to 3.5x10-8
Torr, rise time 8 ms, decay time 0.5 s
Bk 94, p 95
33
Vacuum Pressure Rise from electron cloud

• Electron Stimulated Desorption (ESD) by e-cloud
  striking the vacuum chamber wall
• Pressure rise ( 8ms) on previous slide is
  consistent with conductance of 10 cm beam pipe
  and housing of ion gauge
• Pressure decay (0.3-0.5 s) is consistent with
  conductance of 10 cm beam pipe and pumping speed
  of 500 L pumps
• Pressure rise implies we would have a problem at
  20 or 30 Hz
• Implies pressure would be 10-6 Torr
• Beam scrubbing since 2000 reduces it
  considerably(factor of 10)
• Pressure rise compared with electron flux hitting
  the walls implies 1 molecule desorbed per 70
  electrons hitting the wall
• Consistent with ESD cross-section 10-17 cm2 and
  coverage of 1 (full monolayer)

34
Ion pump current pulse

• A vacuum pressure rise was historically one of
  the first indicators for beam-induced
  multipacting
• Multipacting electrons striking the wall desorb
  gases
• An ion pump current pulse is observed during
  accumulation of intense beam pulses in PSR
• Pulse amplitude tracks the peak RFA signal
  amplitude e.g.,
• When it changes with beam intensity
• As the cloud diminishes over time with beam
  scrubbing
• Simple diagnostic to implement

Gas pulse for 8 mC/pulse stable beam
35
SRIP11 Ion Pump Pulse Current (7.1 mC/pulse,
10/04/03)
ED22Y filtered signal
IP11 pulse, 1ms decay time
Bk 103, p 95
36
Correlation of Ion Pump Pulse (IP13) with nearest
Electron Signal (ED22Y) (effect of beam
intensity)
8 mC/pulse 6/29/02
ED22Y Signal Amplitudes at end of accumulation
and store
5 mC
8 mC
5 mC/pulse 6/29/02
Ratio of ED22Y Amp. (8mC/5mC) 4.7
Ratio of IP13 Amp. (8mC/5mC) 5.1
Ion pump pulse scales with beam intensity the
same as the RFA signals
Bk 94, p 95
37
Correlation of Ion Pump Pulse with Electron
Signal and beam scrubbing
38
Pump signal at various locations in PSR
8 mC/pulse beam _at_ 1Hz, 6/29/02
39
Comparison of data 11/28/03 with 6/29/02
40
Ion Pump layout
SRIP11
Screened Pump Port
41
Model for ion pump pulse

• ESD gas molecules are estimated to come off with
  0.2 eV
• Those with line of sight to pump interior will
  arrive in 50 250 ms (at typical pump),
  depending on molecular weight.
• Some multipacting electrons may also get through
  the screen and desorb gas closer to the pump
  interior.
• Sorption pumping speed of interior of pump is
  4000 l/s
• Implies decay of 35/4000 or 1ms
• Bulk of desorbed gas is thermalized and pumped
  through conductance limited beam pipe by 400 l/s
  ion pumps
• Decay time of 0.3-0.5 s

Pumping Screens
Beam
Desorbed molecule
Anode
e.g. N2O on Ru see Z.W. Gortel and Z.
Wierzbicki, Phys. Rev. B 43 (1991) 7487.
42
Other evidence pertinent to this model

• No signals where line-of-sight is blocked (IP02,
  IP31, IP32)
• Strong signal at IP52
• 15 cm beam pipe, only one screen
• Short drop (larger solid angle)
• Changes with added store time
• Pump signal tracks electron detector signal
• As intensity changes
• Over time (beam scrubbing)
• Another possibility is that multipacting
  electrons reach the ion pump innards and desorb
  gas to generate the observed ion pump pulse
• SLAC people believe they see electron cloud
  getting into ion pumps
• Electrons must have enough energy to get through
  the magnetic fields at the entrance to the pump

43
Layouts for Ion Pumps IP02 and IP51/52
IP02 at Stripper Foil Location
IP51/52 layout
44
Summary of PSR e-cloud diagnostics

• The RFA and ESD are our most useful EC
  diagnostics and have provided immensely useful
  information on the EC in PSR
• RFA provides data on flux, energy spectra and
  time structure of electrons striking the wall
  with minimal perturbation of the EC
• ESD is an RFA that can also measure electrons
  swept from a beam-free time interval
• Biased electrode signals provide some information
  but are difficult to understand and interpret
• Only devices we have in magnetic field regions
• Ion pump gas pulse signal provides a useful
  relative measure of gas desorption and correlates
  well with other measures of the EC
• Simple to implement
• More informative than the DC average pump current
• Provides good sampling over all regions of the
  ring
• Foil current measures secondary and thermionic
  emission from foil

45
Results of various e-cloud studies at PSR
46
Studies of beam parameters affecting EC formation

• Beam intensity
• Longitudinal and transverse beam profiles
• Beam in gap
• High frequency longitudinal structure
• Coherent beam centroid motion

47
Prompt and Swept Electrons vs Beam Intensity
Bk 98, p 132-3
48
Swept and Prompt es vs intensity (near
threshold)
Bk 99, p 72-8
49
Comparison
50
Saturated Swept and Prompt es vs local losses
Bk 98, p 142-3
51
Influence of Bunch Shape

• From changing buncher phase
• From use of notch in injected bunch
• Effect of transverse beam shape

52
Electron signals vs bunch shape

• Data obtained for 7.4 mC/pulse beam

Bk95, p38
53
Comparison of effect of bunch shape at two
locations
54
Comparison to Beam Injected with 50 ns Notch

• Inductors at 190º C, enough inductance to
  over-compensate longitudinal space charge by
  50
• Use 50 ns notch in 305 ns injected pulse
• Stored intensity down 16
• Bunching factor up 50
• Losses down a factor of 2
• Electrons up 10-20

WallCurrentMonitor
Bk92, p123-132
55
Influence of transverse beam profile
spot size estimates ?y 11 mm, ?x 5 mm
56
Effect of added beam in the gap
57
Effect of Added Beam in the Gap on Instability
Threshold
Bk70, p 76-92
58
Effect on e-cloud of 1 added beam-in-the gap

• Added beam in the gap by injecting a number of
  turns (2) with no 2.8 MHz chopping at the end of
  accumulation
• Give beam in gap that is 1 of peak
• Measured beam in gap and prompt and swept
  e-signals for 7 mC/pulse beam intensity

Bk 99, p 14-8
59
Comparison of prompt es with and without beam in
gap

• 7.1 mC/pulse beam intensity
• Both traces have the same beam parameters except
  for beam in gap
• See a factor of 3 more prompt es with beam in
  gap
• Somewhat surprising that it is this large

Bk 99, p 14-8
60
Swept and prompt es with beam in gap

• Recovery behavior basically unchanged with added
  beam in gap
• See larger e-signals than without beam in gap,
  however
• Quantitative comparison compromised by
  significant change in beam pulse shape unrelated
  to beam in the gap and not discovered until after
  the experiment was analyzed
• Data from another day showed a factor 2-3 more
  swept es for 1 beam in gap
• More or less consistent with the expectation that
  swept es would increase to neutralize beam added
  to the gap
• More less explains effect on instability
  threshold shown earlier

Bk 99, p 14-8
61
Effect of high frequency longitudinal structure
62
Longitudinal instability appears at low buncher
voltage

• At low intensity (2.8 mC/pulse) production beams
  and for low buncher voltage, we see a
  microwave-like longitudinal instability that is
  correlated with a large increase in prompt es
  and bursts
• Signals below are for single accumulated
  macropulse (no averaging)

Bk 99, p 40-1
63
Large increase in es when longitudinal
structure appears

• Data below for low intensity, production beam
  (2.8 mC/pulse)
• Have bursts but data below is averaged over 128
  repeated macropulses

Bk 99, p 40-1
64
Standard Deviation of RFA signals

• Standard deviation (s) bin-by-bin is a
  convenient, quantitative measure of fluctuations
  from the bursts
• s is the same size as the average
• Data below for 2.8 mC/pulse production beam

Bk 99, p 40-1
65
Effect of coherent beam motion
66
Prompt electron signal for unstable beam

• Data (1999) for 4.4 mC store, higher intensity
  saturated detector electronics
• E-signal much larger (gt10) than for stable beam
  of same intensity
• Electron pulse shape (at end of each turn) is
  similar to stable pulses

Bk91, p48
67
Unstable beam expanded
Bk 91, p 48
68
Unstable beam turn-by-turn
Bk 91, p 48
69
Electron cloud dissipation
70
Studies of electron cloud dissipation after beam
pulse passes

• Cloud at end of gap is key to understanding the
  e-p instability in PSR
• These are captured by the next pulse and
  oscillate against the protons during the bunch
  passage and thus can drive the instability
• Have measured dissipation as function of beam
  intensity, conditioning, TiN coating and in the
  extraction line (single pass)
• Have found that the decay time is 200 ns and
  insensitive to all of these variables
• Implies that d ? 0.5 for low energy electrons
  (lt20 eV) which is surprisingly high and
  insensitive to conditioning and TiN coatings

71
Electron survival vs intensity
Contemporaneous electron dissipation curves at 3
beam intensities. Data was collected 8/24/02.
72
Electron survival at various beam intensities
Swept electron survival curves (ES41Y) at various
intensities including a 2.7 mC/pulse data set
collected during production running at reduced
intensity (55 mA at 20 Hz.)
73
Studies of the sources of seed electrons

• Crucial input for simulations of what we can
  measure at PSR
• Issues regarding the source terms
• Losses
• Vacuum
• Stripper foil

74
Some complications regarding seed electrons from
losses

• Trailing edge multipactor has highest gain for
  electrons born near the wall at peak of beam
  pulse
• Assumption that initial es are from grazing
  angle losses, uniformly distributed around the
  ring with 100 e/lost proton is a good first
  approximation but likely too simplistic for
  accurate simulations of PSR experiments
• The 100e/proton comes from model by Sternglass
  for grazing angle (cos ? lt0.002) scrapping at a
  surface and is supported by measurements of
  Thieberger etal

e/proton goes with cos(?)-1

• Losses are anything but uniform, rate can vary by
  factor of 1000 around the ring
• Grazing angle losses occur mainly in the quads
  (10 of circumference) and here it is mostly
  confined to those in the region of injection and
  extraction (25 of the quads)
• Only scattered beam reaches the regions where
  electron detectors are located (drift spaces).
  This strikes the walls at 10s of mr.
• e/scattered particle down factor 10 or more
• Electrons from residual gas ionization are often
  neglected as being few in number and born near
  the beam not at the walls

75
PSR Layout
Circumference 90m Beam energy 798
MeV Revolution frequency 2.8 MHz Bunch length
250 ns (63 m) Accumulation time 750 ms
2000 turns
76
Ring Beam Loss and Activation

• Ring losses are from
• Foil scattering (60-70)
• Nuclear and Large Angle Coulomb
• Lost in sect 0, 1 and at extraction region
• Production of excited states of H0 (n3,4..) that
  field strip part way into first dipole d.s. of
  stripper
• Lost in first 2-3 sections after foil
• Ring Losses concentrated at injection and
  extraction
• Ring Loss Monitors
• Max 22.4
• Min 0.2
• Ratio Max/Min 112
• Ring Activation _at_ 30 cm
• Max 5000 mRad/h
• Min 10 mRad/h
• Ratio Max/Min 500

77
Other Problems with Uniform Loss Model

• We consistently see more prompt electrons in
  section 4 than in sections 2 and 9 where losses
  are considerably higher
• What could be different?
• Loss monitor signal and activation may not track
  local seed electron production very well
• SEY is not measured, hence something of an
  unknown for the simulations
• Transverse profile of the beam
• Vacuum (section 4 is worse by factor of 5-10)

section Ratio of electrons to section 4 Ratio of losses to section 4 Ratio of activation to section 4
9 1/3 17 7 - 35
2 1/2.5 7 2
1 6 55 50
78
Experiments on effect of beam losses and vacuum

• Changed beam losses two ways
• Move stripper foil into the beam
• Changes amount of foil scattering but all other
  beam parameters fixed
• Monitor foil current
• Introduce local closed orbit bumps, measure
  losses with local loss monitor (scintillator with
  10 ns resolution, if desired)
• Find that prompt electron signal in RFA is linear
  in losses over considerable range
• Changed vacuum in several sections by turning off
  ion pumps
• Find that prompt electron signal in RFA is linear
  over range of 10-1000 nTorr
• Electrons surviving the gap
• Note that ions from residual gas ionization are
  driven to the wall in 1-3 turns and hit with 2
  keV. These can create secondaries electrons at
  the wall. Effect not in simulations.

79
Effect of losses (moving foil into beam)
5.8 mC/pulse beam
Bk 100, p 68-75
80
Effect of changing losses by local bump
Signals from horizontal and vertical RFAs plotted
as function of local loss monitor as horizontal
bump was varied from -6 to 8 mm.Beam intensity
was 8.1 mC/pulse
Bk 98, p 142-63
81
Effect of changing vacuum pressure in Sect 4
8.2 mC/pulse beam intensity
Bk 101, p 10-13
82
Effect of changing vacuum pressure in Sect 2 5
At beam intensity of 8.2 mC/pulse
Bk 101, p 10-13
83
Seed electrons from the foil

• Data showing effect of stripped electrons
• Data showing effect of thermionic emission

84
Effect of electrons stripped at foil
ED02X downstream of foil
Expanded Near EOI
85
Summary of seed electron issue

• Beam losses certainly are a source of seed
  electrons for trailing edge multipactor
• Need simulation studies of angle that scattered
  particles (scattered from losses in quads) make
  with walls in drift spaces and downstream
  elements
• Vacuum pressure above 10-7 Torr makes a
  significant contribution
• Is it from ions driven to the wall?
• Need to know precisely the angle scattered
  radiation makes with chamber walls in elements
  downstream of local losses
• LAHET or MCNPX simulations?
• We see stripped electrons and secondaries from
  the foil influencing the ED02X signal but the
  situation is complicated by the change in chamber
  geometry at this detector and the distance from
  the stripper foil

86
Electron bursts, recovery from sweeping
87
Electron burst phenomenon (110 turns)
Bk 98, p 53
88
What can cause burst phenomena for prompt es?

• Fluctuations in the seed electrons
• local losses dont seem to be strongly
  implicated
• Correlations of ED42Y and ED92Y suggest that it
  might be fluctuations on the beam but dont see
  anything obvious on wall current monitor signal
  above 4-5 mC/pulse
• Situation changes at lower intensity (i.e 3
  mC/pulse)
• See sudden increase in es as buncher voltage is
  lowered (also no added store time)
• Beam stable transversely during accumulation
  while below the standard threshold definition
  (which uses 500 ms store)
• See a microwave-like longitudinal instability and
  some evidence for beam in gap

89
Correlations of bursts in sec 4 and 9
Bk 99, p 102
90
Recovery after Clearing Gap of electrons
Bk 98, p 50-1
91
Correlations in Recovery
Bk xx, p yy
92
Possible explanations for recovery phenomenon

• Captured electrons (mechanism 1) may be more
  important than indicated by simulations
• Sub-threshold coherent motion may be playing a
  role

93
Mechanism 1 production by captured electrons
94
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 es 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 es seen in dipoles and quads using
  biased collection plates but lacking the details
  obtained from RFA or e-sweeper
• Highest pressure rise associated with inductor
  region of section 5
• Seems reasonable to assume that line density of
  es surviving the gap in section 4 represent a
  lower limit on the average density around the ring

95
Studies of electron suppression
96
Studies of suppressing electron generation

• Previous studies (prior to direct H- injection
  upgrade) using electrostatic clearing fields in
  the injection section and 3 other straight
  sections showed a slight improvement in
  instability threshold (10-20)
• TiN coatings gave mixed results
• suppressed prompt electrons by a factor of 100
  or more in tests in section 5 of PSR in 1999,
• perhaps a factor of 40 in section 9 but
• no improvement in section 4 in 2002 tests
• Weak solenoid magnetic field suppressed prompt
  electrons by factor of 50 in a 0.5 m section in
  PSR
• Beam conditioning over time reduced prompt
  electron signals and improved the instability
  threshold curves
• Do the swept electrons change with conditioning?
• Lower beam losses and better vacuum ?

97
Electron Clearing Devices in Injection Section
(Pre H- Injection Upgrade)
98
Effect of TiN Coating on Electron Flux (1999 test)
Bk xx, p yy
99
Picture of Solenoid Section with
RFA
Bk 98, p 27-8
100
RFA Signals in a Weak Solenoid Field
Bk 98, p 27-8
101
Effect of Solenoid on RFA Signal Amplitude
Bk 98, p 27-8
102
Summary of Beam Scrubbing at PSR
Effect on e-p instability threshold curves
103
Electron suppression has not yet provided a cure
for e-p at PSR

• In 2002 tried weak solenoids over 10 of the
  circumference in drift spaces with no effect on
  instability threshold
• Beam scrubbing in 2000-2001 was effective in
  improving e-p instability threshold and has
  continued to reduce prompt electrons in 2002 and
  2003 but no improvement in e-p threshold in
  2002/3
• Experience with TiN gave mixed results

104
Summary/Conclusions

• Trailing edge multipactor is clearly responsible
  for much of the prompt electron signal in the
  drift spaces
• Low energy electrons dissipate rather slowly in
  beam free regions such as the gap between bunch
  passages
• Now have better knowledge of electrons surviving
  the gap
• To complete the picture we need data in dipoles
  and quads
• Simulate biased electrodes to interpret data
• Have some ideas on detectors but now resources
• Simulations are hampered by uncertainties on SEY
  for actual surfaces and the source terms for seed
  electrons esp from losses
• TiN coatings gave mixed results on electron
  suppression, why?
• Effect of beam scrubbing (conditioning) on e-p
  threshold intensity curves is our best evidence
  that global e-cloud suppression is a cure for e-p
  instability
• Have a lot of data on effect of various factors
  on e-cloud formation which could be compared with
  simulations
• Bunch shape changes, beam in gap, beam intensity
  etc

105
Remaining Issues and ideas for the future work

• Seed electron source terms are a troubling issue
  that needs work
• Simulations of the scattered radiation from
  primary loss points to get the distributions of
  scattered particles striking the walls downstream
  would be very informative and a needed input to
  the cloud formation simulations
• Various simulations for quads seem to be in some
  disagreement
• Suitable RFA and sweeping detectors for quads and
  dipoles would provide important test of the
  simulations
• Have some design ideas
• Cloud buildup simulations with biased collection
  electrodes included and measurable quantities
  simulated could be tested with existing and new
  data from these detectors
• L. Wang was planning to do this for the PSR
  pinger electrodes

106
Proposed sweeping detector for PSR quad
107
Simulations for e-cloud in PSR quad
Courtesy M. Pivi
108
Snapshot of e-cloud in quad 5 ms after beam
passage
Courtesy M. Pivi
109
Backups
110
scope pictures of charge collection
Coasting beam
Bunched beam
LB62, p. 30
LB62, p. 16
111
PSR Layout with EC Diagnostics
Circumference 90m Beam energy 798
MeV Revolution frequency 2.8 MHz Bunch length
250 ns (63 m) Accumulation time 750 ms
2000 turns
112
RFA electronics
113
Experimental studies of factors affecting e-cloud
formation

• Beam intensity has strong effect
• Longitudinal bunch shape, transverse beam profile
• Variation in primary (seed) electrons
• Losses, vacuum, foil emission
• Changes in SEY at chamber walls
• TiN coating
• Beam Scrubbing
• Beam in gap
• Circulating beam vs single pass
• High frequency longitudinal beam structure
• Weak solenoidal fields
• Coherent motion of beam centroid
• Location in ring

114
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
  doesnt change the threshold
• Is the picture complete?
• Model doesnt have a threshold dependence on
  bunch length
• 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
• Do these need to be explained beyond the academic
  interest?
• 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

115
Conditioning effect
Bk95, p125-7