Electron cloud simulations for the SIS18 and SIS100/300

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ECLOUD04 Schedule and Agenda Tuesday April 20,
2004 Session C Surface Properties, Measurements
and Treatments (Chair, M. Pivi Secretary, F. Le
Pimpec) 0800-0830 Vacuum and electron cloud
issues at the GSI present

and future facilities (30)
G. Rumolo, GSI 0830-0900 Surface
related properties as an essential ingredient


to e-cloud simulations (30) R. Cimino,
LNF-INFN 0900-930 Instrumental Effects in
Secondary Electron Yield

and Energy Distribution
Measurements (30) R. Kirby, SLAC 0930-1000
SEY and electron cloud build-up with NEG
materials (30) A. Rossi, CERN

1000-1030 BREAK 1030-1100 Design and
Implementation of SNS Accumulator Ring Vacuum

System with Suppression of
Electron Cloud Instability (30) H. Hseuh,
BNL 1100-1130 Experimental Results of a LHC
Type Cryogenic Vacuum System

Subjected to an Electron Cloud (30)
V. Baglin, CERN 1130-1150 Suppression of
the effective SEY for agrooved metal surface (20)

G. Stupakov,
SLAC 1150-1230 DISCUSSION (40)
Summary
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Vacuum and electron cloud issues at the GSI
present and future facilitiesGiovanni Rumolo,O.
Boine-Frankenheim, I. Hofmann, E. Mustafin

• Overview on the GSI existing machines and the
  future International Accelerator Project
• Vacuum requirements and beam life time due to the
  dynamic vacuum
• Measurements of the ion desorption yield
• The e-cloud in the SIS18 and SIS100/300
  thresholds for build up and instability
• Conclusions and outlook

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Summarizing (I) Is vacuum a serious concern for
the GSI future project ? Yes

• The number of U28 that we can inject into the
  SIS18 is limited by a vacuum instability driven
  by ion losses (Ion Stimulated Desorption)
• Presently the threshold is still well below the
  goal value of 2.5 x 1011 required to inject 1012
  U28 into the SIS100
• Studies of coating efficiency and/or collimators
  as possible solutions are still in progress and
  need to be more conclusive
• The use of U73 is also considered as a possible
  alternative, even if the intensity would be lower
  due to space charge

TiZrV NEG coating of one superperiod, next
shutdown
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• E-cloud build up with 4 bunches in a field-free
  region (left) and in a dipole (right)
• The threshold for the build up is 2.1 in
  field-free and 1.9 in a dipole
• Saturation values are in the order of 1011
  e-/m3, enough to cause beam instability
• Cimino-Collins parametrization for elastic
  reflection used.

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It is interesting to show how the recently
proposed parametrization for elastically
reflected electrons makes a significant
difference in the predictions of electron cloud
build up.
Can low energy electrons affect high energy
physics accelerators ? R. Cimino, I. Collins, et
al. accepted for publication in Phis. Rev. Letters
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Summarizing (II) can electron cloud harm the
performance of GSI machines ?

• SIS18 could suffer from electron cloud when
  upgraded to become an injector for SIS100.
• The threshold for e-cloud formation is rather
  high (dmax2.1), thus it can be relatively easy
  to have an inner pipe wall with a SEY below this
  value.
• In the SIS100/300, too, the threshold for e-cloud
  formation is quite high, especially for smaller
  chamber radii.
• Due to beam loss, there might be accumulation of
  electrons up to relatively high densities.
• Both in SIS18 and SIS100/300 thresholds are lower
  in dipoles.

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SPS
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Surface related properties as an essential
ingredient to e-cloud simulations.

• R. Cimino
• LNF-INFN Frascati (Roma) Italy.

• The e- cloud problem a SS-oriented review.
• Surface science techniques to provide input
  parameters.
• Some selected results
• Future work and implications

ECLOUD04, Napa, April 19-23, 04.
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R. Cimino
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Measure of Secondary e- YIELD

• Each single point in a DELTA plot gives the total
  number of electrons emitted at a give primary
  energy.

The energy distribution of such emitted electrons
is important for the simulations.
R. Cimino et al Phys. Rev. Lett. in print
ECLOUD04, Napa, April 19-23, 04.
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R. Cimino
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What has been measured on f.s. Cu

• Energy Distribution Curves as function of Ep

Ep11 eV
Ep3.7eV
Secondaries
Kin. En. (eV)
Kin. En. (eV)
R. Cimino et al Phys. Rev. Lett. in print
ECLOUD04, Napa, April 19-23, 04.
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R. Cimino
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Simulated average heat load in an LHC DM as a
function of Nb, considering the elastic
reflection (dashed line) or ignoring it (full
line)
Calculation done extrapolating measured SEY to
dM1.7 EM240 eV
R. Cimino et al Phys. Rev. Lett. in print
ECLOUD04, Napa, April 19-23, 04.
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R. Cimino
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Implication

• Low energy electrons have a long survival time
  (in agreement with observation on PSR at LANL).
• In FELs a low repetition rate is supposed to
  ensure no e- cloud problems. BIEM has to be
  considered.
• BIEM simulations need to be updated for the LHC
  and other machines.

• Reflected el. are NOT absorbed and do not
  directly contribute to heat load !!!
• However they will be accelerated by the following
  bunches, gaining energy to be deposited on the
  LHC BS.

ECLOUD04, Napa, April 19-23, 04.
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R. Cimino
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Instrumental Effects in Secondary Electron Yield
and Energy Distribution Measurements
SLAC-PUB- April 20, 2004

• Robert E. Kirby
• Physical Electronics Group
• The Stanford Linear Accelerator Center
• Stanford University, Stanford, CA 94309

Work supported by the U.S.
Department of Energy under contract number
DE-AC03-76SF00515 (SLAC).
Presented at the 31st ICFA Advanced Beam
Dynamics Workshop on Electron-Cloud Effects,
April 19- 23, 2004, Napa,
Ca.
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Effects

• Secondary primary electrons generated inside
  the source
• Secondary electrons generated inside detector or
  from chamber

• Surface modification by incident electrons
  (desorption, carburization, oxidation, damage)
• Substrate effects
• Near-zero energy

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SEY Measurement - RFA
Strengths Angular, energy distribution
measurements possible Weaknesses
Grid/collimator tertiaries, gun space charge
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Secondary Primary Electrons
Electron-gun Anode
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RFA Tertiary Electrons
From J.A. Cross, J.Phys.D6, 622 (1973)
Inner grid
Collector Current
Secondary Energy (eV)
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Substrate Effect
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Primary Electron Range (TiN)(All axes in
angstroms)
? 0o
? 83o
Ep 500 eV
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Yield From Sputtered (But Disordered) Surfaces
SEY may go to 1 because of the difference
between the work function of the e- gun-cathode
and the target repel primary e- beam
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Rely on scrubbing to decrease SEY
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Experimental Results of a LHC Type Cryogenic
Vacuum System
V. Baglin, B. Jenninger
CERN AT-VAC, Geneva

• 1. Introduction
• LHC Electron Cloud
• LHC cryogenic vacuum system
• 2. Electron cloud in a cryogenic environment
• COLDEX set up
• Long term beam circulation
• Condensed gas
• Operating temperature / filling parameters /
  comparison with other detectors
• 3. Some implications for the LHC
• Scrubbing period (pressure/heat load)
• Cooling / filling schemes
• Beam screens warming up against quench and end
  effects
• 4. Conclusions and future work

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2. Electron cloud in a cryogenic environment 2.1
COLDEX set-up (1) Field free region (SPS Long
straight section 4), closed geometry, 2.2 m
long Pressure gas composition measurements,
heat load measurement (temperature, flow)
BS from 5 to 150 K CB from 2 to 5 K
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• 2.2 Long term beam circulation (3) heat load
• Scrubbing run 2003 12 A.h
• Normalised to 4 batches with 1.1 1011
  protons/bunch, 95 duty cycle
• Heat load (HL) onto the BS is decreasing with
  electron dose beam conditioning
• Final heat load 1.5 W/m
• HL 1.9 exp(-D/70)
• Electron dose 20 mC/mm2
• for estimated lt85gt eV

BS 13 K CB 3 K
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2.2 Long term beam circulation (5) Comparison
to ECLOUD simulations Courtesy of D. Schulte and
F. Zimmermann
The heat load observed in COLDEX and WAMPAC 3
during the 2003 scrubbing run is compatible with
?max 1.2 - 1.3
Reasonable agreement between simulations and
experiments
Measurements Simulation with ?max 1.3
Heat load 1.6 W/m 1.6 W/m
Current 17 mA/m 3 mA/m
ltEgt 85 eV 100 eV
Fraction gt30 eV n.a 70
Dose 20 mC/mm2 10 mC/mm2
?max 1.1 1.2
Disagreement !
V. Baglin et al. Chamonix 2001
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• 3. Some implications to the LHC
• 3.1 Scrubbing period (1) heat load
• Conditioning exist ONLY when an electron cloud
  is present
• Dedicated period are required to perform the
  conditioning
• Conditioning shall be performed at injection (
  1.5 W/m available)
• Conditioning might be lost, over-conditioning
  would be helpful
• Rough estimate
• Based on the previous fit, HL 1.9 exp(-D/70),
  and assuming that 1.5 W/m could be dissipated
  onto the BS, a dose of 200 A.h would be required
  to reduce the dissipated heat load at nominal
  current to 1 W/m

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• 2.5 Filling parameters
• 75 ns bunch separation
• A single MD in 2003
• Preliminary result indicates a reduction of the
  dissipated heat load by at least a factor 2
• More studies this year
• Effect of number of batches and bunch current
• During scrubbing run 2002
• consecutives batches separated by 225 ns, 95
  duty cycle, 4 ns bunch length
• At nominal bunch current heat load
• proportional to the number of batches
• i.e. few bunches are required to
• equilibrate the electron cloud
• Threshold at 4 1010 protons/bunch

Simulations for LHC indicate that increasing
bunch spacing helps reducing the heat load
BS and CB increase to 20 K. Measured from 50 to
75 h
Nominal
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• 4. Conclusions future work
• In the SPS, the electron cloud stimulates
  molecular desorption 10-7 to 10-6 Torr
• A vacuum cleaning is observed at cryogenic
  temperature
• The dynamic pressure is initially dominated by
  H2, then by H2 and CO
• In the SPS, a significant heat load is observed
  at cryogenic temperature 2 W/m
• A conditioning is observed at cryogenic
  temperature (is ?max 1.2-1.3 in COLDEX ?)
• BUT , for LHC, other means to reduce the
  electron cloud shall be studied and be validated
  in existing machines
• COLDEX observations are in a rather good
  agreement with the ECLOUD code
• Thick layers of condensed gas induce large heat
  load (up to 8 W/m) and vacuum transients which
  have consequences onto the LHC design and
  operation
• More laboratory and machine data related to beam
  conditioning and condensed gases are required to
  benchmark the codes and predict more accurately
  the LHC behaviour
• The operation with the LHC requires a deep
  understanding of the electron cloud phenomena to
  control the radiation level, the emittance blow
  up and the vacuum life times

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Should check angular emission of secondary
emitted electrons in presence of strong external
field (cosine distribution ?!)
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1 mm
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A.A.Krasnov, BINP, Novosibirsk, Russia,
sent a contribution presented
by Frank Zimmermann
Effective secondary electron emission from
toothed surface.
Figure 2. The suppression of the secondary
electron emission by the toothed surface. Solid
curve without magnetic field. Dotted curve with
magnetic field.
?!
1.     J.Kawata, K.Ohya, K.Nishimura, Simulation
of secondary electron emission from rough
surfaces, Journal of Nuclear Materials 220-222
(1995) pp.997-1000. 2.     V.Baglin, J.Bojko,
O.Gröbner, B.Henrist, N.Hilleret, C.Scheuerlein
and M.Taborelli, The secondary electron yield of
technical materials and its variation with
surface treatments, Proceedings of EPAC 2000,
Vienna, Austria.
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Thin film coating and a few SEY results F. Le
Pimpec SLAC \ NLC R. Kirby, F. King, M. Pivi
Napa Valley, California, April 2004

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SEY Ti30Zr18V52 NEG after different process
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Summarizing the Summary ...

• Threshold for ecloud is quite high in future
  SIS100/300 at GSI

• Evidence in SPS that TiZrV NEG coating will
  limit cloud build-up

• CERN SEY measurements indicate high e-
  reflectivity, implicating larger e- cloud
  densities than previously estimated

• Instrumental effects and artifacts in SEY
  measurements may cause misleading results

• Extensive remedies applied to prevent cloud
  build up in SNS

• Electron cloud heat loads and e- conditioning
  measured at cold temperature in CERN SPS,
  consistent with simulations

• Promising studies and measurements on groove
  surface design aiming to reduce SEY