Beam Loss Measurements at the Los Alamos Proton Storage Ring

1
Beam Loss Measurements at the Los Alamos Proton
Storage Ring

• T. Spickermann, LANSCE
• ICFA HB-2004, Bensheim, Germany
• October 19, 2004

2
Outline

• PSR Layout and Injection Scheme
• Stripper Foil related Losses
• Loss Detection
• Simulation
• Summary and Outlook

3
PSR Layout Injection Scheme

• Charge-Exchange injection of H- via merging
  dipole and stripper foil.
• Injection with offset and closed orbit bump to
  fill acceptance and reduce foil hits.

4
PSR Layout and Injection Scheme

• Injected beam is matched to ring acceptance.
• Closed orbit bump in vertical plane.

5
PSR Layout and Injection Scheme

• Stripper foil is made of 4 layers of carbon,
  mounted on 3-5 micron fibers.
• About 1.2 of injected beam passing through foil
  is not completely stripped.
• Excited states of H0 may be field-stripped in
  downstream magnets ? First-Turn losses.
• First-Turn losses usually decrease during first
  few days of new foil operation, because foil
  wrinkles up, improving stripping efficiency.
• Thickness of foil, 400 mg/cm2, chosen to
  minimize sum of first-turn and circulating-beam
  losses.

6
Stripper Foil Related Losses

• We believe that foil related losses dominate in
  PSR.
• Biggest remedy reduce number of foil hits.
• Foil hits also hurt the foil itself, most likely
  by overheating.

7
Stripper Foil Related Losses

• Circulating protons keep hitting the foil.
• For 125 ?A at 20 Hz or 6.25 ?C/pulse, injected
  over 625 ?s a proton hits the foil on average up
  to 80 times.
• Beam-Foil interactions that cause losses include
• Large Angle Single Coulomb scattering.
• Plural and Multiple Coulomb scattering.
• Nuclear scattering.
• Other causes of losses are
• Production of excited states of H0 (First-Turn
  losses).
• Emittance growth due to space charge,
  non-linearities, etc.
• Beam scraping.
• Extraction losses.

8
Stripper Foil Related Losses

• Large Angle Single Coulomb Scattering
• Cross-Section in Small-Angle Approximation (see
  J.D. Jackson, Classical Electrodynamics, Elastic
  Scattering of fast particles by atoms)

• Equation is valid for scattering angles between

• For PSR (800 MeV protons)

9
Stripper Foil Related Losses

• For a pencil beam, injected on-axis
• Courant-Snyder invariants
• Limiting aperture XA or YA will be hit if for
  scattered particle
• Thus, limiting scattering angles
• For present PSR

10
Stripper Foil Related Losses

• Integrating cross section over angles larger than
  ?xl,?yl gives probability for a proton to be lost
  after 1 foil traversal P ? 410-6.
• With up to 80 foil traversals per proton
  (average) one finds the probability that a proton
  is lost is about 0.03 , i.e. single large angle
  Coulomb scattering accounts for about 20 of the
  typical loss rate of 0.15 .
• For more accurate numbers one needs to take into
  account finite beam sizes, dispersion etc.
  
• ? use ring optics and tracking codes (e.g. MAD,
  ORBIT).

11
Stripper Foil Related Losses

• Nuclear scattering
• Collision length 60.2 g/cm2
• Foil thickness 400 ?g/cm
• ? probability of loss 6.610-6 per foil
  traversal.
• Assuming 80 hits per proton one finds the
  probability for a proton to be lost due to
  nuclear scattering to be about 0.05 , i.e.
  nuclear scattering accounts for about 33 of the
  typical loss rate of 0.15 .
• Measurements show that first-turn losses account
  for about 15 of the overall loss rate.
• The remaining 32 are probably due to multiple
  and plural coulomb scattering, space charge
  growth, etc..

12
Loss Detection

• Photomultiplier tubes (SRLMs)
• 10 detectors located around the ring.
• Viewed with a scope.
• Can view signal from individual detector
  (multiplexed) or sum signal.
• Allow to measure loss behavior over the course of
  one injection pulse.

Extraction Loss Peak
13
Loss Detection

• Ionization Chambers (SRIRs)
• 20 detectors equally spaced around the ring.
• Used to fast-protect the ring, can turn off beam
  in 35 ?s if loss rates above thresholds.

14
Loss Detection

• Stripper foil current
• Protons hitting the foil cause secondary emission
  of electrons
• Simulated SE current can be calculated from total
  number of foil hits with Sternglass formula

Thermionic Emission (TE) Peak only observed at
high intensities.
where Y SE yield (e- / p) P Probability
0.5 ds average depth from which secondaries
arise 1 nm E average amount of kinetic
energy lost by an ion per ionization 25
eV dE/dx eV / nm
15
Loss Detection

• Example vary the foil position to change amount
  of beam under the foil (incl. 1.2 not
  completely stripped).

Measurement of March 9, 2004. Initial Ring
Current 100 ?A.
16
Simulation

• Beam envelopes and TWISS parameters from MAD.
• 5?x (to allow for orbit distortion), 4?y.
• Use ORBIT to track particles around the ring,
  including apertures.

17
Simulation

• ORBIT offers two choices for foil scattering
• A Coulomb scattering, adaptation of ACCSIM model
  that simulates plural scattering with single
  scattering, using a cumulative distribution
  function for the scattering angle.
• B Coulomb, Rutherford and nuclear scattering
  (el. inel.), adaptation of ORBIT collimator
  model.

B
Fraction of ?y gt 3 mrad 810-6 for method A.
A
18
Simulation

• Compare simulated loss rate with radiation survey

SRBM91 (Bender with ding and y offset)
SRQU11
SRQF11 SRYM11 (bump magnet)
RODM01 (Septum)
19
Simulation

• Foil current and losses, measured and simulated.
• Measurement of March 9, 2004.
• Beam under Foil 1.4 , 3.0 , 3.5 and 5.5 .

20
Summary and Outlook

• We have varied several ring parameters to study
  loss tolerances. I have concentrated here on
  stripper-foil related losses.
• Stripper foil related losses dominate the loss
  rate in PSR. The key to reduce loss rate is to
  reduce the number of foil hits per circulating
  proton.
• We have begun to try to simulate foil related
  losses, but lots of work lies obviously ahead. We
  need to refine the simulations, supported by
  measurements in PSR.
• Other loss contributors will eventually be
  included in the study, e.g. emittance growth,
  limitations to dynamic aperture, etc..