Beam characteristics

1
Beam characteristics
RAL

• What is a perfect beam?
• It comes from the Injector.
• It is affected by many factors
• A few highlights from contributed talks
• Beam Environment
• Capillary discharge (S. Hooker) scaling matched
  wo wall ultimately comes into play at small
  radii or low ne.
• Hollow channel (N. Andreev) multimode not
  detrimental to wakefield phase relation between
  Ez and Ey changes in a channel.
• Few-cycle driver (M. Geissler) very rapid
  evolution of wake effects of density ramps
  sharp edge needed for external injection, Nbunch
  gt Nion.
• Non-linear effects Beam Loading
• Beam loading (A. Reitsma) h lt-gt dE/E tradeoff
  f slippage to flatten dE/E function of
  Lbunch/lp.
• Beam loading in PWFA (K. Lotov) minimize
  residual energy flux --gt optimal witness pulse
  shape, linear to blowout regimes.
• Dark current (T. Katsouleas) Seen in SLAC PWFA
  experiment Ecrit SQRT(k) Dawson cold
  wavebreaking for blowout conditions.

2
Beam characteristics (cont.)
RAL

• What is a perfect beam?
• It comes from the Injector.
• It is affected by many factors
• A few highlights from contributed talks (cont.)
• Beam Environment
• Non-linear effects Beam Loading
• Transport and Staging
• Transport and focusing (Y. Saveliev) finite
  path length differences in divergent beam -gt
  temporal stretch curved photocathode. General
  issue for all transport optics.
• Head-tail coupling (part of T. Katsouleas
  talk) for long bunchs in plasma, head defines a
  structure and head-tail coupling is similar to
  RF structures. For short bunch (blowout/bubble),
  equivalent structure is time-dependent -gt
  head-tail coupling is damped.
• Transverse dynamics (A. Reitsma) strong beam
  loading lt-gt transverse field modification -gt
  damping of head-tail breakup helps
  slice-dependent beta matching.

3
Related topics (Participant input)
RAL

• Is it possible to marry the bubble/blowout
  structure with external injection?
• Questions are Can a witness beam load enough
  to distort the bubble and prevent self-injection?
  How to precisely place the bunch initially or at
  the next stage? (see W. Leemans, session1 W. Lu
  M. Tzoufras, session2 M. Geissler, session3,
  A. Pukov, session2)
• Beam breakup instability (BBI) in a linac -gt
  betatron motion couples head-to-tail.
• Is this the same as hosing? Is there a damping
  mechanism (e.g., BNS)?
• Yes, similar. Yes, ideas for damping (see T.
  Katsouleas session3 P. Muggli, session12).
• Electromagnetic Magnus Effect -gt non-ideal
  driver -gt meandering of wake vector.
• Seen in self-sidescatter (RAL, LBNL). Due to
  asymmetries in transverse ponderomotive force.
  Related to laser hosing, but not an instability.
  Stabilized in plasma channel(?) (B. Bingham,
  session3 W. Leemans comment).
• Synchotron-damping is larger off axis -gt halo
  reduction? Emittance damping?
• Effective damping for E TeV. Synchrotron
  radiation is getting into the codes. For positron
  emittance damping? (see P. Muggli, ibid).

4
Related topics (Participant input, cont.)
RAL

• Axicon channel between acceleration stages -gt
  minimize temporal dispersion.
• temporal distortion -gt
  minimize q (see Y. Saveliev session3 N. Lopes,
  slide 5).
• Laser shaping
• Plasma mirror to setup a matched beam
  (pre-erode the head)
• Works in simulation. (see W. Lu, session2) Need
  FROG measurements from experiments.
• Short length of plasma to increase a0 via photon
  deceleration.
• Seen in simulations responsible for the Dream
  Beams (L. Silva, slide 6).
• Diagnostics and feedback
• sub-micron BPM -gt Thomson scattering off wake
  collection of expelled e-
• Technologies could be developed/tested in
  near-term experiments.
• coherent THz -gt current profile
• Multi-shot autocorrelator (see W. Leemans,
  session1), single shot electrooptic (see D.
  Jaroszynski, session4).

5
Prevent e- bunch expansionion-channel guiding
Maryland, Texas, Oxford, IST, UCLA etc.
technologies
Nelson Lopes
6
Nonlinear evolution of laser - a0 amplifier
Conservation of number of photons classical wave
action
Initially, no wave breaking
a/a0
1/?
a0 3
ctL/lp0 1/2
Photon deceleration/frequency downshift
?
?
?
Higher a0 leads to wave breaking
Nonlinear evolution of laser pulse for long
propagation distances leads to single cycle laser
pulse with amplified a0 F. Tsung et al, Proc.
Natl. Acad. Sci. v. 99, 29 (2002)