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15 Νοε 2013 (πριν από 3 χρόνια και 4 μήνες)

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Beam characteristics

UCLA



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 w
o
; wall ultimately
comes into play at small radii or low n
e
.



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, N
bunch

> N
ion
.



“Non
-
linear” effects; Beam Loading



Beam loading (A. Reitsma);
h

<
-
>
d
E/E tradeoff; f slippage to flatten
d
E/E; function of L
bunch
/
l
p
.



Beam loading in PWFA (K. Lotov); minimize residual energy flux
--
>
optimal witness pulse shape, linear to blowout regimes.



Dark current (T. Katsouleas); Seen in SLAC PWFA experiment; E
crit

~
SQRT(k’) = Dawson cold wavebreaking for blowout conditions.

Beam characteristics (cont.)

UCLA



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
-
> 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
-
> head
-
tail coupling is damped.



Transverse dynamics (A. Reitsma); strong beam loading <
-
>
transverse field modification
-
> damping of head
-
tail breakup; helps
slice
-
dependent ‘beta matching’.

Related topics (Participant input)…

UCLA



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
-
> 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,
session1&2)
.



Electromagnetic Magnus Effect
-
> non
-
ideal driver
-
> 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
-
> halo reduction? Emittance damping?



Effective damping for E ~ TeV. Synchrotron radiation is getting into the codes.
For positron emittance damping?
(see P. Muggli, ibid).

Related topics (Participant input, cont.)…

UCLA



Axicon channel between acceleration stages
-
> minimize temporal dispersion.



= temporal distortion
-
> 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 a
0

via photon deceleration.



Seen in simulations; responsible for the “Dream Beams”
(L. Silva, slide 6).



Diagnostics and feedback



sub
-
micron BPM
-
> Thomson scattering off wake; collection of expelled e
-



Technologies could be developed/tested in near
-
term experiments.



coherent THz
-
> current profile



Multi
-
shot autocorrelator
(see W. Leemans, session1)
, single shot electrooptic
(see
D. Jaroszynski, session4).

Prevent e
-

bunch expansion…ion
-
channel guiding

Maryland, Texas, Oxford, IST, UCLA etc. technologies

Nelson Lopes

Nonlinear evolution of laser
-

a
0

amplifier

UCLA

Initially
, no wave breaking

Conservation of number of photons

classical wave action

Photon deceleration/frequency downshift

Nonlinear evolution of laser pulse for long propagation
distances leads to single cycle laser pulse with amplified a
0

F. Tsung et al, Proc. Natl. Acad. Sci. v. 99, 29 (2002)

Higher a
0

leads to
wave breaking







1/


|a|/a
0

a
0
= 3

c
t
L
/
l
p0
= 1/2