The future of high bunch charge channeling studies beyond A0
Steps toward progress
Two advances are required to move channeling studies fully into the
plasma acceleration regime. One is to increase bunch charge per
unit area by a factor on the order of 107. Part of this can
be accomplished by using more focused beams. The second is to use pulse
lengths in the 10 fs regime. This is challenging but there have
recently been significant developments in femtosecond laser technology
that could help. Higher beam energies might reduce beam size and
perhaps also help for channeling studies. However high energy
channeling experiments are different and often harder to arrange than
the A0 study.
SLAC 30 GeV FFTB?
An example of a potential facility for investigating channeling
radiation is E164 at the SLAC 30 GeV FFTB
. This is being used for continuing plasma acceleration studies.
By adding a crystal and a high energy gamma ray detector it might be
possible to do a channeling radiation study there along the lines of
experiments carried out at Serpukhov and CERN . The
relative beam charge at SLAC is less than at A0 but the beam cross
section is substantially smaller so that the bunch charge per unit area
would be 500 times larger. The potential reach of SLAC E164 relative to
A0 is shown schematically in the results
(schematically because the graph is expressed per bunch while the
relevant factor is bunch charge per unit area). The 300 femtosecond
pulse length at the facility is a step forward but not all the way to
the plasma acceleration regime.
Liveremore 100 TW laser?
A second possibility is to use the 100 TW laser facilities at Livermore
to get extremely high beam currents. A 100 fs
laser capable of producing a 50 micron spot with a beam power density
of 5*1018 W/cm2 is used at Livermore to generate
protons by a pseudo solid state acceleration process in the first foil.
The proton beam produces 4 eV plasma conditions in a second foil.
Could one do channeling studies with this geometry by replacing the
second aluminum foil with a crystal? One possibility might be to try
Rutherford back scattering although it is not obvious how the
backscattering detector could be incorporated in the geometry. Another
possibility might be a blocking experiment. Lattice behavior with time
could be studied using pump and probe and streak camera techniques.
“Available” petawatt lasers could get up into the 1014
Dynamic behavior of crystal lattices
There have also been recent studies in other fields that suggest
different paths to follow for investigations of crystal lattices in
dynamic situations. An intriguing example is a study of laser melting
at Toronto using sub-picosecond electron diffraction [(Siwick
et al., Science 302, 1382 (2003)]. They use a special 500 fs
electron gun to
study electron diffraction as a very thin polycrystalline aluminum foil
is heated by a laser that is weak by the standards discussed above. The
transition from the electronic plasma stage to phonon melting is
clearly indicated in the successive Bragg diffraction pictures in their
These are three recent illustrations of potential experimental
approaches from a rapidly emerging field studying behavior of solids
under dynamic conditions. Many of the possibilities are driven by
developments in laser technology. Channeling may have something to
offer these studies. Conversely, one might learn interesting things
about channeling. And maybe, one may be able to take a step toward solid-state acceleration
employing oriented crystals.