Future for high intensity channeling       updated October 25, 2007  D. Carrigan carrigan@fnal.gov (subject line must be sensible)

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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.


An example of a potential facility for investigating channeling radiation is E164 at the SLAC 30 GeV FFTB facility .  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 figure (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 protons/bunch regime.

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 publication.

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.