Exotic AcceleratorsModern accelerators have been able to reach extremely high energies but at a cost. With available technology they must be very large and expensive to reach the ever-rising energy frontier. Currently they are limited by available magnetic fields, roughly 10 Tesla, and by RF gradients, at best 100 MeV/m.There are several possibilities for breaking the acceleration bottleneck. The first thought is to use the fantastic capabilities of lasers directly. A second possibility is to use a plasma wave to accelerate particles.The ideas below are expanded in the accompanying Power Point presentation .
The gradient, G, in a plasma is equal to
Laser accelerationThe good news about lasers is that they can give very high electric fields. The bad news is that the electric vectors point in the wrong direction (transverse to the laser beam direction) and optical frequencies make the construction of an electromagnetic cavity outside of the reach of conventional technology. Several ingenious ideas have been advanced to overcome this problem including the use of gratings, exotic boundary conditions at metallic surfaces, and other special modes. In the last several years there has been some real progress on this at the STELLA facility at Brookhaven. STELLA was built to investigate the challenges of cascading laser staging. A powerful laser system is required for STELLA with 24 MW instantaneous for the first stage and 300 MW for the second. Initially this device has been used to demonstrate rephasing but not acceleration.
A large-scale laser accelerator R&D program, LEAP, is now underway at Stanford/SLAC
Solid State accelerators
(Stanford HEPL-560-1968) mused on the limitations of conventional
accelerators and speculated on
early version of a channeling accelerator. In his words "To anyone who has carried out experiments with
large modern accelerator there always comes a moment when he wishes
powerful spatial compression of his equipment could take place. If only the very large and massive pieces
could fit in a small room!” What
Hofstadter imagined was a tabletop
accelerator he called Miniac. The device
would consist of a
single crystal driven by an x-ray laser. Channeling would be used to
beam. Hofstadter realized the device might be an after-burner to boost
conventional accelerator beam that was already up into the relativistic
somewhat in the spirit of the recent SLAC studies. At the time
channeling was a
new subject and x-ray lasers were distant dreams.
The basic solid-state acceleration paradigm is to excite a plasma
Pseudo solid state acceleratorsIn the last years there has been progress on something that might at first glance be considered solid-state acceleration. Groups from
Channeling for crystal acceleratorsIn a crystal accelerator channeling would be used to reduce energy loss, to focus the particles, and perhaps even cool the beam. However there are significant problems. The required electron or laser driver beam is so powerful that the crystal would probably be blown away. In addition there is the classical problem of dechanneling where the channeled particle is scattered out of the channel. Interestingly, models of "dynamic channeling" suggest crystals may channel until they vaporize.
In the late 1990s Helen Edwards’ group at Fermilab built a prototype photoinjector at A0 to work on development of the Tesla injector (Tesla has now become part of the (International Linear Collider). The Tesla photoinjector is basically a gigantic photocathode powered by a laser and followed by warm and cold RF stations. The accelerator can deliver very large 14 MeV picosecond electron pulses on the order of 10 nanocoulombs or 105 A/cm2.
The Fermilab A0 photoinjector offered a
means to probe in
the direction of channeling conditions characteristic of those needed
solid-state acceleration and do observations of channeling under
never studied before. With the facility it was possible to study
behavior as the bunch charge increased and go several orders of
beyond earlier measurements. Our Darmsatdt-Fermilab group has
used A0 to study channeling at the highest bunch charges ever channeling at
highest bunch charge ever.