Compact high average gradient particle accelerators utilizing photoconductive switches

The drive for high average gradient particle accelerators is on two fronts: the first is in the high energy physics and the second in medical and industrial applications. The conventional approach utilizes Klystron pumped cavities, which are pumped slowly (microseconds) and dumped fast (nanoseconds). These accelerators operate at under ~MV/cm gradient. Both breakdown electric field and cavity losses due to field emission are time dependent. This results in an upper limit on the working field in the cavity and thus on the average gradient. [1] There have been a number of concepts proposed which do not have the above mentioned limitations; key among them are the direct acceleration in a focused laser beam [2], laser plasma based devices [3,4] and the photoconductively switched arrays of radial transmission lines (RTL). The RTL approach depends on photoconductive switches, which makes this approach viable because they represent the highest power per unit volume and the highest switching speed available. In all RTL approaches, the accelerator is composed of a series of centrally apertured radial disks where adjacent pairs constitute radial transmission lines. A radially convergent TEM wave is launched at the radial periphery, which in turn increases in field as a result of the radial convergence. Such a concept has the high field only at the aperture and lasts only for the temporal length of the wave which is also the duration of the acceleration. The RTL approach described here is but one of a variety of potential approaches based on switched RTLs. The approach described here has a number of innovations over the more common RTL approaches which are: 1. The photoconductive switch orientation allows both switch and dielectric transmission lines to operate at their respective breakdown fields allowing increased power and average gradient by choosing a different geometry. 2. Matching the pulse length and electrode material (gold) to obtain the highest accelerating field i- the aperture by allowing the field emission current to operate just below the EM wave current. The resulting concept has the potential of pushing the average field of accelerators to the 5MV/cm range.