Pulse line ion accelerator concept

Abstract

April 3, 2006 LBNL-59492 The pulse line ion accelerator concept Richard J. Briggs Science Applications International Corporation, Alamo, California, 94507 The Pulse Line Ion Accelerator concept was motivated by the desire for an inexpensive way to accelerate intense short pulse heavy ion beams to regimes of interest for studies of High Energy Density Physics and Warm Dense Matter. A pulse power driver applied at one end of a helical pulse line creates a traveling wave pulse that accelerates and axially confines the heavy ion beam pulse. Acceleration scenarios with constant parameter helical lines are described which result in output energies of a single stage much larger than the several hundred kilovolt peak voltages on the line, with a goal of 3-5 MeV/meter acceleration gradients. The concept might be described crudely as an “air core” induction linac where the PFN is integrated into the beam line so the accelerating voltage pulse can move along with the ions to get voltage multiplication. 1. Introduction In the “Pulse Line Ion Accelerator” (PLIA) concept described in this paper a ramped high voltage pulse is applied at the input end of a helical pulse line structure. The resulting traveling wave pulse on the line can accelerate an ion bunch to energies much greater than the peak voltage applied to the line. It should also be possible to achieve an axial acceleration gradient of several MeV per meter with realistic helix parameters. The development of this concept was originally motivated by a proposal to use moderate energy intense ion beams to heat matter to regimes of interest for studies of High Energy Density Physics (HEDP) and Warm Dense Matter (WDM) [ 1 ] . In this approach, very short pulses (~ a nanosecond or less) of “medium mass” ions with energies slightly above the Bragg peak are focused to mm scale spot sizes onto thin target “foils”, which may in fact be foams with mean densities of order 10% solid density or less. The high degree of uniformity and efficiency with volumetric and shock-less heating of the target to temperatures of order 1 to 10 eV are attractive features in this approach. The concept presented in this paper is an excellent fit to the accelerator requirements for this HEDP/WDM application. A helical pulse line inserted into a large bore 5 - 10T superconducting solenoid can transport and accelerate a singly charged Ne or Na ion bunch with a total charge of order 0.1 to 1 micro-coulombs, an axial bunch length of 10’s of cm, and a radius of a few cm. The required axial compression to bunch lengths of order one cm in a neutralized drift compression region following the accelerator is then only a factor of 10-30, which greatly eases the requirements on longitudinal emittance, velocity tilt, and drift distance.