High-gradient two-beam accelerator structure

Abstract

A novel cavity structure is described that could be the basis for a two-beam, high-gradient, accelerator. Versions of the structure could be used for acceleration of beams of electrons, positrons, muons, protons, or heavier ions; with either electron or proton drive beams. The structure embodies cavities that are excited in several harmonically related eigenmodes, such that rf fields reach their peak values only during small portions of each basic rf period. This feature could help raise breakdown and pulse heating thresholds. The two-beam accelerator structure comprises chains of these cavities. In this configuration, no transfer elements are needed to couple rf energy from the drive beam to the accelerated beam, since both beams traverse the same cavities. Purposeful cavity detuning is used to provide much smaller deceleration for a high-current drive beam, than acceleration for a low-current accelerated beam, i.e., to provide a high transformer ratio. A self-consistent theory is presented to calculate idealized acceleration gradient, transformer ratio, and efficiency for energy transfer from the drive beam to the accelerated beam, for either parallel or antiparallel motion of the beams. The theory has been cast in dimensionless quantities so as to facilitate optimization with respect to efficiency, acceleration gradient, or transformer ratio, and to illuminate the interdependence of these parameters. Means for dramatically shortening the structure fill time are also described. However, no beam dynamics analysis is presented, so the range of parameters within which this new acceleration concept can be used will remain uncertain until it is established that stable beam transport along the structure using an appropriate focusing system is possible.

A multi-frequency RF photocathode gun

“…The acceleration field is proportional to a total number N of modes, because an accelerated bunch sees all modes in phase. The necessary RF power to provide a given gradient is asymptotically reduced by a factor of N in comparison with a single mode gun [5,6]. An application of several harmonically related modes in RF injection gun cell thus promises less loss and less input power in comparison with traditional single mode gun of the same bunch energy, length, charge, and emittance.…”

High-power free-electron maser operated in a two-mode frequency-multiplying regime

“…Moreover, the coherent bichromatic radiation generated by an FEM multiplier is beneficial itself, in particular, for powering a double-frequency accelerating structure. Such a structure of the novel type was proposed in [17] and [18]. The addition of a high harmonic to the main accelerating field allows, first, to reach a higher accelerating gradient due to the reduction of the structure surface exposed to rf fields and the exposure time, as well as due to the reduction of those surface field components which are responsible for the autoelectron emission [17,18].…”

“…Such a structure of the novel type was proposed in [17] and [18]. The addition of a high harmonic to the main accelerating field allows, first, to reach a higher accelerating gradient due to the reduction of the structure surface exposed to rf fields and the exposure time, as well as due to the reduction of those surface field components which are responsible for the autoelectron emission [17,18]. Second, the use of the higher harmonics can also reduce the consumed rf power for a given value of the accelerating gradient due to the enhancement of the structure's shunt impedance, as was shown in [19].…”

Concept and Numerical Simulations of Multi-Beam Linear Accelerators EVT with Depressed Collector for Drive Beam

“…Therefore, synchronous energy transfer from the drive beams to the accelerated beam is achieved by correct choices of distances ∆z between the adjacent cavity gaps. A similar condition, of two-beam acceleration in inductive tuned cavity, is considered by Kazakov et al [5], where, in contrast to EVT, the authors have placed the drive beam and the accelerated beam on the same axis. Dolbilov G has discussed a two-beam accelerator of protons, where the use of inductive tuned cavities for an electron drive beam is considered [6,7].…”