Spiral Line Induction Accelerator Research Articles

Control of emittance growth due to mismatches in accelerators that use stellarator transport

In the spiral line induction accelerator, an intense electron beam is transported along an open-ended beam pipe that makes multiple passes through the accelerating cavities. In the straight sections of the beam line, solenoidal focusing is used; in the bends, an l=2 stellarator field is used. At the solenoid/stellarator transition, where the beam equilibrium changes, a mismatch can occur, exciting oscillations of the beam envelope. Numerical simulation is used to show that the frequency, damping rate, and emittance growth associated with these oscillations are sensitive to nonlinear space-charge forces that depend significantly on the radial profile of the beam. Comparisons between simulation and experimental results illustrate this sensitivity. It is shown that mismatch oscillations can be avoided by using a single thick quadrupole lens at the solenoid/stellarator transition. Simulation and experimental results show excellent agreement.

Further studies on beam breakup growth reduction by cavity cross-couplings in recirculating accelerators: effects of long pulse length and multiturn recirculation

Cavity cross-coupling was recently found to reduce beam breakup (BBU) growth in a recirculating accelerator known as the Spiral Line Induction Accelerator (SLIA). Here, we extend the analyses in two respects: long beam pulse lengths and a SLIA upgrade geometry which accelerates a 10 kA, 35 ns beam to 25 MeV via a 70 cavity, 7 turn recirculation. We found that when the beam pulse length τ exceeds the beam's transit time τ′ between cross-coupled cavities. BBU growth may be worsened as a result of the cross-couplings among cavities. This situation is not unlike other long pulse recirculating accelerators where beam recirculation leads to beam breakup of a regenative type. Thus, the advantage of BBU reduction by cavity cross-coupling is restricted primarily to beams with τ<τ′, a condition envisioned for all SLIA geometries. For the 70 gap, 7 turn SLIA upgrade, we found that cavity cross-coupling may reduce BBU growth up to factors of a thousand when the quality factor Q of the deflecting modes are relatively high (like 100). In these high Q cases, the amount of growth reduction depends on the arrangement and sequence of beam recirculation. For Q⪅20, BBU growth reduction by factors of hundreds is observed, but this reduction is insensitive to the sequence of beam recirculation. The above conclusions were based on simple models of cavity coupling that have been used in conventional microwave literature. Not addressed is the detail design consideration that leads to the desired degree of cavity coupling.

Electromagnetic beam stability in a betatron with helical quadrupole magnetic field

A Vlasov treatment is developed to investigate the beam stability in a betatron with stellarator windings. The general formulation includes effects of finite beam temperature and radius. An analytic dispersion relation is derived. In the limit of cold beam and small beam radius, the general dispersion relation reduces to simpler form, which exhibits instability due to frequency matching between the transverse electric (TE) beam pipe cavity modes and the Doppler shifted electron oscillations in a combined longitudinal and helical quadrupole magnetic field. In particular, an internal beam mode with azimuthal mode number ν=0 is found to be unstable for typical parameters of the Spiral Line Induction Accelerator design [Bull. Am. Phys. Soc. 34, 2160 (1989)]