Abstract
An extensive experimental study is performed to confirm fundamental resonance bands of an intense hadron beam propagating through an alternating gradient linear transport channel. The present work focuses on the most common lattice geometry called ``FODO'' or ``doublet'' that consists of two quadrupoles of opposite polarities. The tabletop ion-trap system ``S-POD'' (Simulator of Particle Orbit Dynamics) developed at Hiroshima University is employed to clarify the parameter-dependence of coherent beam instability. S-POD can provide a non-neutral plasma physically equivalent to a charged-particle beam in a periodic focusing potential. In contrast with conventional experimental approaches relying on large-scale machines, it is straightforward in S-POD to control the doublet geometry characterized by the quadrupole filling factor and drift-space ratio. We verify that the resonance feature does not essentially change depending on these geometric factors. A few clear stop bands of low-order resonances always appear in the same pattern as previously found with the sinusoidal focusing model. All stop bands become widened and shift to the higher-tune side as the beam density is increased. In the space-charge-dominated regime, the most dangerous stop band is located at the bare betatron phase advance slightly above 90 degrees. Experimental data from S-POD suggest that this severe resonance is driven mainly by the linear self-field potential rather than by nonlinear external imperfections and, therefore, unavoidable at high beam density. The instability of the third-order coherent mode generates relatively weak but noticeable stop bands near the phase advances of 60 and 120 degrees. The latter sextupole stop band is considerably enhanced by lattice imperfections. In a strongly asymmetric focusing channel, extra attention may have to be paid to some coupling resonance lines induced by the Coulomb potential. Our interpretations of experimental data are supported by theoretical predictions and systematic multiparticle simulations.
Highlights
Modern particle accelerators have relied, almost without exception, on the principle of strong focusing [1,2]
More nonlinear resonance lines have been successfully identified in S-POD III, but coupling instabilities of the fourth or higher orders are generally very weak and difficult to locate unless we considerably extend the period of plasma storage to enhance ion losses
Systematic experiments have been conducted to investigate the stability of an intense hadron beam traveling in a periodic alternating gradient (AG) focusing channel
Summary
Modern particle accelerators have relied, almost without exception, on the principle of strong focusing [1,2]. Previous experiments on the resonant instability of space-charge-dominated beams have relied on a particular machine with a particular lattice configuration [5,6,7,8,9,10,11] To overcome this practical limitation, we here employ the novel tabletop apparatus called “S-POD” (Simulator of Particle Orbit Dynamics) [12,13,14,15,16,17,18,19]. This experiment is based on an isomorphism between the equations of non-neutral plasma motion in a linear Paul trap (LPT) and the equations of intense beam motion in a strong focusing channel [20].
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