Mechanisms and Control of Beam Halo Formation in Intense Microwave Sources and Accelerators

Halo formation and control in space-charge-dominated electron and ion beams are investigated in parameter regimes relevant to the development of high-power microwave (HPM) sources and highintensity electron and ion linear accelerators. In particular, a mechanism for electron beam halo formation is identified in high-power periodic permanent magnet (PPM) focusing klystron amplifiers. It is found in self-consistent simulations that large-amplitude current oscillations induce mismatched beam envelope oscillations and electron beam halo formation. Qualitative agreement is found between simulations and the 50 MW 11.4 GHz PPM focusing klystron experiment at Stanford Linear Accelerator Center (SLAC) [D. Sprehn, G. Caryotakis, E. Jongewaard, and R. M. Phillips, “Periodic permanent magnetic development for linear collider X-band klystrons,” Proc. XIX International Linac Conf. (Argonne National Laboratory Report ANL-98/28, 1998), p. 689.] Moreover, a new class of cold-fluid corkscrewing elliptic beam equilibria is discovered for ultrahigh-brightness, space-chargedominated electron or ion beam propagation through a linear focusing channel consisting of uniform solenoidal magnetic focusing fields, periodic solenoidal magnetic focusing fields, and/or alternatinggradient quadrupole magnetic focusing fields in an arbitrary arrangement including field tapering. As an important application of such new cold-fluid corkscrewing elliptic beam equilibria, a technique is developed and demonstrated for controlling of halo formation and beam hollowing in an rms-matched ultrahigh-brightness ion beam as it is injected from an axisymmetric Pierce diode into an alternatinggradient magnetic quadrupole focusing channel. PACS numbers: 29.27, 52.75.V