Charge compensation and acceleration of a thick-walled high-current ion beam in induction linac

Abstract

INTRODUCTIONAt present, several methods of obtaining highcurrent ion beam, which are based on the use of induction accelerators and are applied to inertial controlled fusion (ICF) research, are being considered [1][2][3]. To date, kiloampere ion beams with energies of several hundred keV have been produced in highcurrent linear induction accelerators (linac) with collective focusing [4][5][6]. The power of the hollow highcurrent ion beam (HHCIB) for ICF purposes must be several orders greater, with rather stringent requirements on beam brightness. Therefore, when developing a driver for ICF on the basis of a highcurrent linac, it is necessary to investigate a number of important physical problems: (1) the formation of highcurrent beams in injector; (2) the provision of efficient magnetic insulation for accelerating gaps; (3) charge compensation of the ion beam in the transport channel and in the magneto-insulated accelerating gaps; (4) effective acceleration and stability of the ion beam in accelerating channel; and (5) transport, focusing, and space-time compression of HHCIBs.In linac, the conventional way of charge and current compensation [7,8] is inefficient. In [9, 10], a new mechanism for the neutralization of HHCIBs in axially symmetric magneto-insulated gaps was proposed. Its physical meaning is that a specially injected compensating electron beam drifts through the cusp due to self-consistent azimuthal magnetic field and an electric field caused by a small radial separation of ion and electron beams.In [11][12][13], the investigation results of the acceleration, and the charge and current compensation of HHCIBs in one and two linac cusps are reported. These results have shown that both in the presence and in the absence of an accelerating electric field, the following effects take place: (1) charge and current compensation of HHCIB in the accelerating gaps; (2) stability of the ion beam during times that substantially exceed the inverse ion Langmuir and Larmor frequencies. The performed numerical simulations have also shown that in the drift space between two accelerating gaps the current and charge compensation of the ion beam proved to be insufficient because of a substantial difference in the electron and ion velocities acquired up to the time of the transit of the beams through the drift gap. As a result, the positive potential of the self-consistent field in the drift space leads to the spread and deceleration of the ion beam and, consequently, to the degradation of the beam brightness. The positive space charge in the drift gap can be compensated by injecting thermal electrons into it. It was shown that a preliminary injection of cold electrons permits to eliminate broadening and decelerating an ion beam in the drift gap and to provide its additional focusing.