Beam initiated discharge channels for neutralized ion-beam transport

High current discharge channels are an interesting alternative for the transport of intense ion beams. A sufficiently dense plasma can neutralize both beam-space charge and current and provides a strong focusing magnetic field all the way along the channel. Laser initiated discharge channels were subject to detailed investigations in the past few years at GSI [1, 2]. Recently, stable discharges were alternativley initiated by an ion-beam pulse from the UNILAC, which creates a trace of seed electrons in a gas fill of a few mbar along the desired path of breakdown. The discharge chamber is integrated into the beamline by a multi-stage differential pumping system. A beam mask with a single 2 mm diameter circular hole, aligned exactly to the channel axis prepares a narrow ion-beamlet of 11.4 AMeV Ni ions, which defines the size of the ionization channel. First experiments produced stable, free-standing 50 cm long discharge channels in ammonia and argon with currents of up to 55 kA. The discharge is triggered 20 μs after the beginning of a 50 μs long macro pulse with an electric current of less than 5 μA inside the metallic 60 cm diameter discharge chamber. A low energy electric prepulse can optionally be applied before the high current discharge, to stabilize the channels. An upper limit for the electron density induced by the beam, can be estimated from the total energy loss of the beam in the gas. At this current and a gas pressure of 10 mbar the total energy loss of the beam in ammonia during 20 μs is around 10 eV/cm. Neglecting recombination of free electrons and assuming that the whole energy is used to ionize only electrons with the lowest ionization potential, the maximum electron density becomes roughly 10 cm. Although the real density can be one or two orders of magnitude lower, the value is comparable or even higher than the electron densities produced by UV-laser induced ionization of organic molecules, which was used so far for the initiation of discharge channels [3] The dis-