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Vacuum Arc Ion Sources: Charge State Enhancement and Arc Voltage

The Metal Vapor Vacuum ion source (MEWA) has been developed for the production of ion beams for a wide range of metal ions. The ion charge states for most heavy elements created by the high current MEVVA ion source, which typically generates ions with a mean charge state up to three, have to be elevated to higher charge states for accelerator reasons at the GSI accelerator facility. Several methods are known which elevate charge states. Especially for the generation of U4+ ions a strong magnetic field and a high arc current has been used at GSI. Both methods increase the arc voltage which in turn influences the electron energy distribution and thus the mean ion charge state. Arc voltages have been measured for different settings of arc current, magnetic flux density and discharge geometry indicating the dependencies between these quantities and the arc voltage.   more...

Author: M. Galonska et al.

Accel–decel extraction system for PIG sources

Increasing the brightness of the ion beam is a typical demand for any accelerator. In order to fill the heavy ion synchrotron (SIS) up to the space charge limit, higher pulse currents are necessary. The UNILAC is operated with a pulse length up to 6 ms and a repetition rate up to 50 s−1. For the injection into the SIS a pulse length of 300 μs at 0.3 s−1 is required. The transverse acceptance of the RFQ is 138 π mm mrad. According to Child’s law, the ion current density which can be extracted from a plasma source is for the space charge limited flow proportional to Φ1.5/d2. With Φ and d denoting the extraction voltage and the width of the extraction gap. One approach to generate higher ion currents is to increase the extraction voltage in the regularly used extraction system. However, the required beam velocity is fixed by the RFQ structure with 2.2 keV/u. The mass-to-charge ratio which is to be accelerated can be between 1 and 65, resulting in a total voltage drop from 2.2 up to 143 kV (extraction and postacceleration voltage). The other approach is to decrease the gap width in the extraction system, but there is an optimum in the aspect ratio. To generate an ion beam with a higher current at low energy, a triode extraction system operated in accel–decel (Uacc,Udec) mode is a possible solution. By applying a negative potential at the second electrode a higher extraction field strength can be achieved. The effects of extraction field, arc current, and ion energy on the extracted ion beam current and its emittance were investigated.   more...

 2002 American Institute of Physics.

Author: P. Spädtke et al.