Preprint for the HIDIF 2000 Meeting

Low energy beam transport (LEBT) for a future heavy ion driven inertial fusion (HIDIF [1]) facility is a crucial point using a Bi beam of 40 mA at 156 keV. High space charge forces (generalised perveance K=3.6*10) restrict the use of electrostatic focussing systems. On the other hand magnetic lenses using space charge compensation suffer from the low particle velocity. Additionally the emittance requirements are very high in order to avoid particle losses in the linac and at ring injection [2]. Furthermore source noise and rise time of space charge compensation [3] might enhance particle losses and emittance. Gabor lenses [4] using a continuous space charge cloud for focussing could be a serious alternative to conventional LEBT systems. They combine strong cylinder symmetric focussing with partly space charge compensation and low emittance growth due to lower non linear fields. A high tolerance against source noise and current fluctuations and reduced investment costs are other possible advantages. The proof of principle has already been shown [5, 6]. To broaden the experiences an experimental program was started. Therefrom the first experimental results using a double Gabor lens (DGPL, see fig. 1 ) LEBT system for transporting an high perveance Xe beam will be presented and the results of numerical simulations will be shown. 1. Theory and simulations The charge density and the expanse of the space charge cloud determine the focusing strength of a Gabor lens. The space charge cloud can only be established if transversal and longitudinal enclosure conditions are simultaneously fulfilled [7]. Gabor showed that by absence of external electric fields the transversal enclosure condition (first criteria) is given by the Hull-Brillouin flow and therefrom the maximum electron density can be calculated by: ρ ε e rad z