Space Charge Lens for Focusing Heavy Ion Beams

Space charge lenses use a confined electron cloud for the focusing of ion beams. Due to the electric space charge field, the focusing is independent of the particle mass. For this reason the application of space charge lenses especially in the field of heavy ion beams is advantageous. Moreover, the trapped non neutral plasma cloud compensates the space charge forces of the ion beam. The focusing strength is given by the confined electron density whereas the density distribution influences the mapping quality of the space charge lens. An important parameter for the focusing capability of the space charge lens besides the homogeneity is a high electron density. In ongoing theoretical and experimental work methods have been developed to determine the most important parameters like electron temperature and electron density distribution for an optimized lens design. Based on experimental results a new space charge lens has been designed to focus low energy heavy ion beams like a 2.2 keV/u U4+ at the low energy transport section into the GSI High Current Injector [1]. Experimental results will be presented in comparison with numerical simulations. SPACE CHARGE LENS FOR THE HSI-UPGRADE The beam transport of high perveance heavy ion beams is influenced by high space charge forces and high ion mass. The introduction of trapped electrons into the low energy beam transport section (LEBT) leads to space charge compensation and focusing of the ion beam at drastically reduced magnetic and electric field strength compared to conventional ion optics. The high current injector (HSI) at GSI provides experimental conditions to investigate the performance of the space charge lens. As an alternative LEBT concept which is described in [2] a new space charge lens has been designed for the possible application at the HSI upgrade. The length of the prototype is 436 mm with an aperture of 150 mm (fig. 1). The device is optimized for a 2.2 keV/u U4+-beam with a maximum beam radius of 50 mm. For the mapping quality of the space charge lens the electron density distribution due to the confining fields is very important. Numerical simulations by the use of ΦA=30 kV electrode potential and the magnetic field of Bz=13 mT for the confinement of the non neutral plasma results in a nearly homogeneous electron density distribution of ne,max=2.7·10 m−3. This yields a linear selffield ∗Work supported by HIC for FAIR, BMBF No. 06FY90891 † [email protected] Figure 1: The design of the HSI lens prototype (Bmax=0.1 T, ΦA,max=50 kV). of Er=117 kV/m at r=50 mm (fig. 2). The magnetic field Figure 2: Calculated electron density distribution (left) and the resulting radial electrostatic selffield of the non neutral plasma in the lens midplane (right). produced by the coils is not uniform in the z direction. In comparison to an ideal homogeneous magnetic field it leads only to minor changes in the radial electron density distribution (fig. 3). Figure 3: a) Measured (I=5A) and calculated magnetic field of the coil system, b) comparison of the electron density distribution, and c) difference of the electron density calculated for the homogeneous field and the field of the magnetic coils. BEAM TRANSPORT SIMULATION With respect to the design of the prototype the transport of a 2.2 keV/u U4+-beam as a function of the beam radius was simulated using the code LINTRA [3]. For these calculations a homogeneous phase space distribution at the entrance of the beam line and a space charge compensation of 100% was assumed. Fig. 4 shows the beam envelope along the transport section that consists of a first drift section with different lengths, the space charge lens, and a second drift of 650 mm. The output emittance at the end of the second drift for different beam radii (r1,beam=38 mm, r2,beam=51 mm, r3,beam=72 mm) is presented as well. As Figure 4: Beam envelope along the transport section, input emittance, and output emittance for selected beam radii. a consequence of the linear electric field in radial direction aberrations only occur at the edge of the non neutral plasma column. At the edge of the electron cloud the density drops off over a distance on the order of the Debye length (λD=1.9 cm) which leads to an emittance growth (fig. 5). However, the emittance growth up to rbeam=58 mm Figure 5: Emittance growth (100% ellipse) as a function of the beam radius. The colour code corresponds to the frames in fig. 4. is below 2%. For an optimized lens design and in order to determine the operation mode the plasma parameters and the dynamics have to be further investigated experimentally. DIAGNOSTICS AND EXPERIMENTS To determine the important plasma parameters like electron temperature, electron density, and to investigate diagnostic techniques on a non neutral plasma three experimental setups have been established (fig. 6). In the following Figure 6: Experimental setup (not scaled) for a) density measurement, b) the study of optical technique to determine electron temperature, and c) pepperpot emittance measurement. sections the methods to determine the plasma parameters are described.