M. Droba

Characterization of volume type ion source for p, H+2 and H+3 beams

Abstract Recently, there is an increasing need for H 2 + and H 3 + ion sources. One example is ion therapy facilities, where C 4 + and H 3 + ion beams along the linac are of great interest. Another example is a H 2 + test beam for linacs finally operated with intense deuteron beams. At Frankfurt, a simple proton ion source is needed to test a new kind of beam injection into a magnetic storage ring [N. Joshi, et al., Beam transport in toroidal magnetic field, EPAC’08, Genoa, June 2008 〈 http://www.jacow.org 〉 , to be published [1] ; M. Droba, et al., Design studies on a novel stellarator type high current ion storage ring, EPAC’06, Edinburgh, June 2006, pp. 297–299 〈 http://www.jacow.org 〉 [2] ]. This article describes a volume type ion source which can deliver up to 3.05 mA beam current at 10 keV in stable dc operation. It is a hot filament driven ion source which can provide high fractions of p, H 2 + or H 3 + , depending on the operation settings.

Experimental studies of stable confined electron clouds using Gabor lenses

Based on the idea of D. Gabor [1] space charge lenses are under investigation to be a powerful focussing device for intense ion beams. A stable confined electron column is used to provide strong radially symmetric electrostatic focussing, e.g. for positively charged ion beams. The advantages of Gabor lenses are a mass independent focussing strength, space charge compensation of the ion beam and reduced magnetic or electric fields compared to conventional focussing devices. Collective phenomena of the electron cloud result in aberrations and emittance growth of the ion beam. The knowledge of the behaviour of the electron cloud prevents a decrease of the beam brilliance. Numerical models developed to describe the electron confinement and dynamics within a Gabor lens help to understand the interaction of the ion beam with the electron column and show the causes of non-neutral plasma instabilities. The diagnosis of the electron cloud properties helps to evaluate the numerical models and to investigate the influence of the ion beam on the confined non-neutral plasma.

JACoW : Space-Charge Compensation in the Transition Area Between LEBT and RFQ

The transition from a space-charge compensated beam in the LEBT to an uncompensated beam in the RFQ will influence the beam parameters. In order to investigate the impact of the electric fields on the space charge compensation, an insulated cone is used as a repeller electrode in front of the RFQ. Depending on the time dependent potential of the RFQ rods respectively to the beam potential, the compensation electrons may be prevented from moving into the RF field which oozes out of the RFQ entrance. The simulation studies are performed with the particle-in-cell code bender [1]. The simulations may substantiate measurements at the CW-operated RFQ in Frankfurt University [2] as well as at the foreseen MYRRHA LEBT-RFQ interface. [3]. In this contribution, a study on a LEBT-RFQ interface is shown.

Beam transport and space charge compensation strategies (invited)

The transport of intense ion beams is affected by the collective behavior of this kind of multi-particle and multi-species system. The space charge expressed by the generalized perveance dominates the dynamical process of thermalisation, which leads to emittance growth. To prevent changes of intrinsic beam properties and to reduce the intensity dependent focusing forces, space charge compensation seems to be an adequate solution. In the case of positively charged ion beams, electrons produced by residual gas ionization and secondary electrons provide the space charge compensation. The influence of the compensation particles on the beam transport and the local degree of space charge compensation is given by different beam properties as well as the ion beam optics. Especially for highly charged ion beams, space charge compensation in combination with poor vacuum conditions leads to recombination processes and therefore increased beam losses. Strategies for providing a compensation-electron reservoir at very low residual gas pressures will be discussed.

Heavy Ion Linac for Synchrotron Injectors

The heavy ion high intensity injector linacs for rings are characterized by a low duty cycle and by high beam intensities. Main disadvantages are the very low particle velocity after passing the extraction system and terminal as well as the high voltages needed to accelerate heavy ions to a specified energy per nucleon. Especially for a low energy acceleration part the H-mode cavity design has some advantages. An overview of ion sources, 4-rod Radio Frequency Quadrupoles (RFQs) and room temperature H-mode cavities will be given. Specific features like the beam quality, the efficiency, the beam dynamic and energy schemes will be discussed.

Magnetic high current ion storage ring

A novel magnetic storage ring for an accumulation of low energy (W ~ 100 keV-1 MeV) high current (~10 A) proton beams is under investigation at Frankfurt University. It offers various possibilities for example to investigate the (p+B11) fusion reaction under different non-neutral plasma state conditions. The proposed ring structure based on a toroidal magnetic field possesses different potential concepts for space charge compensation, accumulation of intense proton beams, and colliding beam operation. The beam life time, reaction cross sections, reaction products from interaction with gas jet and ring operation should be studied in a stellarator-type storage ring for the first time. The theory of thermal and non-neutral plasma confinement on magnetic surfaces is translated to numerical simulations on circulating ion beams. The stability conditions, space charge effects and constraints of the numerical model are studied and will be presented. An additional experimental setup consisting of two 30deg toroidal segments and ion sources is suited to study the nontrivial beam injection into such a ring configuration.