Beam optics of a 10-CM diameter high current heavy ion diode

BEAM OPTICS OF A 10-CM DIAMETER HIGH CURRENT HEAVY ION DIODE * J.W. Kwan, J.L. Vay, F.M. Bieniosek, Lawrence Berkeley National Laboratory; E. Halaxa, G. Westenskow, Lawrence Livermore National Laboratory; I. Haber, Univ. of Maryland Abstract Typically a large diameter surface ionization source is used to produce > 0.5 A K + current with emittance < 1 π-mm-mrad for heavy ion fusion experiments. So far we have observed aberrations that are slightly different from those predicted by computer simulations. We have now set up an experiment to study in detail the beam optics of such a large diameter ion diode and to benchmark the simulation code. for fusion drivers, lighter ions such as K + can be useful in the near future because they provide an opportunity to do experiments at high ion velocities on medium length accelerator facilities during the early development phases. Figure 1 shows the 2-MV injector that was developed for the previous ILSE project [3] which consists of a 750 kV triode followed by an electrostatic quadruple (ESQ) section. Recently the ion source and extraction gap was modified in order to improve the beam optics for meeting the requirements of the High Current Transport (HCX) experiment [4]. So far the experimental results from the HCX experiments have shown qualitative agreement with the computer simulation. The discrepancy in beam current and in beam optics between the measurement and simulation predictions were significant enough to cause uncertainty in future beamline designs. This is especially true if the design relies heavily on end-to-end simulation or the experiment requires a high degree of phase space control accuracy. INTRODUCTION Heavy ion driven inertial fusion (HIF) requires about 3- 7 MJ to achieve ignition with a D-T target [1]. At 2-4 GeV ion kinetic energy, the corresponding beam charge is ~1 mC. Induction linacs can accelerate and compress these beams from ~10 µs at injection to ~300 ns by the end of the driver and further drift compress the duration down to ~10 ns at the target. The total beam current from the ion source is ~ 50-100 A. In order to overcome the space charge problem associated with high current heavy ion beams, an HIF driver is usually designed to contain an array of N ~ 100 parallel ion beam channels at ~ 0.5 A each. In order to focus the ion beams onto a mm-size fusion target the beam emittance must be small, thus HIF requires beams with both large current and high brightness. Since the beam brightness is proportional to J/T, where J is the current density and T is the effective ion temperature, high brightness demands either high current density and/or low effective ion temperature. Furthermore, a heavy ion injector must have an adequate low energy beam transport (LEBT) system that can handle the severe space charge force. The LEBT often limits the maximum current density in the injector and therefore dictates the type of ion source that can be used. According to high voltage breakdown and Child- Langmuir space-charge flow scalings, the current density of a diode decreases as the beam current increases [2]. Thus producing large current and high brightness from a single large aperture implies low ion temperature because the current density must be low. This condition can be met by a surface ionization sources because their typical effective ion temperature is < 1 eV and the solid emitter surface provides a way to design large diameter beam optics. Although heavy ions such as Cs + are ultimately needed **This work is supported by the Office of Fusion Energy Science, US DOE under contract No. DE-AC03-76SF00098 (LBNL) and W-7405- ENG-48 (LLNL). Email: jwkwan@lbl.gov Fig. 1: Schematic Diagram of the 2-MV injector. At present, our strategy is to use a two-pronged approach for the injector program [2]. On one hand we are exploring a new approach to built compact injectors, that are suitable for multiple beams HIF drivers, using an array of high current density beamlets. On the other hand, we continue to improve the large diameter surface source so it can provide a single-beam injector for near-term HIF projects such as the Integrated Beam Experiment (IBX). Recently we have mastered the techniques to fabricate large diameter alumino-silicate sources with a uniform emitting surface [5]. The main objective of the experiment described in this paper is to study how to produce a high quality ion beam from a large diameter surface ionization source. By carefully comparing the results from experiment against that from computer simulation, we will benchmark the simulation code(s) and also determine if the necessary physics are included in the simulations.