High Current Heavy Ion Beams Research Articles

A Study on the Dielectric Design of the High-Voltage Platform for Developing the 28 GHz ECRIS at the KBSI

Currently, the 28 GHz ECRIS (electron cyclotron resonance ion source) was developed to produce a high-current heavy-ion beam at the KBSI (Korea Basic Science Institute). The high-voltage platform of the 28 GHz ECRIS is essential to deliver an ion beam to the next acceleration stage. The high-voltage platform was designed considering dielectric characteristics in order to ensure electrical safety. In this paper, a study on the dielectric characteristics of GFRP (glass- fiber-reinforced plastic) is performed to determine the thickness of the GFRP cylindrical shell located between the plasma chamber and the warm bore of the superconducting magnet cryostat. The dielectric experiments on a GFRP cylindrical shell were conducted under a DC voltage. Sphere-to-plane electrode systems were used to examine the dielectric characteristics. Also, the relationship between the dielectric characteristics of the GFRP cylindrical shell and the distribution of the electric field intensity was calculated and analyzed by using FEM (Finite Elements Method) simulations.

Efficient focusing, bunching, and acceleration of high current heavy ion beams at low energy

The efficient focusing, bunching and acceleration of high current, low energy heavy ion beams in an RFQ accelerator have been investigated. The considerations on the choice of the working frequency for increasing the beam current are introduced. Designed using an unconventional procedure, this 20 emA RFQ will be able to work with A/q≤ 60 ion beams in the β range of 0.002–0.016. The beam dynamics simulation shows that the design fulfills all requirements with good beam quality and a compact structure.

Investigation of ion beam space charge compensation with a 4-grid analyzer.

Experiments to investigate the space charge compensation of pulsed high-current heavy ion beams are performed at the GSI ion source text benches with a 4-grid analyzer provided by CEA/Saclay. The technical design of the 4-grid analyzer is revised to verify its functionality for measurements at pulsed high-current heavy ion beams. The experimental investigation of space charge compensation processes is needed to increase the performance and quality of current and future accelerator facilities. Measurements are performed directly downstream a triode extraction system mounted to a multi-cusp ion source at a high-current test bench as well as downstream the post-acceleration system of the high-current test injector (HOSTI) with ion energies up to 120 keV/u for helium and argon. At HOSTI, a cold or hot reflex discharge ion source is used to change the conditions for the measurements. The measurements were performed with helium, argon, and xenon and are presented. Results from measurements with single aperture extraction systems are shown.

Fabrication of the plasma chamber for the SC-ECRIS of the RAON accelerator

The RAON is the heavy ion accelerator being built in Korea. A 3rd generation SC-ECRIS (Super Conducting Electron Cyclotron Resonance Ion Source) which uses 28 GHz/18 GHz microwave power is used to extract a high-current heavy-ion beam. The plasma chamber for that ECRIS was fabricated and assembled. It is made of aluminum to increase the beam current. It also contains a cooling channel, an X-ray shield, and high voltage insulation. The extraction electrodes are aligned through precise machining and assembly. The plasma chamber has been installed at the test site. The first plasma ignition and beam extraction will be done in the near future.

Plasma shape control by pulsed solenoid on laser ion source

A Laser ion source (LIS) provides high current heavy ion beams with a very simple mechanical structure. Plasma is produced by a pulsed laser ablation of a solid state target and ions are extracted by an electric field. However, it was difficult to manipulate the beam parameters of a LIS, since the plasma condition could only be adjusted by the laser irradiation condition. To enhance flexibility of LIS operation, we employed a pulsed solenoid in the plasma drift section and investigated the effect of the solenoid field on singly charged iron beams. The experimentally obtained current profile was satisfactorily controlled by the pulsed magnetic field. This approach may also be useful to reduce beam emittance of a LIS.

Laser ablation ion source for heavy ion inertial fusion

Laser ion source (LIS) is one of the promising candidates for the front end of heavy ion inertial fusion power plant. A LIS can provide low emittance high current heavy ion beams. Based on the performance of an existing LIS, the feasibilities of the ion source of heavy ion inertial fusion plant are investigated assuming both induction accelerator scheme and radio frequency (RF) accelerator scheme. By combining recently developed techniques, we can design LIS both for the induction and RF accelerator schemes.

Recent advances in plasma devices based on plasma lens configuration for manipulating high-current heavy ion beams

We describe new results of development of novel generation cylindrical plasma devices based on the electrostatic plasma lens configuration and concept of electrons magnetic insulation. The crossed electric and magnetic fields plasma lens configuration provides us with the attractive and suitable method for establishing a stable plasma discharge at low pressure. Using plasma lens configuration in this way some cost-effective plasma devices were developed for ion treatment and deposition of exotic coatings and the effective lens was first proposed for manipulating high-current beams of negatively charged particles. Here we describe operation and features of these plasma devices, and results of theoretical consideration of mechanisms determining their optimal operation conditions.

Beam behavior under a non-stationary state in high-current heavy ion beams

Three-dimensional beam dynamics in a space-charge-dominated regime during a longitudinal pulse compression is investigated numerically using a multi-particle code developed. Results are compared with those of a two-dimensional particle simulation including a longitudinal current increase model. The rms transverse emittance additionally increases along the drift distance due to the longitudinal motion of the beam particles. The three-dimensional beam behavior can be also compared with a longitudinal one-dimensional calculation under a condition of a constant geometry factor for the transverse direction. Results indicate that the longitudinal beam dynamics are not affected by the transverse motions in the parameter regime.

Beam dynamics during longitudinal bunch compression of high-current heavy-ion beams

Dynamics of high-current heavy-ion beams are studied during longitudinal bunch compression. Operation regions of particle beams are plotted on a tune depression–wave velocity diagram. Emittance growth due to the transverse focusing field error is investigated during the final beam bunching. The beam bunch is longitudinally compressed during the transport with the field error in continuous focusing (CF) or alternating gradient (AG) field lattices. Numerical calculation results show the difference of the emittance growth is only 2% between the cases with and without field error in the CF lattice. On the other hand, in the case of AG lattice with field error of 10%, the emittance growth of 2.4 times is estimated. The beam behaviors in the CF and AG lattice transport are discussed based on the numerical simulations.

Transport of heavy-ion beams in a 1 m free-standing plasma channel

The transport of high-current heavy-ion beams in plasma channels is a promising option for the final transport in a heavy-ion fusion reactor, since it simplifies the construction of the reactor chamber significantly. Our experiments at the Gesellschaft für Schwerionenforschung demonstrate the creation of 1 m long stable plasma channels and the transport of heavy-ion beams. The article outlines the experimental setup used at GSI and reports the results of beam transport measurements using these long channels. The experiments demonstrate good beam transport properties of the channel, indicating that channel transport is a viable alternative to neutralized-ballistic transport.

Options for integrated beam experiments for inertial fusion energy and high-energy density physics research

The Heavy Ion Fusion Virtual National Laboratory (HIF–VNL), a collaboration among LBNL, LLNL, and PPPL, is presently focused on separate smaller-scale scientific experiments addressing key issues of future Inertial Fusion Energy (IFE) and High-Energy-Density-Physics (HEDP) drivers: the injection, transport, and focusing of intense heavy ion beams at currents from 25 to 600 mA. As a next major step in the HIF–VNL program, we aim for a fully integrated beam physics experiment, which allows integrated source-to-target physics research with a high-current heavy ion beam of IFE-relevant brightness with the goal of optimizing target focusing. This paper describes two rather different options for such an integrated experiment, the Integrated Beam Experiment (IBX) and the Neutralized Drift Compression Experiment (NDCX). Both proposals put emphasis on the unique capability for integrated injection, acceleration, compression, and focusing of a high-current, space-charge-dominated heavy ion beam.

Effect of the electrostatic plasma lens on the emittance of a high-current heavy ion beam

We describe measurements we have made of the emittance of a high-current, moderate-energy ion beam after transport through a permanent-magnet electrostatic plasma lens. The results indicate the absence of emittance growth due to the lens, when the lens is adjusted for optimal beam focusing. The measured emittance for a 16 keV Cu{sup 2+} ion beam formed by a vacuum arc ion source was about 0.4 {pi} {center_dot} mm {center_dot} mrad at a beam current of 50 mA rising more-or-less linearly to 1.5 {pi} {center_dot} mm {center_dot} mrad at 250 mA, and was conserved in beam transport through the lens. These results have significance for the application of high-current ion sources and the electrostatic plasma lens to particle accelerator injection.

Electron-cloud simulation and theory for high-current heavy-ion beams

Stray electrons can arise in positive-ion accelerators for heavy-ion fusion or other applications as a result of ionization of ambient gas or gas released from walls due to halo-ion impact, or as a result of secondary-electron emission. We summarize the distinguishing features of electron-cloud issues in heavy-ion-fusion accelerators and a plan for developing a self-consistent simulation capability for heavy-ion beams and electron clouds (also applicable to other accelerators). We also present results from several ingredients in this capability. (1) We calculate the electron cloud produced by electron desorption from computed beam-ion loss, which illustrates the importance of retaining ion reflection at the walls. (2) We simulate the effect of specified electron-cloud distributions on ion beam dynamics. We consider here electron distributions with axially varying density, centroid location, or radial shape, and examine both random and sinusoidally varying perturbations. We find that amplitude variations are most effective in spoiling ion beam quality, though for sinusoidal variations which match the natural ion beam centroid oscillation or breathing-mode frequencies, the centroid and shape perturbations can also have significant impact. We identify an instability associated with a resonance between the beam-envelope ``breathing'' mode and the electron perturbation. We estimate its growth rate, which is moderate (compared to the reciprocal of a typical pulse duration). One conclusion from this study is that heavy-ion beams are surprisingly robust to electron clouds, compared to a priori expectations. (3) We report first results from a long-time-step algorithm for electron dynamics, which holds promise for efficient simultaneous solution of electron and ion dynamics.

High-Current Heavy Ion Beams in the Electrostatic Plasma Lens

We describe applications of the electrostatic plasma lens for manipulating and focusing moderate-energy, high-current, broad, heavy ion beams. Use of a plasma lens in this way has been successfully demonstrated in a series of experiments carried out collaboratively between the Institute of Physics, National Academy of Sciences, Kiev, Ukraine, and the Lawrence Berkeley National Laboratory, Berkeley, CA, in recent years. Here, we briefly review the plasma lens fundamentals, peculiarities of focusing heavy ion beams, and summarize some recent developments (experiments, computer simulations, theory). We show that there is a very narrow range of low magnetic field for which the optical properties of the lens improve markedly. This opens up some attractive possibilities for the development of a new-generation compact lens based on permanent magnets. Preliminary experimental results obtained at Kiev and Berkeley on the operation of a permanent magnet plasma lens for manipulating wide aperture high-current heavy ion beams are presented and summarized.

Numerical simulations of a high-current plasma lens

The dynamic processes by which an electrostatic plasma lens with a wide-aperture ion beam and electrons produced from the secondary ion-electron emission relaxes to a steady state is investigated for the first time by the particle-in-cell method. The parameters of a two-dimensional mathematical model were chosen to correspond to those of actual plasma lenses used in experimental studies on the focusing of high-current heavy-ion beams at the Institute of Physics of the National Academy of Sciences of Ukraine (Kiev, Ukraine) and the Lawrence Berkeley National Laboratory (Berkeley, USA). It is revealed that the ion background plays a fundamental role in the formation of a high potential relief in the cross section of a plasma lens. It is established that, in the volume of the plasma lens, a stratified electron structure appears that is governed by the nonuniform distribution of the external potential over the fixing electrodes and the insulating magnetic field. The stratification is very pronounced because of the finite sizes of the cylindrical fixing electrodes of the lens. It is shown that the presence of such a structure limits the maximum compression ratio for an ion beam to values that agree with those observed experimentally.

Status of the electrostatic plasma lens

The electrostatic plasma lens provides an important tool for the manipulation of high current heavy ion beams with moderate energy of 1–100 keV. Applications can include, for example, high dose ion implantation and particle accelerator injection. Here we briefly review the plasma lens (PL) fundamentals and summarize some recent developments and experiments performed at the Institute of Physics NASU (Kiev) and LBNL (Berkeley). We also describe progress in experimental investigations of the nonlinear electron vortex structures that arise due to the drift instability in the high current plasma lens, and lens operation in the low magnetic field regime. We show that there is a very narrow range of low magnetic field for which the focusing properties of the PL improve markedly. Under these conditions a significant decrease (by more than an order of magnitude) in the amplitude of oscillations in the lens volume and focused ion beam are observed.

Electron trapping in high-current ion beam pipes

The space charge voltage depression in a drifting heavy ion beam during the final stages of current pulse compression can be hundreds of kV. For example, a 1 kA beam of ions at β= v/ c=0.4 would have a beam center-to-edge potential difference of 75 kV. With suitable clearance from beam edge to the beam pipe, this amount is typically increased by a factor of 2–3 by the (1+2 ln( b/ a)) term that accounts for the ratio of pipe radius, b, to beam radius, a. Such high voltages, and resulting high electric fields at the pipe wall, will result in electrons being pulled into the beam pipe. These electrons which are emitted from the grounded beam pipe, will pass through the ion beam at high velocity and then turn around without (usually) striking the wall and continue to pass through the beam on repeated oscillations. It is possible to control the longitudinal motion of these trapped electrons by suitably varying the pipe size while considering the beam diameter. A segment of the beam pipe that has a larger diameter will result in a potential well that traps the electrons longitudinally. In a constant current scenario in a uniform pipe, the electrons will drift in the direction of the beam. However, the head and especially the tail of the ion beam will have a dramatic effect on the electrons, causing them to be pulled into the ion beam. These complex processes will continue until the ion beam passes through an optical element such as a beam transport magnet that will effectively block the motion of the electron clouds following the ions. In this paper, we will show examples of how electrons can be trapped and controlled by varying the conditions determining their emission and confinement. Ray tracing simulations using the EGN2 (Herrmannsfeldt, SLAC Electron Trajectory Program, SLAC-331 (1988)) computer code will be used to model the electron trajectories in the presence of a high current heavy ion beam. The self-magnetic field of the ion beam, while not sufficient to affect the ions themselves significantly, has a strong influence on the electron orbits.

Magnet and cryostat configurations for a multi-port quadrupole array

This report describes the results of a study of arrays of up to sixteen quadrupoles in a single cryostat surrounded by an induction accelerator that is used for accelerating high current heavy ion beams for fusion. Each quadrupole in the array can have a gradient of 72 T/m, when the quadrupole has a warm bore diameter of 90 mm. An array of sixteen quadrupoles can be made to fit into a round cryostat vacuum vessel with a diameter of 850 mm. If the number of quadrupoles in the array is reduced to nine, the outer diameter of the cryostat is 700 mm. It is proposed that the quadrupole array be conduction cooled using either a 4 K cryocooler or two phase liquid helium in pipes around the magnet array. The two-phase helium can be supplied to a string of multi-bore quadrupoles using a large refrigerator.

Focusing of heavy ion beams by a high-current plasma lens

Results are presented from studies of the focusing of wide-aperture low-energy (100–400 eV) and moderate-energy (5–25 keV) beams of heavy-metal ions by a high-current electrostatic plasma lens. It is found experimentally that, because of the significant electron losses, the efficient focusing of such beams can be achieved only if the external potentials at the plasma-lens electrodes are maintained constant. Static and dynamic characteristics of the lens are studied under these conditions. It is shown that, as the beam current and the electrode voltage increase, the maximum electrostatic field in the lens tends to a certain limiting value because of the increase in the spatial potential near the lens axis. The role of spherical and moment aberrations in the focusing of wide-aperture low-divergence ion beams is revealed. It is shown that, even when spherical aberrations are minimized, unremovable moment aberrations decrease the maximum compression ratio of a low-energy heavy-ion beam because of the charge separation of multiply charged ions in the focal region. At the same time, as the ion energy increases, the role of the moment aberrations decreases and the focusing of high-current heavy-ion beams by a plasma lens becomes more efficient than the focusing of light-ion (hydrogen) beams. This opens up the possibility of using electrostatic plasma lenses to control ion beams in high-dose ion implanters and high-current accelerators of heavy ions.

Development of a high-current plasma lens for focusing broad beams of heavy metal ions

We describe the results of investigations of the manipulation of moderate energy, large area beams of heavy metal ions by a high-current electrostatic plasma lens. Electrostatic plasma lenses are essential for the focusing of high-current heavy ion beams with moderate energies of 10–100 keV. In our experiments, beams of carbon, copper, zinc, and tantalum (separately) were formed by a repetitively pulsed vacuum arc ion source with energy in the range 10–50 keV, beam current up to 0.5 A, and initial diameter 10 cm. The characteristics of the focusing of the ion beam passing through the lens were measured by a radially movable, magnetically suppressed Faraday cup. The plasma lens focusing properties were determined for a number of different distributions of the lens ring-electrode potentials. We have shown that by changing the lens electrode potential distribution we can control the lens focusing, in both the convergent and divergent regimes. Some features of heavy metal ion beam focusing under these conditions are discussed. The experiments demonstrate the versatile possibilities of the plasma lens for use with moderate-energy, high-current, heavy ion beams.