Development of a 1-m plasma source for heavy ion beam charge neutralization

Highly ionized plasmas are being used as a medium for charge neutralizing heavy ion beams in order to focus the ion beam to a small spot size. A radio frequency (RF) plasma source has been built at the Princeton Plasma Physics Laboratory (PPPL) in support of the joint Neutralized Transport Experiment (NTX) at the Lawrence Berkeley National Laboratory (LBNL) to study ion beam neutralization with plasma. The goal is to operate the source at pressures /spl sim/10/sup -5/ Torr at full ionization. The initial operation of the source has been at pressures of 10/sup -4/-10/sup -5/ Torr and electron densities in the range of 10/sup 8/-10/sup 11/ cm/sup -3/. Recently, pulsed operation of the source has enabled operation at pressures in the 10/sup -6/ Torr range with densities of 10/sup 11/ cm/sup -3/. Near 100% ionization has been achieved. The source has been integrated with the NTX facility and experiments have begun.

Long plasma source for heavy ion beam charge neutralization

Summary form only given, as follows. The transport of space charge dominated particle beams is of interest to many applications. The most challenging demands are found in the field of ion beam driven inertial confinement fusion. One of the possible solutions for final ion beam focusing and transport through the reactor chamber is discharge channel transport. This technique has the advantage of providing complete space charge and current neutralization of the ion beam. To be applicable to an ion beam driven fusion reactor, the final transport system has to avoid destructible structures in the reactor chamber. This is the primary reason to study laser initiated discharge channels. One important feature of final focusing and transport based on a plasma discharge is the large acceptance in the transverse phase space, which allows the transmission of beams with different initial angles to the channel axis. This provides the possibility to merge several individual beams in a neutralizing environment, reducing the number of beam entrances into the reactor chamber. A further advantage of a plasma based system is the large acceptance in the longitudinal phase space. This will allow to use simultaneously beams with different energy to drive the target, as assumed in different target designs for heavy ion beam fusion. Therefore plasma based focusing and transport relaxes the requirements for the accelerator significantly. Motivated by these advantages, neutralized beam transport and focusing in gas discharge plasmas is under investigation for heavy ion beam fusion at the Lawrence Berkeley National Laboratory (LBNL) and at GSI Darmstadt. The program is outlined and some experimental results of plasma diagnostics on the laser initiated discharge channel and of heavy ion beam transport experiments in these channels are discussed.

This article provides an overview of current U.S. research on accelerators for Heavy Ion Fusion, that is, inertial fusion driven by intense beams of heavy ions with the goal of energy production. The concept, beam requirements, approach, and major issues are introduced. An overview of a number of new experiments is presented. These include: the High Current Experiment now underway at Lawrence Berkeley National Laboratory; studies of advanced injectors (and in particular an approach based on the merging of multiple beamlets), being investigated experimentally at Lawrence Livermore National Laboratory); the Neutralized (chamber) Transport Experiment being assembled at Lawrence Berkeley National Laboratory; and smaller experiments at the University of Maryland and at Princeton Plasma Physics Laboratory. The comprehensive program of beam simulations and theory is outlined. Finally, prospects and plans for further development of this promising approach to fusion energy are discussed.

A nonperturbing electron-beam diagnostic system for measuring the charge distribution of an ion beam is developed for heavy ion fusion beam physics studies. Conventional diagnostics require temporary insertion of sensors into the beam, but such diagnostics stop the beam, or significantly alter its properties. In this diagnostic a low energy, low current electron beam is swept transversely across the ion beam; the measured electron-beam deflection is used to infer the charge density profile of the ion beam. The initial application of this diagnostic is to the neutralized transport experiment (NTX), which is exploring the physics of space-charge-dominated beam focusing onto a small spot using a neutralizing plasma. Design and development of this diagnostic and performance with the NTX ion beamline is presented.

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.

A drift chamber telescope for heavy ion track detection

Stable plasma production in a radio-frequency plasma source and steady-state high-neutral-pressure environment in a test region has been achieved simultaneously with backflow suppression components such as orifices and a differential pumping system. Neutral pressure in the test region is changed by two orders, resulting in changes in the electron temperature and density in the test region. Conversely, neutral pressure, electron temperature, and electron density near a plasma production region showed little change. The result indicates that the experimental device provides us independent control knobs of source plasma and neutral pressure in the test region.

Negative hydrogen ion beam can be compensated by the trapping of ions into the beam potential. When the beam propagates through a neutral gas, these ions arise due to gas ionization by the beam ions. However, the high neutral gas pressure may cause serious negative hydrogen ion beam loss, while low neutral gas pressure may lead to ion-ion instability and decompensation. To better understand the space charge compensation processes within a negative hydrogen beam, experimental study and numerical simulation were carried out at Peking University (PKU). The simulation code for negative hydrogen ion beam is improved from a 2D particle-in-cell-Monte Carlo collision code which has been successfully applied to H(+) beam compensated with Ar gas. Impacts among ions, electrons, and neutral gases in negative hydrogen beam compensation processes are carefully treated. The results of the beam simulations were compared with current and emittance measurements of an H(-) beam from a 2.45 GHz microwave driven H(-) ion source in PKU. Compensation gas was injected directly into the beam transport region to modify the space charge compensation degree. The experimental results were in good agreement with the simulation results.

Here is a brief review some new results of development novel generation cylindrical plasma devices based on an idea of magnetic insulation electrons and an electrostatic plasma lens configuration. The axial-symmetric plasma lens is a well-explored device for manipulating high current, heavy ion beams, that has been used to preference to focus wide-aperture, heavy metal ion beams such as produced by vacuum arc ion sources (like MEVVA ion sources). The plasma lens configuration of crossed electric and magnetic fields provides an attractive and suitable method for establishing a stable plasma discharge at low pressure. Use of plasma lens configuration in this way was elaborated, investigated and developed some cost efficiency, low maintenance plasma devices based on this approach, for ion treatment and deposition of exotic coatings with given functional properties. These devices make using of permanent magnets and possess considerable flexibility with respect to spatial configuration. We describe the new results of theoretical calculations and experimental investigations physical mechanisms, determining optimal operation conditions cylindrical plasma devices based on plasma lens configuration.

In order to produce large-diameter uniform plasmas, we have proposed two plasma sources: a plane electron-cyclotron-resonance (ECR) slotted antenna (PESA) source for ECR plasma production and a modified magnetron-type (MMT) radio frequency (RF) source for RF plasma production. The PESA has magnet rings behind the slotted antenna to provide the magnetic field for the ECR. The MMT RF source provides high-density uniform plasmas with the help of the LC resonance of auxiliary electrode placed parallel to the substrate. Both plasma sources produce large-diameter uniform plasmas exceeding 30 cm in diameter. We have proposed a grid method for electron temperature control. By applying a negative dc voltage to the grid which separates a plasma-processing region from a plasma production region, the electron temperature is decreased by one order of magnitude in the processing region. The same effect is obtained by changing the mesh size of a floating grid. The change of a slit between two grids also causes variation of the electron temperature. These techniques are quite effective for chemical reaction control in processing plasmas. © 1999 Scripta Technica, Electr Eng Jpn, 130(1): 1–7, 2000

In heavy ion inertial confinement fusion systems, intense beams of ions must be transported from the exit of the final focus magnet system through the target chamber to hit millimeter spot sizes on the target. Effective plasma neutralization of intense ion beams through the target chamber is essential for the viability of an economically competitive heavy ion fusion power plant. The physics of neutralized drift has been studied extensively with PIC simulations. To provide quantitative comparisons of theoretical predictions with experiment, the Heavy Ion Fusion Virtual National Laboratory has completed the construction and has begun experimentation with the NTX (Neutralized Transport Experiment) as shown in Figure 1. The experiment consists of 3 phases, each with physics issues of its own. Phase 1 is designed to generate a very high brightness potassium beam with variable perveance, using a beam aperturing technique. Phase 2 consists of magnetic transport through four pulsed quadrupoles. Here, beam tuning as well as the effects of phase space dilution through higher order nonlinear fields must be understood. In Phase 3, a converging ion beam at the exit of the magnetic section is transported through a drift section with plasma sources for beam neutralization, and the final spot size is measured under various conditions of neutralization. In this paper, we present first results from all 3 phases of the experiment.

A new luminescence beam profile monitor is realized in the polarized hydrogen gas jet target at the Relativistic Heavy Ion Collider (RHIC) facility. In addition to the spin polarization of the proton beam being routinely measured by the hydrogen gas jet, the luminescence produced by beam-hydrogen excitation leads to a strong Balmer series lines emission. A selected hydrogen Balmer line is spectrally filtered and imaged to produce the transverse RHIC proton beam shape with unprecedented details on the RHIC beam profile. Alternatively, when the passage of the high energy RHIC gold ion beam excited only the residual gas molecules in the beam path, sufficient ion beam induced luminescence is produced and the transverse gold ion beam profile is obtained. The measured transverse beam sizes and the calculated emittances provide an independent confirmation of the RHIC beam characteristics and to verify the emittance conservation along the RHIC accelerator. This optical beam diagnostic technique by making use of the beam induced fluorescence from injected or residual gas offers a truly noninvasive particle beam characterization, and provides a visual observation of proton and heavy ion beams. Combined with a longitudinal bunch measurement system, a 3-dimensional spatial particle beam profile can be reconstructed tomographically.

Ferroelectric Plasma Sources (FEPSs) can generate plasma that provides effective space-charge neutralization of intense high-perveance ion beams, as has been demonstrated on the Neutralized Drift Compression Experiment NDCX-I and NDCX-II. This article presents experimental results on charge neutralization of a high-perveance 38 keV Ar+ beam by a plasma produced in a FEPS discharge. By comparing the measured beam radius with the envelope model for space-charge expansion, it is shown that a charge neutralization fraction of 98% is attainable with sufficiently dense FEPS plasma. The transverse electrostatic potential of the ion beam is reduced from 15 V before neutralization to 0.3 V, implying that the energy of the neutralizing electrons is below 0.3 eV. Measurements of the time-evolution of beam radius show that near-complete charge neutralization is established ∼5 μs after the driving pulse is applied to the FEPS and can last for 35 μs. It is argued that the duration of neutralization is much longer than a reasonable lifetime of the plasma produced in the sub-μs surface discharge. Measurements of current flow in the driving circuit of the FEPS show the existence of electron emission into vacuum, which lasts for tens of μs after the high voltage pulse is applied. It is argued that the beam is neutralized by the plasma produced by this process and not by a surface discharge plasma that is produced at the instant the high-voltage pulse is applied.

In heavy-ion inertial-confinement fusion systems, intense beams of ions must be transported from the exit of the final focus magnet system through the fusion chamber to hit millimeter-sized spots on the target. Effective plasma neutralization of intense ion beams in this final transport is essential for a heavy-ion fusion power plant to be economically competitive. The physics of neutralized drift has been studied extensively with particle-in-cell simulations. To provide quantitative comparisons of theoretical predictions with experiment, the Virtual National Laboratory for Heavy Ion Fusion has completed the construction and has begun experimentation with the Neutralized Transport Experiment (NTX). The experiment consists of three main sections, each with its own physics issues. The injector is designed to generate a very high-brightness, space-charge-dominated potassium beam while still allowing variable perveance by a beam aperturing technique. The magnetic-focusing section, consisting of four pulsed magnetic quadrupoles, permits the study of beam tuning, as well as the effects of phase space dilution due to higher-order nonlinear fields. In the final section, the converging ion beam exiting the magnetic section is transported through a drift region with plasma sources for beam neutralization, and the final spot size is measured under various conditions of neutralization. In this paper, we discuss the design and characterization of the three sections in detail and present initial results from the experiment.