RF plasma source for heavy ion beam charge neutralization

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

Use of optical emission intensity to characterize an RF plasma source for MBE growth of GaAsN

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.

The properties of a new plasma source, i.e. a large-scale modified-magnetron typed (MMT) radio frequency (RF) plasma source, are investigated for producing a uniform high-density plasma with a large diameter of the order of 1 m. A plasma with uniformity within a few percent is produced over 120 cm in diameter in front of the substrate under an argon pressure of 1 mTorr. The plasma density is 6×1010 cm-3 at the axial center of the RF electrode when the incident RF power is 2.8 kW. The plasma density is almost proportional to the RF power. Since the structure of MMT plasma source is quite simple, it is useful for large area plasma processing.

Gallium nitride layers were grown by plasma-assisted molecular-beam epitaxy on (0001)-oriented sapphire substrates using an electron cyclotron resonance (ECR) and a radio frequency (rf) plasma source. An applied substrate bias was varied from -200 to +250 V, resulting in a change of the density and energy of nitrogen ions impinging the growth surface. The layers were investigated by high-resolution x-ray diffractometry and high-resolution transmission electron microscopy (HRTEM). Applying a negative bias during growth has a marked detrimental effect on the crystal perfection of the layers grown with an ECR plasma source. This is indicated by a change in shape and width of (0002) and (202\ifmmode\bar\else\textasciimacron\fi{}5) reciprocal lattice points as monitored by triple axis x-ray measurements. In HRTEM images, isolated basal plane stacking faults were found, which probably result from precipitation of interstitial atoms. The crystal damage in layers grown with a highly negative substrate bias is comparable to that observed for ion implantation processes at orders of magnitude larger ion energies. This is attributed to the impact of ions on the growing surface. None of the described phenomena was observed for the samples grown with the rf plasma source.

This experiment investigates a novel technique of hydrogen production by dissociating water molecules in a radio-frequency (rf) plasma. This plasma source has a helicon antenna, capable of operating in capacitive, inductive, or helicon mode when operating conditions match those required to excite these modes. Hydrogen is produced by injecting water vapor into the plasma source. The species identified in the plasma from data obtained via a residual gas analyze are hydrogen, oxygen, water, hydroxyl, and nitrogen. Partial pressures of these gases are also obtained from the residual gas analyzer. In this paper, we show that this plasma source is capable of dissociating water molecules into their constituent species when operating in inductive mode.

The space plasma has relatively low energy but is dense in the low Earth orbit (LEO). In this study, we prepared various samples of anodic alloy surfaces with coating thicknesses of 20, 25, 35, and 45 μm to identify the most suitable characteristics for space applications. Ground-based tests were conducted at the Laboratory of Lean Satellite Enterprises and In-Orbit Experiments at the Kyushu Institute of Technology. A radio frequency (RF) plasma source was used to generate a simulated LEO plasma using argon gas in a vacuum chamber. The plasma properties were measured with a Langmuir probe under different test conditions. A negatively biased voltage of –450 V was applied to the samples to study charging/discharging phenomena. The samples were exposed to Ar-plasma for 1–2 h. The physical properties and structural morphology of various alloy samples were analyzed before and after exposure to plasma. This analysis involved ultraviolet (UV)/visible (Vis)/ Near-Infrared (NIR) absorption spectra, Energy Dispersive X-ray (EDX) analysis, and surface roughness testing. The results showed that space plasma notably impacts the physical properties and morphology of the alloys. A coated thickness of around 25 μm is considered more suitable for spacecraft surface structures due to its improved optical stability and resistance to plasma degradation, as indicated by the experimental results.

Summary form only given. In recent years, carbon dioxide (CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) plasma discharges have gained considerable interest due to rising concerns of global climate change. The increasing dependence on fossil fuels to serve as the world's primary energy source has lead to nontrivial effects on the overall CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> content in the atmosphere. One possible solution to mitigating the effects of high atmospheric concentrations of CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is the use of a plasma source to break apart the molecule into carbon monoxide (CO) and oxygen. The CO can be filtered out for use in the chemical industry and as a fuel in industrial applications, or CO can be further dissociated to produce carbon and oxygen.This work experimentally investigates the dissociation of C02 and CO in a 1 kW radio frequency plasma source operating at 13.56 MHz in the low pressure regime of about 100-300 mTorr. Mass spectrometry, optical emission spectroscopy, and x-ray photoelectron spectroscopy diagnostics are used to determine the species present in the discharge. For plasmaassisted C02 dissociation to be a practical and cost-effective solution for the climate change challenges, the process must meet certain standards of energy efficiency. Therefore the energy efficiency of the radio frequency plasma source is examined for both C02 and CO dissociation, and is determined too low for large-scale industrial applications. Improvements must be made to enhance the energy deposition into specific modes of dissociation through changing the type of plasma source and/or adding a catalyst into the system to increase the rate of dissociative reactions.

Radio frequency (RF) plasma in water was used for the degradation of methylene blue. The fraction of decomposition of methylene blue and the intensity of the spectral line from OH radical increased with RF power. RF plasma in water also produced hydrogen peroxide. The density of hydrogen peroxide increased with RF power and exposure time. When pure water (300 mL) is exposed to plasma at 310 W for 15 min, density of hydrogen peroxide reaches to 120 mg/L. Methylene blue after exposed to plasma degraded gradually for three weeks. This degradation may be due to chemical processes via hydrogen peroxide and tungsten. The comparison between the experimental and calculated spectral lines of OH radical (A–X) shows that the temperature of the radical is around 3,500 K. Electron density is evaluated to be ≃3.5 × 1020 m−3 from the stark broadening of the Hβ line.

Long plasma source for heavy ion beam charge neutralization

Keeping compatibility between steady state gas puffing and stable radio frequency (RF) discharge, helium recombining plasma production was achieved in an RF plasma device. In this experiment, axial position of orifice, which suppresses backflow of secondary gas, was modified to increase electron density at a test region. Changing neutral pressure at the test region from 11 Pa to 21 Pa, the electron temperature, the electron density and the wavelength spectrum were measured. The electron temperature decreased with increasing neutral pressure and finally becomes about 3 eV. The electron density shows similar pressure dependence as the electron temperature. When the neutral pressure increases to 15 Pa, the line spectra from highly excited helium atoms were clearly observed. The electron temperature estimated from these line spectral intensities is about 0.05 eV, which indicates that the electron density reduction is caused by volumetric recombination occurring at the periphery of the plasma column.

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.

The radio frequency (RF) powered source is the most stable and capable analytical source available for glow discharge optical emission spectroscopy (GD-OES). It allows the direct analysis of both conductive and non-conductive samples on the same source, without changing the source. Recent developments in RF generators and components have further improved the performance of these sources, and now allow the routine measurement of additional RF parameters such as applied voltage and DC bias voltage. Theoretical studies of the fundamental characteristics of the RF source are providing deeper understanding of how they work, and combined with the new measurement tools will lead to further improvements in analytical performance. Recent theoretical and experimental work on the RF plasma source will be presented. These studies will be illustrated with some recent applications: in metals analysis, plus a hard coating and a non-conductive coating.