RF-driven Source Research Articles

High voltage power supply for steady state operation of the neutral beam test facility ELISE

The Neutral Beam Injection (NBI) system of ITER shall provide 33 MW heating power to the plasma for up to one hour by two injectors. The beam lines will be based on large RF-driven sources for negative hydrogen or deuterium ions. The test facility ELISE (Extraction from a Large Ion Source Experiment) at IPP Garching is supporting the ion source development for ITER with an ion source of 1 × 1 m2 (half the size of the ITER source). While plasma operation of ELISE is possible for up to one hour, the extraction and acceleration of a negative ion beam (up to 60 kV) was initially possible only for ≈10 s every ≈150 s with the old power supply. During such pulses the temporal behavior of the co-extracted electrons significantly depends on whether beam extraction is switched on or off. Thus, continuous wave (CW) extraction is mandatory for developing fully ITER relevant operational scenarios and recently, a steady state power supply for beam extraction has been installed at ELISE. This power supply consists of two stages connected in series: a first stage with 12 kV / 70 A, providing 35 A for the extracted negative ion current, with a 35 A overhead for co-extracted electrons. The second stage of 50 kV / 35 A is used for the further acceleration of only the ions to a total potential of 60 kV. The two stages have an electrical active power of 1.75 MW and 0.84 MW respectively. The power supply operates with pulse step modulation. Bipolar transistors with insulated gate electrodes (IGBT) are used as electrical switches on the DC side. The 50 kV stage consists of 64 modules in series and the 12 kV stage has 24 modules. Both stages are controlled synchronously and must detect breakdowns between the grids in a µs range.

Record productions establish RF-driven sources as the standard for generating high-duty-factor, high-current H- beams for accelerators (Winner of the ICIS 2017 Brightness Award).

The Spallation Neutron Source operates reliably at 1.2 MW and will gradually ramp to 1.4 MW. This paper briefly recalls some of the struggles when the unprecedented project was started and ramped to 1 MW over a 3½ year period. This was challenging, especially for the H- ion source and the low-energy beam transport system, which make up the H- injector. It took several more years to push the H- injector to the 1.4 MW requirements, and even longer to reach close to 100% injector availability. An additional breakthrough was the carefully staged, successful extension of the H- source service cycle so that disruptive source changes became rare events. More than 7 A·h of extracted H- ions have been demonstrated with a single source without maintenance, more than twice the single-source quantity of ions produced by any other high-current H- accelerator facility. Achieving the 1.4 MW requirements with close to 100% availability and record-breaking source service cycles were the basis for the 2017 Brightness Award.

Numerical Simulations of the Radio-Frequency–driven Toroidal Current in Tokamaks

Radio-frequency (rf) waves are a powerful tool for improving the performance and stability of tokamak plasmas through heating and current drive mechanisms, allowing current density profile control and steady-state operation. From first principles, and taking advantage from the ordering between the various time and space scales, fast and powerful numerical tools have been developed to calculate the rf-driven current. The current drive problem in tokamaks is first introduced with the purpose of maintaining a steady-state self-organized toroidal magnetohydrodynamic equilibrium, such that a minimal amount of the fusion power has to be recycled to control the plasma current. The strict criterion that characterizes a steady-state discharge is derived from the response of the tokamak, considered as a transformer, and of the plasma, when an external source of current is applied. The calculation of a rf-driven source of current requires solving self-consistently a set of equations describing the dynamics of wave fields and charged particles in an inhomogeneous magnetized plasma. The range of applicability of these equations is discussed, as well as numerical methods developed to solve them, such as the ray-tracing code C3PO and the three-dimensional linearized relativistic bounce-averaged electron Fokker-Planck solver LUKE. Simulations of current drive by lower-hybrid waves are presented to illustrate the applications of our numerical tools. Current drive modeling includes the effect of electron density fluctuations at the plasma edge, and the case of electron cyclotron waves used for stabilization of the 3/2 neoclassical tearing modes in ITER is studied in detail. Finally, ongoing developments, including cross effects between momentum and configuration spaces, aiming at improving current drive calculations are discussed.

Negative ion source development for fusion application (invited)

Giant negative ion sources, producing high-current of several tens amps with high energy of several hundreds keV to 1 MeV, are required for a neutral beam injector (NBI) in a fusion device. The giant negative ion sources are cesium-seeded plasma sources, in which the negative ions are produced on the cesium-covered surface. Their characteristic features are discussed with the views of large-volume plasma production, large-area beam acceleration, and high-voltage dc holding. The international thermonuclear experimental reactor NBI employs a 1 MeV-40 A of deuterium negative ion source, and intensive development programs for the rf-driven source plasma production and the multistage electrostatic acceleration are in progress, including the long pulse operation for 3600 s. Present status of the development, as well as the achievements of the giant negative ion sources in the working injectors, is also summarized.

Rf-driven ion sources for industrial applications (invited) (abstract)

The Plasma and Ion Source Technology Group at the Lawrence Berkeley National Laboratory have been developing rf-driven ion sources for the last two decades. These sources are being used to generate both positive and negative ion beams. Some of these sources are operating in particle accelerators such as the Spallation Neutron Source (SNS) at Oak Ridge, while others are being employed in various industrial ion beam systems. There are four areas where the rf-driven ion sources are commonly used in industry. (1) In semiconductor manufacturing, rf-driven sources have found important applications in plasma etching, ion beam implantation, and ion beam lithography. (2) In material analysis and surface modification, miniature rf-ion sources can be found in focused ion beam systems. They can provide ion beams of essentially any element in the Periodic Table. The newly developed combined rf ion-electron beam unit improves greatly the performance of the secondary ion mass spectrometry tool. (3) For neutron production, rf ion source is a major component of compact, high flux D-D, D-T, or T-T neutron generators. These neutron sources are now being employed in boron neutron capture therapy (BNCT) as well as in neutron imaging and material interrogation. (4) Large area rf-driven ion source will be used in an industrial design neutral beam diagnostic system for probing fusion plasmas. Such sources can be easily scaled to provide large ion beam current for future fusion reactor applications.

Ion energy spread and current measurements of the rf-driven multicusp ion source

Axial energy spread and useful beam current of positive ion beams have been carried out using a radio frequency (rf)-driven multicusp ion source. Operating the source with a 13.56 MHz induction discharge, the axial energy spread is found to be approximately 3.2 eV. The extractable beam current of the rf-driven source is found to be comparable to that of filament-discharge sources. With a 0.6 mm diameter extraction aperture, a positive hydrogen ion beam current density of 80 mA/cm2 can be obtained at a rf input power of 2.5 kW. The expected source lifetime is much longer than that of filament discharges.

Performance characterization of rf-driven multicusp ion sources

Radio-frequency (rf)-driven multicusp ion sources have been developed extensively at Lawrence Berkeley National Laboratory (LBNL) for many applications, each requiring specific source designs. These uses have ranged from large ion sources for neutral-beam injectors—several tens of centimeters in size—to small sources for oil-well logging neutron tubes—a few centimeters in diameter. The advantages associated with internal antenna, rf-driven ion sources include reliability, long component life, ease of operation, and the ability to generate plasmas free of the impurities commonly found in hot-filament discharge sources. We have investigated and characterized the performance of rf-driven sources with respect to the rf operating frequency and ion source size for hydrogen ion species and current density. Furthermore, we have included in this study the aspects of proper coupling of the rf generator to the antenna through an impedance matching network. Finally, critical issues pertaining to general rf operation including beam extraction, rf shielding, and cooling of transformer cores are discussed.

Low-energy injector design for SSC

An injector is being designed for SSC that will deliver 30 mA of H− at 35 keV, matched to the RFQ input. The H− ions are produced by a new rf-driven multicusp source that uses no cesium. The ions are extracted by a high-perveance accelerator designed to handle the large electron content of this type of source. The low-energy beam transport system (LEBT) uses electrostatic focusing; gas-neutralized magnetic focusing is ruled out because pulse lengths (7–35 μs) are shorter than plasma buildup times and because pulse intervals are longer than plasma decay times. Plasma buildup is prevented by the LEBT electric fields, so the system is noise free and stable. The advantages of electrostatic LEBTs were demonstrated by a previous design tested at LBL, which showed negligible emittance growth, confirming the results of computer modeling. The object of our new LEBT design is to preserve the emittance of the rf-driven source, which is less than 0.01 π mrad cm (normalized rms), and thereby produce a beam well within the SSC RFQ acceptance of 0.018 π mrad cm.