G. C. Barber

Quasi-steady-state multimegawatt ion source for neutral beam injection

A quasi‐steady‐state (pulse duration of 30 s) ion source of the duoPIGatron type has been developed for fusion applications. It was designed to deliver an 80‐keV hydrogen ion beam of low beamlet divergence (Θrms= 0.26°) at a current density of 0.19 A cm−2. Hydrogen ion beams of 40 to 48 A were extracted at beam energies of 77 to 80 keV for 30‐s‐long pulses. The reliability and stability of the ion source operation were demonstrated by extracting about 600 beam pulses at full power and full pulse length. The ion source was also operated with deuterium as the working gas, and the optimum current at 80 keV was found to be about 33 A, in agreement with the expected inverse square‐root scaling of current density with atomic mass.

Eight‐shot pneumatic pellet injection system for the tokamak fusion test reactor

An eight‐shot pneumatic pellet injection system has been developed for plasma fueling of the tokamak fusion test reactor (TFTR). The active cryogenic mechanisms consist of a solid hydrogen extruder and a rotating pellet wheel that are cooled by flowing liquid‐helium refrigerant. The extruder provides solid hydrogen for stepwise loading of eight holes located circumferentially around the pellet wheel. This design allows for three different pellet diameters: 3.0 mm (three pellets), 3.5 mm (three pellets), and 4.0 mm (two pellets) in the present configuration. Each of the eight pellets can be shot independently. Deuterium pellets are accelerated in 1.0‐m‐long gun barrels with compressed hydrogen gas (at pressures from 70 to 105 bar) to velocities in the range 1.0–1.5 km/s. The pellets are transported to the plasma in an injection line that incorporates two stages of guide tubes with intermediate vacuum pumping stations. A remote, stand‐alone control and data‐acquisition system is used for injector and vacuum...

Properties of an intense 50‐kV neutral‐beam injection system

The properties of an intense 50‐kV neutral‐beam system are discussed. The salient features of this system are a transmission efficiency of 76% of the extracted ion beam through a 30×34 cm aperture that is 4.5 m from the ion source, a transmitted neutral power of 1.8 MW H0 (2.0 MW D0) at extraction parameters of 50 kV/100 A/0.1 s (53 kV/85 A/0.1 s), a proton fraction of ∼80%, an ion‐source arc efficiency of ∼1.3 A/kW, an ion‐source gas efficiency of ∼35%, and a reliability of ≳90%.

Power transmission characteristics of a two‐stage multiaperture neutral beam source

Beam power transmission and grid loading characteristics of a two‐stage neutral beam source are presented. The dependence of power deposition on the target, the grids, and the gas cell was studied over a wide range of extraction perveance values with the accel‐to‐extraction gap field ratio as the other parameter. The results show that the power transmission improves remarkably with increasing field ratio. For sufficiently large field ratios (≈2.5), more than 80% of the input IV power was collected on a target located 4 m downstream and subtending 2 ° half angle to the source. The sum of the grid loading is approximately double that of single‐stage accelerators; the plasma grid loading is the highest, followed by ground grid, accel grid, and extraction grid in that order.

Positive-ion recovery scheme based on magnetic blocking of electrons

A method is described for making positive‐ion‐based neutral‐beam injection viable at energies of ≲100 keV per nucleon by recovering the energy of residual charged particles as electrical energy. The concept of transverse magnetic field blocking of electrons has been shown to be successful, and preliminary experimental results are presented.

Ion optics improvements to a multiple aperture ion source

Experimental comparison is made of four plasma grids, each with a specific aperture geometry, in an attempt to improve the ion optics of a multiple aperture ion source. It is clearly shown that a simple notch geometry outperforms the other candidates with a high transmission efficiency (∼68%) to a 2° target at high perveance (∼9.6 μperv).

ISX-B neutral beam injector experiment on a prototype beam line

Two PLT-injector-type duoPIGatron sources, modified further by shaping the beam-forming apertures, have been tested and experimented on a prototype beam line similar to the ISX-B neutral beam injection system. The accelerator column modification has resulted in an increase of the beam power transmission efficiency from that of the straight-bore aperture by 50%. Maximum neutral beam powers achieved on a 28-cm-diam target simulating the ISX-B plasma, located 4.1 m downstream from the source, are congruent to 910 kW of H/sup 0/ at an accelerator power of 42 kV and 61 A and congruent to 1020 kW of D/sup 0/ at 43 kV and 55 A. Measurements have been made to investigate the following: the effects on beam optics of aperture shape, aspect ratio, and different ions (H/sup +/ or D/sup +/); the distribution of beam power deposition along the beam line; ion species compositions; and background pressure behavior due to scraped-off beam particles.

Discharge characteristics of a plasma generator for sitex and vitex ion sources

Surface ionization with Transverse Extraction (SITEX) and Volume Ionization with Transverse Extraction (VITEX) ion sources are being developed to produce intense beams of light negative ions for neutral particle beam applications. The salient feature of these ion sources is their ability to form intense negative‐ion beams. With the objective of improving the performance of these sources, an experimental study of their plasma properties has been conducted. The effects of various electrodes in the plasma generator were investigated. Low electron and ion temperatures (below 1 eV) and positive plasma potential up to +6V have been measured. The measured distributions of plasma density and potential reveal the existence of multichamber characteristics in the source plasma. The significant discharge characteristics and the plasma properties associated with the performance of SITEX and VITEX ion sources are discussed.

Short‐pulse operation with the SITEX negative ion source

The successful high current, long‐pulse SITEX source experiments with H− beams have been extended to D− operation and to short‐pulse H− and D− beam acceleration. Extracted D− beam current densities of 100 mA/cm2 at the plasma grid have been achieved for 3 s at a total beam of 260 mA and 10 keV. The extracted electron‐to‐D− ratio is <5% with ≊100% of the extracted electrons collected at 2 keV. Short‐pulse beams down to 10 ms in length have been accelerated with the minimum pulse length determined by the power supply time constants. Optics of the accelerated beam is θ(1/e)⊥=±0.48°, which corresponds to a normalized emittance perpendicular to the slot of 0.0007π cm‐mrad. These data place an upper limit on the surface conversion D− ion temperature of 0.7 eV.

The ORNL plasma fueling program

Advanced plasma fueling systems for magnetic confinement devices are under development at the Oak Ridge National Laboratory (ORNL). The general approach is to produce and accelerate frozen hydrogen-isotope pellets at speeds ranging from 1 to 2 km/s and higher. Recently, ORNL provided pneumatic-based pellet fueling systems for two of the worlds largest tokamak experiments, the Tokamak Fusion Test Reactor (TFTR) and the Joint European Torus (JET). A new, versatile, centrifuge-type injector is being installed on the Tore Supra tokamak. Also, a new, simplified, eight-shot injector has been developed, and injectors based on this design are operating on the Princeton Beta Experiment (PBX) and the ORNL Advanced Toroidal Facility (ATF). In addition to these confinement-physics-related activities, ORNL is pursuing advanced technologies to achieve pellet velocities significantly in excess of 2 km/s, and has carried out a tritium proof of principle experiment in which the fabrication and acceleration of tritium pellets were demonstrated. These ongoing activities are described.<<ETX>>