M. M. Menon

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.

Properties of an intense 40-kV neutral beam injector.

The properties of an intense neutral beam injector, the modified duoPIGatron ion source, are discussed and compared with other injectors. For this source (a) beam composition for hydrogen is approximately (85+/-5) % monatomic, (b) nucleon gas efficiency is 50%, (c) the electrical efficiency of ion generation is 1.1 A/kW, and (d) up to 52% of the input power is delivered in the ion and neutral beam to a target subtending a half angle of 1.8 degrees x1.4 degrees .

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.

Ion beamlet steering by aperture displacement for a tetrode accelerating structure

Steering of a single beamlet of ions in a tetrode accelerating structure as a function of both aperture displacement and the ratio of acceleration to extraction voltage (Gamma) has been measured, and the results are compared to the steering predicted by a modified linear theory. Although the data contain the predicted features of (1) null steering for a particular value of Gamma and (2) a directional change in steering through this point, the steering was also observed to be nonlinear in nature. A conjecture as to the cause of this nonlinearity is given, and the implications of these findings to beamlet steering in a multibeamlet source are discussed.

Geometrical effects on the beamlet optics of a two‐stage ion accelerator

The effects of geometrical parameters (aspect ratio, aperture size, gaps) and electrical parameters (voltage distribution, beam energy) on single‐beamlet profiles are investigated theoretically and experimentally on a two‐stage ion accelerator over both a broad range of beam conditions and a wide range of angles (0 to ±11.5°). In general, the intensity distribution is non‐Gaussian. Under optimum conditions, divergence angles half‐width at half maximum less than 0.5° could be achieved even with large holes (0.9 mm). Shaping the plasma grid reduced the non‐Gaussian wings and improved the beam transmission efficiency.

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.

Experimental study of ion beam optics in a two‐stage accelerator

Hydrogen ion beam optics in a two-stage linear acceleration system is studied by examining the beam divergence as a function of the voltage and gap distribution, the beam perveance, the background gas pressure, the aspect ratio, and the total accelerating energy (60-110 keV). The system consists of four electrodes with single, cylindrical, straight-bore apertures acting as an extraction-accel-decel column. An optimum relation between the field ratio and the extraction perveance is obtained from measurements for the minimum beam divergence condition. The HWHM divergence angle is <0.3 degrees under optimum conditions. Qualitative agreement between the measurements and a previous theoretical study is noticed. A potential application of the results to high energy neutral beam injectors for fusion research is also discussed.

Long-pulse ion source for neutral-beam applications

A rectangular ion source is being developed for producing 120‐keV/25‐A hydrogen ion beams for pulse durations up to 10 s. It consists of a plasma generator with a rectangular arc chamber (25×35 cm cross section) and an ion accelerator with rectangular grids (10×25 cm hole pattern). The plasma generator is a modified duoPIGatron type. It has been operated at 120 V, 1100 A, and 10 s arc durations to produce a dense and uniform plasma sufficient for supplying a 25‐A ion beam current. The electron emitter used is either a LaB6 hollow cathode or a LM (molybdenum doped with La2O3) indirectly heated cathode. The ion accelerator having four (or three) rectangular grids with multiple circular apertures has been utilized to form high‐energy ion beams above (or below) 80 keV. With substantial improvements in water cooling and mechanical stability, this ion accelerator has been operated reliably to deliver long‐pulse ion beams with energies in excess of 100 keV and pulse lengths of many seconds. The results of measur...