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The injection of neutral hydrogen or deuterium particles continues to be the most promising means of heating magnetically confined fusion plasmas to ignition temperatures. Neutral beam injection systems that employ positive ion sources presently operate at energies of about 40-50 keV/nucleon at 60 A [Princeton Large Torus (PLT)] or 100 A [Princeton Divertor Experiment (PDX) or the Oak Ridge National Laboratory (ORNL) Impurities Study Experiment (ISX)] with about 60% conversion efficiency. However, the desire for multisecond beams in the 80-keV/nucleon energy range at ~ 10 MW/module has emphasized the need for technological advances in several areas. At such beam energies, as much as 75% of the initial beam energy is retained in the unneutralized ion components. As a result, two questions immediately come to mind: (1) how can one dispose of this energy; or better still, (2) how can one efficiently recover this energy? The conventional way of treating such a problem is to deflect the ions out of the neutral beam and onto water-cooled plates or beam dumps. This method has worked satisfactorily for 40-keV/nucleon beams in excess of 1.5 MW and ~0.5 s. However, the power per unit area to be disposed of in the high power, multisecond beams mentioned above is beyond present-day technology.

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.

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%.

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.

We have developed a prototype high‐velocity pneumatic pellet injector that uses hydrogen plasma propellant generated in a high‐current arc discharge. A single‐barrel pneumatic pellet gun has been fitted with a cylindrical arc chamber interposed between the hydrogen propellant inlet valve and the gun breech. The chamber incorporates a ceramic insert for generating vortex flow in the incoming gas stream, which provides azimuthal arc stabilization. The arc is initiated after the propellant valve opens and the breech pressure starts to rise; a typical discharge lasts 150–300 μs with peak currents up to 2 kA. The gun has been operated with 4‐mm‐diam, 6‐ to 11‐mm‐long deuterium and hydrogen pellets. At 100‐bar plenum pressure (hydrogen propellant), the arc characteristics are 〈V〉=350–800 V, 〈I〉=600 A, so that 60–150 J of electrical power is dissipated. Pellet speeds increase by 300 to 600 m/s depending on the projectile mass, which typically represents a 10‐J increment in the pellet kinetic energy. Velocities u...

Multimegawatt neutral beams of hydrogen or deuterium atoms are needed for fusion machine applications such as MFTB-B, TFTR-U, DIII-U, and FED (INTOR or ETR). For these applications, a duoPIGatron ion source is being developed to produce high-brightness deuterium beams at a beam energy of approx. 120 keV for pulse lengths up to 30 s. A long-pulse plasma generator with active water cooling has been operated at an arc level of 1200 A with 30-s pulse durations. The plasma density and uniformity are sufficient for supplying a 60-A beam of hydrogen ions to a 13- by 43-cm accelerator. A 10- by 25-cm tetrode accelerator has been operated to form 120-keV hydrogen ion beams. Using the two-dimensional (2-D) ion extraction code developed at Oak Ridge National Laboratory (ORNL), a 13- by 43-cm tetrode accelerator has been designed and is being fabricated. The aperture shapes of accelerator grids are optimized for 120-keV beam energy.

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.

A proof-of-principle experiment on direct recovery of residual hydrogen ions based on a magnetic electron suppression scheme is described. Ions extracted from a source plasma a few kilovolts above the ground potential (approx. 20 A) are accelerated to 40 keV by a negative potential maintained on a neutralizer gas cell. As the residual ions exit the gas cell, they are deflected from the neutral beam by a magnetic field that also suppresses gas cell electrons and then recovered on a ground-potential surface. Under optimum conditions, a recovery efficiency (the ratio of the net recovered current to the available full-energy ion current) of 80% +- 20% has been obtained. Magnetic suppression of the beam plasma electrons was rather easily achieved; however, handling the fractional-energy ions originating from molecular species (H/sub 2//sup +/ and H/sub 3//sup +/) proved to be extremely important to recovery efficiency.

This paper describes the development of a long-pulse positive ion source that has been designed to provide high brightness deuterium beams (divergence ≈ 0.25° rms, current density ≈ 0.15 A cm-2) of 40-45 A, at a beam energy of 80 keV, for pulse lengths up to 30 s. The design and construction of the ion source components are described with particular emphasis placed on the long-pulse cathode assembly and ion accelerator.

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