Solid Hydrogen Pellet Research Articles

Linear stability analysis for the ablation flow near a cylindrical pellet in a plasma

The stability of a one-dimensional cylindrically symmetric quasi-steady ablation flow near a solid hydrogen pellet is investigated. The gas-dynamic conservation equations describing the flow are linearized in order to look for growing perturbations of the helical type. The problem is treated both analytically and numerically and comparisons are made between both approaches. The analysis reveals that the ablation process itself has no significant instabilities.

Pneumatic hydrogen pellet injection system for the ISX tokamak

We describe the design and operation of the solid hydrogen pellet injection system used in plasma refueling experiments on the ISX tokamak. The gun-type injector operates on the principle of gas dynamic acceleration of cold pellets confined laterally in a tube. The device is cooled by flowing liquid helium refrigerant, and pellets are formed in situ. Room temperature helium gas at moderate pressure is used as the propellant. The prototype device injected single hydrogen pellets into the tokamak discharge at a nominal 330 m/s. The tokamak plasma fuel content was observed to increase by (0.5-1.2) x10(19) particles subsequent to pellet injection. A simple modification to the existing design has extended the performance to 1000 m/s. At higher propellant operating pressures (28 bars), the muzzle velocity is 20% less than predicted by an idealized constant area expansion process.

Auto-irradiation effects in DT pellets

FUSION reactors based on magnetic containment and operating in a quasi steady-state will need to be refuelled during their burn period. The ballistic injection of solid pellets of hydrogen isotopes, first suggested in 19541, still seems to be the most promising method, and two satisfactory techniques for fabricating solid hydrogen pellets have been developed. In one, a continuous extrusion of solid hydrogen is thermally chopped into cylindrical pellets2. In the other, a jet of liquid hydrogen is acoustically broken into uniform droplets which are then frozen by reducing the pressure to below the triple point value3. Both techniques have worked well with deuterium, but fusion reactors will need DT pellets, and we consider here how the radioactivity of tritium will affect the situation. Two effects are apparent with characteristic times which may be short enough to be of practical significance: thermal stress arising from the deposition of decay heat throughout the pellet and electric stress caused by the charging of the pellet due to the escape of β particles from its surface. For a pellet of radius a, the thermal stress varies as a2 and the electric stress as a−2, which leads to both upper and lower size limits at radii where the effective stress approaches the yield stress of solid hydrogen. Fortunately the range of pellet radii envisaged in current reactor concepts (10−2–10−4 m) falls mid-way between the two limits.

Plasma density measurements on refuelling by solid hydrogen pellets in a rotating plasma

The authors used laser interferometry to directly measure the increase in plasma density caused by the ablation of a solid hydrogen pellet situated in a rotating plasma.

Effect of transonic flow in the ablation cloud on the lifetime of a solid hydrogen pellet in a plasma

A knowledge of solid hydrogen pellet lifetimes in a plasma is critical to the design of devices to refuel tokamak fusion reactors. When the pellet is injected into the plasma, the ablated material from the pellet undergoes a transonic flow since it is heated while it expands. Calculations are done on the behavior of the transonic flow for various plasma conditions and pellet sizes. From these calculations, the ablation rate and lifetimes of the pellet are determined. A scaling law is given which allows pellet lifetimes to be easily calculated for any plasma conditions. The results of these calculations give good agreement when compared with experiments.

A Revised Neutral Gas Shielding Model for Pellet-Plasma Interactions

A revised neutral molecule ablation model is derived to describe the evaporation of a solid hydrogen pellet in a tokamak plasma. The approach taken is based on the theory of Parks, Turnbull, and Foster who postulate that a cloud of molecular hydrogen surrounding the pellet shields the surface from incoming energetic electrons and, in so doing, regulates the evaporation rate. This treatment differs from the earlier model in that the hydrodynamic behavior of the molecular cloud is analyzed without invoking the assumption that the flow of material away from the surface is sonic everywhere. Numerical solutions of the fluid dynamic equations, which include the effects of strong electron heating locally in the gas, reveal that the flow of material away from the pellet is initially retarded relative to the solution of Parks et al., and then rapidly accelerated and rarified. This behavior is more pronounced for higher temperature plasmas and the net effect is that pellet life times might be prolonged slightly at the higher temperatures over those predicted by the approximate sonic flow model. A simple injection depth scaling law is derived and estimates of pellet fueling velocity requirements are made for several tokamaks.

Hydrogen pellet-rotating plasma interaction a spectroscopic analysis

Spectroscopic measurements on the interaction between solid hydrogen pellets and rotating plasmas are reported. It was found that the light emitted is specific to the pellet material, and that the velocity of the ablated H-atoms is of the order of 104 m/s. The investigation was carried out with a view to the possible refuelling of fusion reactors by the injection of pellets.

Hydrogen pellet-rotating plasma interaction. A spectroscopic analysis

Spectroscopic measurements on the interaction between solid hydrogen pellets and rotating plasmas are reported. It was found that the light emitted is specific to the pellet material, and that the velocity of the ablated H-atoms is of the order of 104 m/s. The investigation was carried out with a view to the possible refuelling of fusion reactors by the injection of pellets.

Solid hydrogen pellet injection into the Ormak tokamak

Solid hydrogen spheres were injected into the ORMAK tokamak as a test of pellet refuelling for tokamak fusion reactors. Pellets 70 μm and 210 μm in diameter were injected with speeds of 91 m/s and 100 m/s, respectively. Each of the 210-μm pellets added about 1% to the number of particles contained in the plasma. Excited neutrals, ablated from these hydrogen spheres, emitted light, which was monitored either by a photomultiplier or by a high-speed framing camera. From these light signals it was possible to measure pellet lifetimes, ablation rates, and the spatial distribution of hydrogen atoms in the ablation clouds. The average measured lifetime of the 70-μm pellets was 422 μs, and the 210-μm spheres lasted 880 μs under bombardment by the plasma. These lifetimes and measured ablation rates are in good agreement with a theoretical model which takes into account shielding of plasma electrons by hydrogen atoms ablated from spherical hydrogen ice.

Apparatus for producing uniform solid spheres of hydrogen

An apparatus has been constructed which produces uniform spheres of solid hydrogen for experiments testing the feasibility of using the solid hydrogen pellets for refueling fusion reactors. Two versions of the apparatus were developed, one which produced 70-μm-diam pellets at the rate of 105/sec and another which produced 210-μm pellets at the rate of 2.6×104/sec. The first step in the pellet production is to liquefy the hydrogen by flowing it through a liquid-helium-cooled heat exchanger. Then, a liquid hydrogen jet is formed by flowing the hydrogen through a nozzle. The jet is broken up into uniform drops by an acoustical excitation. The drops are frozen by evaporation in a pressure less than the triple point pressure of hydrogen. Finally, the drops are injected into vacuum for the experiments.

A model for the ablation rate of a solid hydrogen pellet in a plasma

It is shown that the ablation of a solid hydrogen pellet subject to a plasma is likely to produce a quasi-steady dense neutral gas cloud. The total integrated density of the cloud is such that the plasma electrons lose essentially all their energy in the cloud. The electron energy flux is degraded by inelastic collisions and elastic backscattering with the neutral molecules, providing local heating and acceleration of the neutral gas. Only a small fraction of the energy flux reaches the surface of the pellet, raising the pellet's surface temperature to a point where the energy flux at the pellet's surface is in balance with the energy lost through vaporization. The vaporization rate, in turn, determines the total integrated neutral gas cloud density. The scaling laws derived from the model indicate that the pellet lifetime varies as: where τp is the lifetime of the pellet and Te, ne, and rp0 are the electron temperature, density of the plasma, and initial pellet radius, respectively. A good agreement is found between this model and the ORMAK pellet injection experiment.

Ablation of hydrogen pellets in hydrogen and helium plasmas

Measurements on the interaction between solid hydrogen pellets and rotating plasmas are reported. The investigations were carried out because of the possibility of refuelling fusion reactors by the injection of pellets. The ablation rate found is higher than expected on the basis of a theory of a shielding neutral gas layer.

The Possibility of Micro-fission Chain-reactions and their Application to the Controlled Release of Thermonuclear Energy

It had been shown 1, 2 recently that it may be possible to reach densities up to 103 g/cm3 by imploding a solid hydrogen pellet with the aid of a symmetric arrangement of multiple laser beams irradiating the pellet simultaneously from all sides. The purpose of this proposed scheme was to reduce the energy input requirements for the controlled release of thermonuclear energy delivered by laser ignited T - D micro-explosion systems. It is shown in this paper that by applying the foregoing concept of high density compression to a pellet consisting of fissionable material such as U235, Pu239, or U223, the critical mass for the initiation of a fission chain reaction in these materials can be reduced by many orders of magnitude raising the possibility of micro-fission-explosions with laser energy inputs for compression within the expected reach of laser technology. Alternatively, the pellet compression may be also achieved by the employment of intense relativistic electron beams. In addition, by surrounding the fis­sionable pellet with a layer of dense T - D material to be compressed together with the pellet, the minimum pellet size can be furthermore reduced by neutron reflection from the high neutron albedo T - D shell. For fissionable pellets without a reflector the critical mass can be as small as ∼ 0.3 g requiring a laser energy input for compression of several megajoule. For a pellet surrounded by a T - D reflector the critical mass can be reducel down to ∼10-3g with a laser energy input for compression of several 105 joule. In addition to the neutron albedo effect of the T - D shell there is a bootstrap mechanism where­by the T - D mantle achieves thermonuclear temperatures resulting in the emission of thermo­nuclear reaction neutrons into the fissionable pellet causing more fission reactions and thereby raising the temperature of the pellet. This then in turn will lead to an additional heating of the T - D blanket resulting in an increased flux of thermonuclear neutrons raising the fission rate even further. Because of the greatly increased material density, a fission chain reaction initiated in it will grow in proportion much faster than in uncompressed fissionable material as it is being used in conventional fission bombs. The system described may have important usefulness as a small fission power plant especially for space propulsion applications but also for a more effective burn in a thermonuclear micro-bomb reactor as a fission supported fusion reaction.

A solid hydrogen pellet launcher

A device is described which produces and launches solid hydrogen pellets in a vacuum, for use in laser irradiation experiments. The pellets have both length and diameter of 0·25 mm, contain approximately 4×1017 atoms, and are ejected with a velocity of 103 cm s−1. Most of the trajectories lie within 30 mrad of the mean. The device operates automatically and launches one pellet every 15 s.

Laser produced plasmas from isolated solid hydrogen pellets

A 500 MW pulsed ruby laser is focussed on to an isolated solid hydrogen pellet in vacuo. The launching and timing mechanisms are described. The resulting plasma expands spherically with a front velocity of 2 × 10 7 cm/sec. The number of ions in the plasma exceeds 5 × 10 16.