Hydrogen Pellet Research Articles

Hydrogen pellet acceleration with a two-stage system consisting of a gas gun and a fuseless electromagnetic railgun

Hydrogen pellets are successfully accelerated for the first time using a two-stage system consisting of a pneumatic gun and an electromagnetic railgun. The pneumatic gun preaccelerator forms cylindrical hydrogen ice pellets (1.6-mm diam×2.15-mm long) and accelerates them with high-pressure helium gas to velocities in excess of 500 m/s. The booster accelerator, which is a fuseless, circular-bore electromagnetic railgun, derives its propulsive force from a plasma arc armature. The plasma arc armature is formed by electrically breaking down the propellant gas which follows the pellet from the gas gun into the railgun. The diagnostics are for the monitoring of the main capacitor bank and rail currents, for the pellet detection and velocity measurements at the breech and muzzle ends of the railgun, for the recording of the plasma-arc-armature movement inside the railgun bore, and for the photographing of the hydrogen pellet exiting the railgun. Using the system, which is a 60-cm long proof-of-principle machine for refueling magnetic fusion devices, hyrogen pellet velocities exceeding 1 km/s have been achieved for pellets exiting the gas gun at velocities of ∼500 m/s.

Operation of a repeating pneumatic hydrogen pellet injector on the Tokamak Fusion Test Reactor

The repeating pneumatic hydrogen pellet injector, which was developed at the Oak Ridge National Laboratory (ORNL), has been installed and operated on the Tokamak Fusion Test Reactor (TFTR). The injector combines high-speed extruder and pneumatic acceleration technologies to propel frozen hydrogen isotope pellets repetitively at high speeds. The pellets are transported to the plasma in an injection line that also serves to minimize the gas loading on the torus; the injection line incorporates a fast shutter valve and 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 system operation. In early pellet fueling experiments on TFTR, the injector has been used to deliver deuterium pellets at speeds ranging from 1.0 to 1.5 km/s into plasma discharges. First, single large (nominal 4 mm diam) pellets provided high densities in TFTR (1.8×1014 cm−3 on axis); after conversion to smaller (nominal 2.7 mm diam) pellets, up to five pellets were injected at 0.25 s intervals into a plasma discharge, giving a line-averaged density of 1×1014 cm−3. Operating characteristics and performance of the injector in initial tests on TFTR are presented.

Plasma shielding of hydrogen pellets

The observation of Hα-radiation extending along the magnetic field lines during pellet injection in ASDEX suggests a renewed discussion of ablation models and, in particular, an investigation of the additional shielding of the pellet by the high-density, medium-temperature plasma generated by the ablated material. In the present paper the authors have simulated the frozen-in gas flowing along the field lines with a 1-D hydrodynamic code, coupled to the neutral-gas shielding model describing the ablation. To take into account long-mean-free-path effects on the electron heat transport in the plasma, it was necessary to replace the classical Spitzer-Harm conductivity by a non-local transport model. – A plasma hose with a density of up to several hundred times that of the background plasma and extending over several metres was found to form along the field lines. As the temperature of this plasma is much lower than that of the background one (in a computation for ASDEX it dropped from 500 to approximately 40 eV), this effect should lead to a distinct decrease in the resulting ablation rate.

Repeating pneumatic hydrogen pellet injector for plasma fueling

A repeating pneumatic pellet injector has been developed for plasma fueling applications. The repetitive device extends pneumatic injector operation to steady state. The active mechanism consists of an extruder and a gun assembly that are cooled by flowing liquid-helium refrigerant. The extruder provides a continuous supply of solid hydrogen to the gun assembly, where a reciprocating gun barrel forms and chambers cylindrical pellet from the extrusion; pellets are then accelerated with compressed hydrogen gas (pressures up to 125 bar) to velocities ≤1.9 km/s (1.6 km/s for deuterium pellets). The gun assembly design can accommodate different pellet sizes and barrel lengths. Steady-state rates of 2 s−1 have been obtained with 2.1- , 3.4- , and 4.0-mm-diameter pellets. The present apparatus operates at higher firing rates in short bursts; for example, a rate of 6 s−1 for 2 s with the larger pellets. These pellet parameters are in the range applicable for fueling large present-day fusion devices such as the Tokamak Fusion Test Reactor (TFTR). Experimental results are presented, including effects of propellant pressure and barrel length on gun performance.

Solid hydrogen pellet injector for T-10

This article describes the first Soviet high-speed hydrogen pellet injector operating on T-10. An experimental investigation was done of the interaction process between the fuel pellet and the tokamak plasma and the diagnostics of the transfer process in the plasma. The system is calculated on the injection during the discharge pulse of a single pellet containing hydrogen with a velocity of up to 700 m/sec. The experiments conducted in T-10 have included an investigation of the limits of stability of the injection, the heat and particle transfer processes in the plasma and the effect of the injection on MHD perturbation.

FIR interferometer/polarimeter system on ISX-B tokamak

Recent results of simultaneous measurements of electron density and Faraday rotation on all five channels of the FIR interferometer/polarimeter system on ISX-B are presented. The time resolution of the interferometer is typically 1 μs. The standard deviation of the output of the interferometer of a constant phase shift is less than 5×10−2 fringe, corresponding to a line electron density of 5×1011 cm−3. The polarimeter shows a sensitivity of the order of 1 mrad and a time resolution of approximately 1 ms. Data are reported on ohmic as well as neutral-beam-heated plasmas including solid hydrogen pellet injections. It is believed that this is the first successful measurement of Faraday rotation in pellet-injected discharges. Data analysis codes have been developed and the results are being compared with other measurements.

The absorption effect of the Lα-line Supplement to the paper ‘On the Correlation Between the Hα-line emission rate and the ablation rate of a hydrogen pellet in tokamak discharges’ – Nuclear Fusion 24 (1984) 697

Several assumptions made in our previous study of the correlation between the Hα-line emission rate and the ablation rate of a hydrogen pellet injected into a tokamak discharge led us to take the emission layer of the ablatant as optically thin with respect to all levels of the principal quantum number, n. While this may be justified for higher values of n, it requires some consideration about the resonance line, Lα, the Lyman alpha line. To study its possible absorption effect, we shall consider the extreme case that the Lα-line is completely absorbed.

Fabrication of planar and cylindrical solid hydrogen pellets for laser interaction experiments

A system is described for fabricating both planar and cylindrical solid hydrogen pellets. The target geometry is determined by the geometry of interchangeable target molds. Planar targets fabricated with this system have dimensions of 3.0×3.0 mm with thicknesses ranging from 0.13 to 0.60 mm, and cylindrical targets were 1.0–3.0 mm in length with diameter of 0.80 mm.

Particle fueling and impurity control in PDX

Fueling requirements and impurity levels in neutral-beam-heated discharges in the PDX tokamak have been compared for plasmas formed with conventional graphite rail limiters, a particle scoop limiter, and an open or closed poloidal divertor. Gas flows necessary to obtain a given density are highest for diverted discharges and lowest for the scoop limiter. Hydrogen pellet injection provides an efficient alternative fueling technique, and a multiple pellet injector has produced high density discharges for an absorbed neutral beam power of up to 600 kW, above which higher speeds or more massive pellets are required for penetration to the plasma core. Power balance studies indicate that 30–40% of the total input power is radiated while ~15% is absorbed by the limiting surface, except in the open divertor case, where 60% flows to the neutralizer plate. In all operating configurations, Zeff usually rises at the onset of neutral beam injection. Both open divertor pl;asmas and those formed on a well conditioned water-cooled limiter have Zeff ⪅ 2 at the end of neutral injection. A definitive comparison of divertors and limiters for impurity control purposes requires longer beam pulses or higher power levels than available on present machines.

Injection of Deuterium Pellets

A discussion is given of the work done at Risø National Laboratory on the design and construction of deuterium pellet injectors. A pellet injection system made for the TFR tokamak at Fontenay-aux-Roses, Paris is described. 0.12-mg pellets are injected with velocities of around 600–700 m/s through a 5-m long guide tube. Next some of the details of a new light gas gun are given; with this gun, hydrogen pellets are accelerated to velocities above 1400 m/s, deuterium pellets to velocities above 1300 m/s and neon pellets to velocities above 550 m/s. Finally, a new acceleration method where a pellet should be accelerated by means of a magnetically stabilised electrical discharge is discussed, and a set up for measuring of the pellet size by means of a microwave cavity is outlined.

Energy Confinement of High-Density Pellet-Fueled Plasmas in the AlcatorCTokamak

A series of pellet-fueling experiments has been carried out on the Alcator C tokamak. High-speed hydrogen pellets penetrate to within a few centimeters of the magnetic axis, raise the plasma density, and produce peaked density profiles. Energy confinement is observed to increase over similar discharges fueled only by gas puffing. In this manner record values of electron density, plasma pressure, and Lawson number (nτ) have been achieved.Received 15 March 1984DOI:https://doi.org/10.1103/PhysRevLett.53.352©1984 American Physical Society

On the correlation between the Hα-line emission rate and the ablation rate of a hydrogen pellet in tokamak discharges

By incorporating a simplified collisional-radiative model into a self-limited ablation model, the correlation between the emission rate of the Hα-line and the ablation rate of a hydrogen pellet in tokamak discharges is studied. In normal Ohmically heated discharges, the fuel deposition profile is reproduced accurately by the Hα-line emission profile, except for a slight delay of the maximum ablation rate with respect to the maximum emission rate.

Pellet acceleration study with a railgun for magnetic fusion reactor refueling

Design, construction, and preliminary testing of a two-stage pellet injection system capable of achieving hydrogen pellet velocities of 5–10 km/s are described. The system, which is intended for the refueling of magnetic fusion devices, combines a gas gun with a small-bore, plasma-arc-driven electromagnetic railgun. The gas gun uses hydrogen gas as the propellant and injects a medium-velocity pellet into the railgun. Once inside the railgun, the propellant gas following the pellet is electrically broken down forming a plasma arc armature. The propulsive force of this plasma arc armature further accelerates the pellet to higher velocities.

ドロップレット法による水素アイス・ペレット・インジェクタの設計と試作

A hydrogen pellet injector by a droplet-method has been constructed and studied, in order to realize a hydrogen-isotope pellet injector for refueling into nuclear fusion reactors, which can inject pellets into plasma repetitively. Preliminary experiments with oxygen gas, instead of hydrogen gas, has been systematically carried out. Assuming the liquid concerned as viscous fluid, theoretical predictions about droplet-diameter, its ejected velocity and optimum frequency of ejecting-nozzle vibration for stable droplet production has been made, and it is found that theoretical results are in good agreement with experimental ones. It is found that the stable droplet train can be obtained when the value of Reynolds number is in the range of 1, 100-1, 300. In the hydrogen experiments based upon the oxygen results, the production of a stable hydrogen liquid-droplet train, their self-solidification and transfer into a vacuum space through an orifice (with the diameter of 1mm) have been successfully realized, by maintaining the gas pressure around the droplets at 45-50 Torr.

Numerical simulation for density increase by hydrogen pellet injection.

Density increase by pellet injection into magnetically confined plasmas is an interesting subject from the point of view of the transport problem. Ablation process of the pellet is analyzed by the shielded neutral ablation model developed by Milora and Foster. A self-limiting ablation process is also taken into account to find a particle deposition profile. It is shown that optimization of both the pellet size and velocity according to plasma parameters of the target plasma is required to increase density efficiently.The particle deposition profile is coupled with the numerical code to study the transport process in a tokamak and a heliotron. The pellet injection into an ohmically heated plasma of ISX-B tokamak is studied by our code and good agreement between the numerical results and the experimental data is obtained.Pelet injection into a currentless NBI heated plasma of Heliotron E is studied and time evolutions of density, temperature and pressure profiles are demonstrated. When beta value increases by the pellet injection, MHD stability against the pressure-driven mode becomes an important criterion to determine the pellet size and velocity. In other words the pressure profile can be controlled by the pellet injection.

Development of Hydrogen Pellet Injectors at ORNL

Pellet injectors that produce and accelerate frozen hydrogen isotope pellets are being developed at Oak Ridge National Laboratory (ORNL) for fueling of present and future plasma fusion devices. The development has focused primarily on two types of injectors: (1) gas guns, which utilize a pneumatic approach to accelerate pellets in a barrel with compressed helium or hydrogen propellant, and (2) centrifuge-type injectors, in which pellets are accelerated by centrifugal forces in a highspeed rotating track. In a single-pellet pneumatic injector, pellet speeds up to 1.4 km/s have been achieved. Three multipellet injection systems (ORNL four-pellet pneumatic design) are now functional, one each on the Poloidal Divertor Experiment (PDX), Alcator-C, and the Impurity Study Experiment (ISX-B). Currently, two repetitive devices (one of each injector type) are in operation to demonstrate steadystate fueling systems in the reactor-relevant parameter ranges of 1-km/s pellet velocity, variable pellet sizes up to 2 mm, and feed rates up to 10-40 pellets/s. The injector designs are described and operating characteristics discussed.

Performance characterization of pneumatic single pellet injection system

The Oak Ridge National Laboratory single-shot pellet injector, which has been used in plasma fueling experiments on ISX and PDX, has been upgraded and extensively instrumented in order to study the gas dyamics of pneumatic pellet injection. An improved pellet transport line was developed which utilizes a 0.3-cm-diam by 100-cm-long guide tube. Pellet gun performance was characterized by measurements of breech and muzzle dynamic pressures and by pellet velocity and mass determinations. Velocities of up to 1.4 km/s were achieved for intact hydrogen pellets using hydrogen propellant at 5-MPa breech pressure. These data have been compared with new pellet acceleration calculations which include the effects of propellant friction, heat transfer, time-dependent boundary conditions, and finite gun geometry. These results provide a basis for the extrapolation of present-day pneumatic injection system performance to velocities in excess of 2 km/s.

Fusion fuel pellet injection with a railgun

Railguns have been nominated as fusion fuel pellet injectors. It appears that a railgun could launch dielectric hydrogen isotope pellets at velocities up to 10 km/s or more at high repetition rates. A railgun consists of two parallel conductors (rails) that carry current to and from an interconnecting link (plasma arc armature). The magnetic field generated by the rail currents accelerates the armature and pellet. In addition to high launch velocity, a railgun offers several potential advantages for pellet injection: (1) pellets can be dielectric, (2) pellets can be fed into the open breech as rapidly as they are launched, (3) current can be controlled to ensure isentropic acceleration, (4) the mass of the propulsive arc is less than the pellet mass, (5) cryogenic and vacuum operation is possible, and (6) the pellet is provided lateral support permitting acceleration stresses in excess of its tensile strength, which results in shorter injectors, acceleration times, and higher repetition rates. The most restrictive limitation on railgun injector operation results from the low strength of the solid hydrogen pellets. An acceleration stress limited to 30 bars would require a launcher length of 3400 times the DT projectile length to achieve 10 km/s; i.e., a 5-mm-long projectile would require a 17-m launcher. A critical area needing investigation is the potentially adverse effect of the high temperature armature on the pellet.

Pellet injection into PDX diverted plasmas

Initial hydrogen pellet injection experiments have been performed in plasmas characterized by a low particle re-cycle fraction resulting from the action of a poloidal magnetic divertor. As in past experiments, the interaction of the fuel with the plasma is observed to be adiabatic. Moreover, the pellet mass is accounted for in the core plasma density increase, indicating only a small loss of fuel while the pellet transits the divertor scrape-off plasma. The effect of edge re-cycling on the density was studied by comparing divertor (low-re-cycle) and limiter (high-re-cycle) plasmas; the distinction between the two cases is clearest in the edge plasma region where the density decay rates differ most. Particle transport subsequent to pellet injection is less ambiguous for divertor cases, and, by comparing the density profile relaxation and the electron temperature recovery with an empirical transport model that closely approximates the pre-injection plasma conditions, it is concluded that the plasma confinement properties do not deteriorate as a result of pellet injection. The principal difference between central and edge fuelling is demonstrated by a peaking of the density profile and an extended decay time for the density perturbation.

Fueling of magnetic confinement devices

A general overview of the fueling of magnetic confinement devices is presented, with particular emphasis on recent experimental results. Various practical fueling mechanisms are considered, such as cold gas inlet (or plasma edge fueling), neutral beam injection, and injection of high speed cryogenic hydrogen pellets. The central role played by charged particle transport and recycle of plasma particles from material surfaces in contact with the plasma is discussed briefly. The various aspects of hydrogen pellet injection are treated in detail, including applications to the production of high purity start‐up plasmas for stellarators and other devices, refueling of tokamak plasmas, pellet ablation theory, and the technology and performance characteristics of low and high speed pellet injectors.