Physics Criteria for a Subscale Plasma Liner Experiment

Spherically imploding plasma liners, formed by merging hypersonic plasma jets, are a proposed standoff driver to compress magnetized target plasmas to fusion conditions (Hsu et al. in IEEE Trans Plasma Sci 40:1287, 2012). In this paper, the parameter space and physics criteria are identified for a subscale, plasma-liner-formation experiment to provide data, e.g., on liner ram-pressure scaling and uniformity, that are relevant for addressing scientific issues of full-scale plasma liners required to achieve fusion conditions. Based on these criteria, we quantitatively estimate the minimum liner kinetic energy and mass needed, which informed the design of a subscale plasma liner experiment now under development.     

C<Subscript>60</Subscript>-Fullerene Hyper-Velocity High-Density Plasma Jets for MIF and Disruption Mitigation

We present an innovative idea to use hyper-velocity (>30 km/s) high-density (>10¹⁷ cm⁻³) plasma jets of D-T/H and C₆₀-fullerene for magneto-inertial fusion (MIF), high energy density laboratory plasma (HEDLP), and disruption mitigation in magnetic fusion plasma devices. The mass (~1–2 g) of sublimated C₆₀ and hydrogen (or D-T fuel) produced in a pulsed power source is ionized and accelerated as a plasma slug in a coaxial plasma accelerator. For MIF/HEDLP we propose to create a magnetized plasma target by injecting two high-Mach number high-density jets with fuel (D-T) and liner (C₆₀/C) structure along the axis of a pulsed magnetic mirror. The magnetized target fusion (MTF) plasma created by head-on collision and stagnation of jets is compressed radially by a metallic liner (Z-pinch) and axially by the C₆₀/C liner. For disruption mitigation, the C₆₀ plasma jets were shown to be able to provide the required impurity mass (J Fusion Energy 27:6, 2008).     

Compression, Heating and Fusion of Colliding Plasmoids by a Z-theta Driven Plasma Liner

A potentially promising approach to fusion employs a plasma shell to radially compress two colliding plasmoids. The presence of the magnetic field in the target plasma suppresses the thermal transport to the confining shell, thus lowering the imploding power needed to compress the target to fusion conditions. With the momentum flux being delivered by an imploding plasma shell, many of the difficulties encountered in imploding a solid metal liner are eliminated or minimized. The best plasma for the target in this approach is the FRC. It has demonstrated both high β, and robustness in translation and compression that is demanded for the target plasma. A high density compressed plasmoid is formed by a staged axial and radial compression of two colliding/merging FRCs where the energy that is required for the implosion compression and heating of the magnetized target plasmoid is stored in the kinetic energy of the plasmas used to compress it. An experimental apparatus is being constructed for the demonstration of both the target plasmoid formation as well as the compression of the plasmoid by a plasma liner. It is believed that with the confinement properties and the high β nature of the FRC, combined with the unique approach to be taken, that an nτ_E T _i triple product ∼5 × 10¹⁷ m⁻³ s keV can be achieved.     

Interaction of imploding shock waves with expanding central plasma in spherical pinch experiments: Simulation analysis

In the spherical pinch scheme, the hot D-T plasma produced in the center of the high pressure spherical vessel is confined by means of imploding shock waves launched from the periphery of the vessel for a time sufficiently long to achieve break-even conditions for plasma fusion. Theoretical studies on spherical pinch made so far have been limited up to the conditions of substantial expansion of the central plasma and the well-defined time delay between the creation of central plasma and the launching of the peripheral shock which led to the conclusion that, in realistic situations of SP experiments, negative time delays should be adopted, i.e., the launching of the imploding shock wave should precede the formation of the central plasma. However, the interaction of converging shock wave with the central plasma causing an additional heating and compression of the central plasma favoring plasma fusion conditions was not taken into account. Starting from the hydrodynamic equations of the system, the proposed simulation code deals with the propagation of converging shock waves and its interaction with the expanding central plasma. Considering the above-mentioned interaction in a self-consistent manner, the temporal evolution of temperature of central plasma is studied. Some results of the numerical simulation on the dynamics of shock wave propagation are also compared with the predictions of point strong explosing theory.     

The Plasma Liner Compression Experiment

Magnetic fusion at high energy density requires compression of a magnetized target in a manner that can be repetitively performed. One approach being investigated in these experiments is the radial compression of a FRC target by a cylindrical plasma liner. A circular array consisting of 16 small coaxial plasma sources were placed at each end of a quartz chamber radially near the wall to form the plasma liner. Liner masses of 3 mg have been formed in Xenon and have been accelerated to a velocity of 50 km/s. An FRC was produced in an adjacent chamber and translated inside the plasma liner where theta-pinch coils were employed to drive the plasma liner radially inward compressing the FRC. Close agreement is found between experimental results and 2D MHD numerical calculations. Efficient coupling (~50%) between the compression bank and the plasma liner has been observed.     

The Role of Magnetized Liner Inertial Fusion as a Pathway to Fusion Energy

We discuss the possible impacts of a new magnetized liner inertial fusion concept on magneto-inertial fusion approaches to fusion energy. Experiments in the last 1.5 years have already shown direct evidence of magnetic flux compression, a highly magnetized fusing fuel, significant compressional heating, a compressed cylindrical fusing plasma, and significant fusion yield. While these exciting results demonstrate several key principles behind magneto-inertial fusion, more work in the coming years will be needed to demonstrate that such targets can scale to ignition and high yield. We argue that justifying significant investment in pulsed inertial fusion energy beyond target development should require well-understood, significant fusion yields to be demonstrated in single-shot experiments. We also caution that even once target ideas and fusion power plants have been demonstrated, historical trends suggest it would still be decades before fusion could materially impact worldwide energy production.     

Pressure Wave Propagation in an Imploding Liquid Metal Liner

The propagation of a pressure wave through an imploding liquid metal liner near turnaround, due to the compression of magnetic flux and plasma is considered, based on which the pressure distribution near turnaround is derived analytically under the assumption of a small Mach number M (defined by the ratio between characteristic radial velocity of liner and the velocity of sound through the medium). It is shown that under certain conditions, the rarefaction wave from the outer (free) surface causes the apparition of a negative pressure in the imploding liner, with consequent liability of cavitation. The criterion for the generation of the negative pressure is found to be MΛ0.6~0.7 for the range of parameters considered (A: liner thickness at turnaround normalized by the inner radius).     

Solving the Stand-off Problem for Magnetized Target Fusion: Plasma Streams as Disposable Electrodes, Together with a Local Spherical Blanket

In a fusion reactor based on the magnetized target fusion approach, the permanent power supply has to deliver currents up to a few mega-amperes to the target dropped into the reaction chamber. All the structures situated around the target will be destroyed after every pulse and have to be replaced at a frequency of 1–10 Hz. In this paper, an approach based on the use of spherical blanket surrounding the target, and pulsed plasma electrodes connecting the target to the power supply, is discussed. A brief analysis of the processes associated with creation of plasma electrodes is presented.     

Field Reversed Configuration Translation and the Magnetized Target Fusion Collaboration

After considerable design and construction, we describe the status of a physics exploration of magnetized target fusion (MTF) that will be carried out with the first flux conserving compression of a high pressure field-reversed configuration (FRC). The upgraded Los Alamos (LANL) high density FRC experiment FRXL has demonstrated that an appropriate FRC plasma target can be created and translated on a time scale fast enough to be useful for MTF. Compression to kilovolt temperature is expected to form a Mbar pressure, high energy density laboratory plasma (HEDLP). Integrated hardware on the new Field Reversed Compression and Heating Experiment (FRCHX) at the Air Force Research Laboratory Shiva Star facility, has formed initial FRC’s and will radially compress them within a cylindrically symmetric aluminum “liner”. FRXL has shown that time scales for FRC translation to the target region are significantly shorter than the typical FRC lifetime. The hardware, diagnostics, and design rationales are presented. Pre-compression plasma formation and trapping experimental data from FRXL and FRCHX are shown.     

Scaling Factor of the Operating Parameters of Z-pinch Liners

Imploding plasma liners in the Z-pinch scheme have been demonstrated to be capable of producing high power radiation in the soft X-ray waveband owing to the conversion of the kinetic energy of imploding liner into thermal energy which in turn is converted into X-ray energy. To obtain largest X-ray power for a certain pulsed-power driving- source, the liner should gain a kinetic energy as great as possible, which imposes an optimal scaling upon the operating parameters of liner in terms of getting largest kinetic energy. This work exposes, by means of numerical calculations based on zero-dimensional quasi-plasma-shell model, the large variation of the scaling factor, which connects the parameters of the initial liner and the driving current, with different driving current waveforms. Also solved in the work is the optimal scaling factor in the sense of producing maximum kinetic energy. Calculations show that maximum kinetic energy is obtained at the current maximum or a little time later. These results ar     

The spherical pinch

This is essentially a review article covering several years of work on the spherical pinch (SP) concept of plasma formation and containment. Central to this concept is the creation of a hot plasma in the center of a sphere, plasma which is then compressed by strong imploding shock waves launched from the periphery of the vessel. The experimental program, which started with the classical cylindrical theta-pinch and continued with the inductive spherical pinch, has taken a turn, in recent times, with the discovery of the scaling laws governing spherical pinch experiments, which prescribe that high gas pressures are required for achieving fusion breakeven conditions. As a consequence, energy deposition in present spherical pinch devices is done through resistive, rather than inductive, discharges. In a pilot experimental program of modest initial condenser bank energy ( 1 KJ), we find that the instantaneous energy deposition in the central plasma can lead to temperatures of the order of 2 KeV, in agreement with the prediction of the Braginskii resistivity for such a plasma, and with the relation to the velocity of the diverging shock wave generated by the sudden deposition of energy into this plasma. Moreover, when the imploding shock waves contain the central plasma, we find the containment time to be as long as 5.4 sec and the plasma to be stable. In discharges in deuterium, neutrons are emitted close to 10⁷ per shot. From the experimental parameters of the plasma, one can derive a particle density for the shocked gas equal to 3.21×10¹⁹ cm^–3, a plasma temperature equal to 730 eV and a productn=1.73 × 10¹⁴ cm^–3· sec.Brief parts of this work are abstracted from previous works of the same author: C.A.S.I. (Canadian Aeronautics and Space Institute)Transaction,2, 21 (1969);Can. J. Phys.,58, 983 (1980);J. Fusion Energy,3, 199 (1983).     

Estimation of plasma density perturbation from dusty plasma injection by laser irradiation on tungsten target in DiPS

To investigate the interaction of dusty plasma with magnetized plasmas at divertor plasma simulator, radial profiles of plasma density(ne) and electron temperature were measured in terms of plasma discharge currents and magnetic flux intensity by using a fast scanning probes system with triple tips. Dusty plasma with dusts(a generation rate of 3 μg s~(-1) and a size of 1–10 μm)was produced via interactions between a high-power laser beam and a full tungsten target. As ne increases, the scale of the effects of dusty plasma injection on magnetized plasmas was decreased. Also, the duration of transient fluctuation was reduced. For numerical estimation of plasma density perturbation due to dusty plasma injection, the result was ~10% at a core region of the magnetized plasma with n_e of(2–5)×10~(11) cm~(-3) at steady state condition.     

Spherical Torus Pathway to Fusion Power

Spherical Torus (ST) as an example of confinement concept innovation to enable a potentially attractive pathway to fusion power is discussed. Given the anticipated high performance in small size, the ST plasma could be used to stimulate innovation also in engineering, technology, and material combinations to provide a smarter, cheaper, faster pathway. This pathway could complement the mainline program based on the tokamak in making the desired progress in fusion energy sciences. The ST pathway could include a small VNS (Volume Neutron Source) with low fusion amplification (Q 1–2) for Fusion Energy Development (energy technology) and a small Pilot Plant with high Q (15–30) to practice Fusion Power Demonstration. Success in these steps also enhances the possibility for competitive non-electric applications of interest to society in time scales shorter than electric power generation. The scientific basis for these possibilities will be tested in the U.S. by the Proof of Principle experiment NSTX (National Spherical Torus Experiment) presently being built, and could be completed by a Proof of Performance and Optimization experiment such as a small DTST (Deuterium-Tritium Spherical Torus). Utilization of facilities and equipment already available in the U.S. would minimize the time and cost for these experiments and accelerate the approach to the stage of Fusion Energy Development.     

Observation of an Enhanced Magnetic Helicity Injection Mode by a Rotating Plasma Annulus

Coaxial plasma guns are commonly used to inject magnetic helicity in innovative confinement concepts (ICC’s) for magnetic fusion. One of the key issues in magnetic helicity injection is to maximize the magnetic helicity injection rate. We have identified experimentally an alternative way to increase the magnetic helicity injection rate through rotating plasmas by extending the length of the inner electrode of a coaxial plasma gun so that an additional E × B region interweaves the standard J//B configuration. In the so-called “enhanced helicity injection” mode, the gun voltage is larger compared with the “normal” mode and decays more slowly. Another signature of the enhanced mode is increased edge magnetic field in conjunction with larger edge rotation. The results indicate that tuning plasma rotation is another way to enhance magnetic helicity injection using coaxial plasma guns. An alternative ICC is proposed based on the experimental observations.     

Operational characteristics of the high flux plasma generator Magnum-PSI

In Magnum-PSI (MAgnetized plasma Generator and NUMerical modeling for Plasma Surface Interactions), the high density, low temperature plasma of a wall stabilized dc cascaded arc is confined to a magnetized plasma beam by a quasi-steady state axial magnetic field up to 1.3 T. It aims at conditions that enable fundamental studies of plasma–surface interactions in the regime relevant for fusion reactors such as ITER: 10²³–10²⁵ m⁻² s⁻¹ hydrogen plasma flux densities at 1–5 eV. To study the effects of transient heat loads on a plasma-facing surface, a high power pulsed magnetized arc discharge has been developed. Additionally, the target surface can be transiently heated with a pulsed laser system during plasma exposure. In this contribution, the current status, capabilities and performance of Magnum-PSI are presented.     

Exploratory studies of tokamaks as fusion test reactors

Studies have been performed to explore various plasma burn scenarios for a tokamak test reactor which could follow the next generation of large tokamak experiments. Tradeoffs between an ignited burning plasma and a sub-ignited driven plasma are examined in terms of device size and performance as a fusion engineering test facility. It is found that plasma performance levels, measured by ignition margin, amplification factorQ, and fusion power output, increase with device size, more optimistic transport scaling laws, lower magnetic field ripple, and higher. The performance of a generally low stress (B ₀=4 T) reference device, with major radiusR=4.5 m and minor radiusa=1.3 m in a D-shaped (=1.6) plasma has been evaluated over a wide range of operating parameters. In particular, a moderate fusion power output of 300 MW is obtained, the driven plasma havingQ 10, an edge ripple of 1%, and a density ranging between 1.0 and 1.5×10¹⁴ cm^–3. The same device operated at a higher general level of stress (B ₀=5.3 T) is predicted to achieve ignition, but is not required for the mission of an engineering test facility and would entail greater technical risk.     

Experimental results of the erosion of molybdenum due to blistering by He+ bombardment

We report some preliminary measurement of the erosion rate of plasma dumps when bombarded with 100 keV He⁺ ions at high power density ( 1 MW/m²). The expected erosion rates, based on measurements of He blistering that were made at lower power density ( 0.3 MW/m²), indicate a potentially serious problem for fusion reactors. Our tests use a reactorlike power density and produce He blisters at a rate that is slower than predicted by 2 to 4 orders of magnitude, depending on the temperature of the molybdenum target.     

A programmatic framework for the Tokamak Physics Experiment (TPX)

Significant advances have been made in the confinement of reactor-grade plasmas, so that we are now preparing for experiments at the power breakeven level in the JET and TFTR experiments. In ITER we will extend the performance of tokamaks into the burning plasma regime, develop the technology of fusion reactors, and produce over a gigawatt of fusion power. Besides taking these crucial steps toward the technical feasibility of fusion, we must also take steps to ensure its economic acceptability. The broad requirements for economically attractive tokamak reactors based on physics advancements have been set forth in a number of studies. An advanced physics data base is emerging from a physics program of concept improvement using existing tokamaks around the world. This concept improvements program is emerging as the primary focus of the U.S. domestic tokamak program, and a key element of that program is the proposed Tokamak Physics Experiment (TPX). With TPX we can develop the scientific data base for compact, continuously-operating fusion reactors, using advanced steady-state control techniques to improve plasma performance. We can develop operating techniques needed to ensure the success of ITER and provide first-time experience with several key fusion reactor technologies. This paper explains the relationships of TPX to the current U.S. fusion physics program, to the ITER program, and to the development of an attractive tokamak demonstration plant for this next stage in the fusion program.Abbreviations used TFTR Tokamak Fusion Test Reactor - JET Joint European Torus - ARIES Advanced Reactor Innovations Evaluation Study - SSTR Steady State Tokamak Reactor - PBXM Princeton Beta Experiment-Modified - DIII-D Doublet III—Dee - JT60-U Japanese Tokamak 60-Upgrade     

Recent progress on the Tokamak Fusion Test Reactor

The deuterium-tritium (D-T) experiments on the Tokamak Fusion Test Reactor (TFTR) have yielded unique information on the confinement, heating and alpha particle physics of reactor scale D-T plasmas as well as the first experience with tritium handling and D-T neutron activation in an experimental environment. The D-T plasmas produced and studied in TFTR have peak fusion power of 10.7 MW with central fusion power densities of 2.8 MWm^–3 which is similar to the 1.7 MWm^–3 fusion power densities projected for 1,500 MW operation of the International Thermonuclear Experimental Reactor (ITER). Detailed alpha particle measurements have confirmed alpha confinement and heating of the D-T plasma by alpha particles as expected. Reversed shear, highl _i and internal barrier advanced tokamak operating modes have been produced in TFTR which have the potential to double the fusion power to 20 MW which would also allow the study of alpha particle effects under conditions very similar to those projected for ITER. TFTR is also investigating two new innovations, alpha channeling and controlled transport barriers, which have the potential to significantly improve the standard advanced tokamak.