Compact Torus Plasma Research Articles

Characterization of fast magnetosonic waves driven by compact toroid plasma injection along a magnetic field

Magnetosonic waves are low-frequency, linearly polarized magnetohydrodynamic (MHD) waves commonly found in space, responsible for many well-known features, such as heating of the solar corona. In this work, we report observations of interesting wave signatures driven by injecting compact toroid (CT) plasmas into a static Helmholtz magnetic field at the Big Red Ball Facility at Wisconsin Plasma Physics Laboratory. By comparing the experimental results with the MHD theory, we identify that these waves are the fast magnetosonic modes propagating perpendicular to the background magnetic field. Additionally, we further investigate how the background field, preapplied poloidal magnetic flux in the CT injector, and the coarse grid placed in the chamber affect the characteristics of the waves. Since this experiment is part of an ongoing effort of creating a target plasma with tangled magnetic fields as a novel fusion fuel for magneto-inertial fusion (MIF), our current results could shed light on future possible paths of forming such a target for MIF.

Design and simulation of 1.5 T conduction-cooled superconducting magnet with flip-over capability

The Compact Torus Injection (CTI) system is a plasma physics device used to create and study compact torus plasmas, which are dense and highly energetic structures with potential applications in fusion energy research and space propulsion. The Experimental Advanced Superconducting Tokamak compact torus injection (EAST-CTI) system is a device used for plasma heating and fueling in fusion experiments. In this paper, a 1.5 T cryocooled superconducting magnet was developed for the EAST-CTI system, which requests the magnet system can work vertically or horizontally to meet different working conditions. The design of the superconducting magnet and cryogenic system is based on the analysis of the thermal and mechanical characteristics of the magnet system. Thermal and mechanical finite element simulation were used to support the design. The 1.5 T magnet superconducting magnet was manufactured according to the design and simulation works. After 35 h cooling, the superconducting magnet stabilized at 3.87 K, and successfully provided a horizontal magnetic field environment for the experiments on EAST-CTI system.

Control system of high-voltage pulse power supply for compact torus injector based on EPICS

The Compact Torus Injector (CTI) is a magnetic confinement fusion device feeding system, which injects the excited compact toroidal plasma into the target device at high speed, and controls the fuel injection and injection speed to achieve optimal control of the density pressure profile of the magnetic confinement fusion device. As the important subsystem of the CTI, the CTI High-Voltage Pulse Power Supply (HVPPS) system consists of five sets of power supply. This paper uses EPCIS as a development platform to design the control system of the CTI HVPPS which realizes timing sequential control, status detection, parameter setting and read-back status of the CTI power supplies with different communication interfaces and protocols. The discharge experiments are performed on the CTI. The experimental results prove that the control system meets the real-time control requirements of CTI devices for HVPPS. (Abstract)

Experimental study of plasma gun-assisted field-reversed configuration formation

The field-reversed configuration (FRC) is a compact toroidal plasma system related to the research of alternative fusion reactors. Improving the FRC formation, which is critical to the improvement of its overall quality, is reported in this paper by using a plasma gun to assist the pre-ionization process. For the single-translated FRC, the poloidal magnetic flux increases around 30%, accompanied by an increase in the separatrix radius and plasma density, while its translation speed is found to decrease slightly. The estimated increase in particle inventory is more than 60%. The improvement of these parameters also extends the lifetime of the FRC.

Investigation of the compact torus plasma motion in the KTX-CTI device based on circuit analyses

Compact torus (CT) injection is one of the most promising methods for the central fuelling of next-generation reactor-grade fusion devices due to its high density, high velocity, and self-contained magnetised structure. A newly compact torus injector (CTI) device in Keda Torus eXperiment (KTX), named KTX-CTI, was successfully developed and tested at the University of Science and Technology in China. In this study, first, we briefly introduce the basic principles and structure of KTX-CTI, and then, present an accurate circuit model that relies on nonlinear regression analysis (NRA) for studying the current waveform of the formation region. The current waveform, displacement, and velocity of CT plasma in the acceleration region are calculated using this NRA-based one-dimensional point model. The model results were in good agreement with the experiments. The next-step upgrading reference scheme of the KTX-CTI device is preliminarily investigated using this NRA-based point model. This research can provide insights for the development of experiments and future upgrades of the device.

Performance improvement with plasma edge biasing on SPECTOR

A description of an edge-biasing experiment conducted on the spherical compact toroid plasma injector is presented with results. The insertion of a disk-shaped molybdenum electrode (probe), biased at up to +100 V, into the edge of the Compact Torus (CT), resulted in up to 1.2 kA radial current being drawn from the probe to the wall. Core electron temperature, as measured with a Thomson-scattering diagnostic, and CT lifetime, more than doubled in the optimal configuration tested. Temperature and density profiles become more centrally peaked with biasing. In contrast to previous biasing experiments, a significant reduction in electron density was observed; this may be due to the effect of a transport barrier impeding CT fueling and associated cooling, where, as verified by MHD simulation, a principal fueling source is neutral gas that remains concentrated around the gas valves after CT formation.

磁化同軸プラズマガン装置におけるCTプラズマの特性評価とパルス高熱負荷模擬試験への適用

A magnetized coaxial plasma gun (MCPG) device at Univ. of Hyogo was used as a simulation facility for pulse heat loads such as ELMs (Edge Localized Modes) onto plasma-facing materials in magnetically confined fusion devices. In the present study, characterization of the compact toroid (CT) plasmas produced by the MCPG device was performed. Dependences of the CT plasma parameters of line-averaged electron density, ion temperature and flow velocity on the gun discharge voltage and the bias magnetic flux were measured. As a result, it was observed that the pulse length, the line-averaged electron density, and the ion temperature and flow velocity were approximately 200 µs, approximately 6×1021 m-3, approximately 30 eV, and approximately 50 km s-1, respectively. The surface energy density measured with a tungsten calorimeter was approximately 0.4 MJ m-2. Thus, it was shown that the ELM-like pulsed plasma heat loads with the energy density and the pulsed length predicted in the ITER divertor can be produced using the MCPG device.

Zero Flux Eigenvalues (ZFE’s) of Tight Aspect Ratio Tokamaks

Compact toroidal magnetized plasmas are an important part of the world’s magnetic fusion and plasma science efforts. These devices can play an integral role in the development of magnetic fusion as a viable commercial energy source, and in our understanding of plasma instabilities, particle and energy transport, and magnetic field transport. In this paper, we are developing a numerical program to study the magnetic dynamo or relaxation of CT’s characterized by arbitrary tight aspect ratio (major to minor radii of tokamak) and arbitrary cross-sections (Multi-pinch and D-Shaped). The lowest ZFE’s has been calculated through the Taylor’s relaxed state (force-free) toroidal plasmas equation. For ZFE’s, we use the toroidal flux vanishing boundary condition along the whole boundary of tokamaks. Several runs of the program for various wave numbers showed that ZFE was very insensitive to the choice of wave numbers. Besides, the CT’s poloidal magnetic field topologies are well represented. It was very interesting to check our methods for the cases when aspect ratio tends to unity (zero tokamak whole). A good fulfillment of the boundary condition is achieved.

Hall probe measurements of the poloidal magnetic field in Compact Toroidal Hybrid plasmas.

A linear array of 16 Hall effect sensors has been developed to directly measure the poloidal magnetic field inside the boundary of a non-axisymmetric hybrid torsatron/tokamak plasma. The array consists of miniature gallium arsenide Hall sensor elements mounted 8 mm apart on a narrow, rotatable printed circuit board inserted into a re-entrant stainless steel tube sheathed in boron nitride. The sensors are calibrated on the bench and in situ to provide accurate local measurements of the magnetic field to aid in reconstructing the equilibrium plasma current density profiles in fully three-dimensional plasmas. Calibrations show that the sensor sensitivities agree with the nominal manufacturers specifications of 1.46 V/T. Poloidal fields measured with the Hall sensor array are found to be within 5% of poloidal fields modeled with a Biot-Savart code.

Development of a low-energy and high-current pulsed neutral beam injector with a washer-gun plasma source for high-beta plasma experiments

We have developed a novel and economical neutral-beam injection system by employing a washer-gun plasma source. It provides a low-cost and maintenance-free ion beam, thus eliminating the need for the filaments and water-cooling systems employed conventionally. In our primary experiments, the washer gun produced a source plasma with an electron temperature of approximately 5 eV and an electron density of 5 × 10(17) m(-3), i.e., conditions suitable for ion-beam extraction. The dependence of the extracted beam current on the acceleration voltage is consistent with space-charge current limitation, because the observed current density is almost proportional to the 3/2 power of the acceleration voltage below approximately 8 kV. By optimizing plasma formation, we successfully achieved beam extraction of up to 40 A at 15 kV and a pulse length in excess of 0.25 ms. Its low-voltage and high-current pulsed-beam properties enable us to apply this high-power neutral beam injection into a high-beta compact torus plasma characterized by a low magnetic field.

Vector and Scalar Tomography of Compact Toroid Plasmas

Multichannel spectroscopic diagnostics have been installed on the TS-4 compact toroid experiment at the University of Tokyo with view chords designed for measuring 2D ion bulk flow velocities and temperatures. Computerized vector tomography reconstructs velocities from the Doppler-shifted line shapes and scalar emissivity from the scalar tomography of the zeroth moment measurements. The first results show bulk ion flow field in a 2D plane with significant cross-field flow possibly due to the slingshot effect during reconnection merging of two counter-helicity spheromaks. The results are limited by a center column and available viewports in the vacuum chamber. This proof-of-principle experiment shows that, with increased access, 3D velocity flow fields can be completely reconstructed in compact toroid plasma experiments.

Monte Carlo Simulation on a Neutralization Process of an Accelerated Compact Torus Plasma

A new concept of particle fueling method is proposed, and its feasibility is studied by a Monte Carlo simulation; by using a compact torus (CT) injection technique and subsequent neutralization of an accelerated CT plasma, ultrahigh-speed neutral-particle flow can be generated. The charge-exchange reaction between ions in a CT plasma and neutral particles in the neutralizer is found to work efficiently to neutralize the CT plasma within a 1-m long neutralizer in a case that the neutral gas exceeds 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> torr.

Observation of a Helical Self-Organized State in a Compact Toroidal Plasma

A nonaxisymmetric stable magnetohydrodynamic (MHD) equilibrium within a prolate cylindrical conducting boundary has been produced experimentally. It has m=1 azimuthal symmetry, helical distortion, and flat lambda profile, all in agreement with the computed magnetically relaxed minimum magnetic energy Taylor state. Despite varied initial conditions determined by two helicity injectors on the device, this same equilibrium consistently emerges as the final state. These results therefore describe a new example of self-organization in an MHD plasma.

Relaxed Plasma State in Compact Tori

Ever since J. B. Taylor (Phys Rev Lett 33:1139, 1974) published his theory with a model to derive the Taylor state \( \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}}{\nabla } \times \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}}{\text{B}} = {{\upmu}}\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}}{\text{B}} \) and to explain the reversed field pinch and spheromak configuration, magnetic helicity has been believed to play an important role as a global invariant in the process of self-organization and relaxation for magnetized plasmas. In the present work, the lowest eigenvalues μmin, which associated with relaxed force-free compact toroidal plasma devices are calculated numerically. We prove the applicability of the collocation method through out a developed numerical programme to investigate such task. We calculate μmin for toroidal compact tori (CT) with arbitrary cross-sections and aspect ratios approaches to unity (\( {{\upalpha}} = {\text{R/a}} \to 1 \), R and a are the major and some sort of minor radius respectively). For a perfect conducting CT wall, we were able to obtain new results concerning satisfaction of the toroidal flux vanishing boundary condition \( \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}}{\text{B}} \cdot \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\rightharpoonup}$}}{\text{n}} = 0 \). Besides, the magnetic field topology inside the CT are well represented. The plots showed a good fulfilment of the boundary condition along the whole boundaries of different cross sections. Dependence of μ on the aspect ratio is also obtained. Several runs of the programme for various wave numbers k showed that μa is very insensitive to the choice of k.

Effects of magnetic field and target plasma on the penetration behaviour of compact toroid plasma by heat load measurements in CPD

Compact toroid (CT) injection experiments have recently been carried out in a compact plasma wall interaction experimental device (CPD) in the presence of toroidal as well as poloidal magnetic field and low density RF target plasma. Spectroscopic studies have been carried out using an IR camera, an IR spectrometer and an Hα monitor to estimate the heat load deposition on the target and also to study the magnetic field as well as RF target plasma effects on CT plasma penetration behaviour. Using IR camera imaging of the target plate, heat load deposition on it has been estimated from the observed temperature rise of the plate and has been compared with the energy content of CT plasma. The estimation suggests that nearly 700 J of energy is uniformly deposited on the target plate under vacuum condition without any magnetic field. On the other hand, post exposure analysis of target plate using scanning electron microscopy has indicated local melting of the target plate. It has been observed that radial penetration of CT towards the target plate is reduced with the increase in the external magnetic field, both toroidal and poloidal. Moreover, the presence of RF target plasma in external magnetic fields seems to change the CT penetration behaviour significantly.

Plasma confinement by circularly polarized electromagnetic field in toroidal geometry

A novel plasma confinement concept based on plasma confinement by electromagnetic pressure of circularly polarized electromagnetic fields is proposed. Practical implementation of this concept in a toroidal device is suggested. In this concept the confining field frequency is in the lower range such that the size of the device is much smaller than the vacuum wavelength. Most of the previous radio-frequency (rf) confinement concepts of unmagnetized plasma were related to confinement in rf cavities which operated at high frequency for which the size of the cavity is comparable to the wavelength. Operation at lower frequencies simplifies rf design, reduces Ohmic losses in the conducting walls and probably makes application of superconductors for wall materials more feasible. It is demonstrated that circular (or nearly circular) polarization of the electromagnetic field is required for confinement from both the equilibrium and stability considerations. Numerical analysis of plasma confinement for magnetohydrodynamic plasma model in two-dimensional toroidal geometry is performed. Within this model plasma is confined by the applied rf fields and its equilibrium is stable. Technically feasible compact and medium size toroidal plasma confinement devices based on this concept are proposed. Application of this approach to the fusion reactor requires use of superconducting materials for the toroidal shell to reduce the Ohmic losses. Further theoretical and experimental studies are required for a more reliable conclusion about the attractiveness of this plasma confinement concept.

Ion kinetic effect on low-n toroidal modes in compact torus plasmas

Ion kinetic effect on the low-n toroidal modes in compact torus (CT) plasmas such as spherical tokamak (ST), spheromak, field reversed configuration (FRC), and compact reversed field pinch (RFP) was investigated by using different ion species. The experimental results indicate that ion kinetic stabilization of the low-n modes was obtained in the low-q95 regime. It was clearly shown that the n = 1 mode was stabilized in the FRC under the low-S* regime (S*: averaged number of ion skin depth). The suppression of the n = 2 mode activity in the low-S* RFP plasma was also observed. In the high-q95(∼4) ST regime, all low-n (1 to 4) toroidal modes were stable for both high S* and low S*. © 2006 Wiley Periodicals, Inc. Electr Eng Jpn, 157(4): 1–6, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20217

Gyroscopic effect in compact toroidal plasmas

The gyroscopic effect in compact toroidal plasmas is investigated using a minimum energy principle in which the angular momentum is added as a constraint. If attention is limited to incompressible plasmas, the flow and field vectors can be expanded in eigenfunctions of the curl (Beltrami functions). Then the flow and field appear, in general, as a spectrum of Beltrami vectors. Adding the angular momentum as a constraint has a strong effect on the energy ordering of the system. Without this constraint, the state with lowest “ordered” energy has a fine structure element that would, in practice, decay, leading to a force-free state. However, if a nonzero angular momentum is specified, then the lowest-energy state has finite pressure and significant flow. For angular momentum of sufficient magnitude, this is a “smooth” state that should have good magnetic confinement. These effects may be an indicator that the gyroscopic effect improves the stability. However, this is by no means certain since a rigorous link between minimum energy and stability in a flowing plasma has not yet been established.

Comparative study of electron density profiles in compact torus plasma merging experiments

AbstractFollowing two previous papers on comparative studies of the electron density distributions for a single compact torus (CT) and a spherical tokamak (ST), and for a single ST and a merged ST, a comparative study on the dynamics of the electron density profile during and after the CT and ST plasma merging process was performed. The sharpness of the peak in the electron density profile around the midplane just after the merging of CT with a low safety factor (q value) such as RFP or spheromak is found to be related to the speed of the magnetic axis during the plasma merging process. It is also found that the electron density gradient near the plasma edge in a high‐q ST is larger than that of a low‐q CT. High‐q ST is found to have a magnetic structure which is able to sustain a large thermal pressure by a strong j × B force. Despite these differences in the electron density profile between CT and ST during merging, the confinement characteristics evaluated from the number of electrons confined within the magnetic separatrix after the completion of the merging are almost similar between the merging CT and the merging ST. For all configurations, the electron density profiles after the completion of the merging are analogous to those of the corresponding single configuration produced without the merging process. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 148(2): 14–21, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10312

Influence of Non-Axisymmetric n=1 Toroidal Mode Instabilities on Reconstruction Measurements of Magnetic Surfaces for Compact Torus Plasmas

Influence of Non-Axisymmetric n=1 Toroidal Mode Instabilities on Reconstruction Measurements of Magnetic Surfaces for Compact Torus Plasmas