Theta-pinch Plasma Research Articles

Time-resolved measurement of the free electron and neutral gas line density in a hydrogen theta-pinch plasma target by two-color interferometry

A hydrogen theta-pinch plasma is diagnosed by two-color interferometry to determine the line density of the free electrons and the hydrogen gas. From the ratio of these line densities, the effective ionization can be calculated. The free electron line density and the effective ionization degree are the most essential quantities for the evaluation of the plasma regarding its applicability as a target in context of plasma ion beam interaction to increase the charge state of the ion beam (plasma stripper). The two-color interferometric diagnostic shows that both line densities exhibit a periodic behavior predefined by the periodic current and at the time the free electron line density reaches a local maximum, the hydrogen line density falls to a local minimum. This occurs, because the plasma axially expands from the coil center by evading the radial compression force of the magnetic field, creating an ionizing wave in the residual gas. The theta-pinch was set up in two versions, with one of them using a cylindrical coil and the other using a spherical coil. For the cylindrical version, the best working point regarding the free electron line density and the effective ionization degree is cm−2 and at 20 Pa and 16 kV. In contrast, for the spherical version, lower values of and at 20 Pa and 18 kV were found. Additionally, a new set-up is proposed for optimizing the plasma target regarding the free electron line density and the effective ionization degree, as well as how to maintain them on a sufficient level for several s.

Measurement of the free electron line density in a spherical theta-pinch plasma target by single wavelength interferometry

A single wavelength heterodyne interferometer has been set up to investigate the free electron density integrated axially along the line of sight (line density) in a theta-pinch plasma to determine its applicability as a plasma target for ion beam stripping. The maximal line density reached in this experiment was cm−2 at 80 Pa and 20 kV. The findings demonstrate the pulsed character of the line density and its increase by raising the load voltage and the working gas pressure. Additionally, the results were compared with spectroscopic free electron density estimations, which were carried out by H-line broadening and peak separation. The time behavior of the line density indicates that its peak value is delayed by about 10 µs compared to the spectroscopic results. This effect is due to the formation of an extended, magnetically compressed plasma column in the vicinity of the current maximum, although the highest volumetric free electron density is reached near the current zero crossing. Since the line density is an essential parameter in describing the stripping capabilities of the plasma target, the interferometric diagnostic is superior to a spectroscopic diagnostic, because it directly provides integrated values along the line of sight. Furthermore, the measurements of the line density in this experiment partially show nonphysical negative values, which is due to gaseous effects and residual shot vibrations.

Automated signal classification in the C-2W fusion experiment.

In TAE Technologies' current experimental fusion device, C-2W (also called "Norman"), record breaking, advanced beam-driven field-reversed configuration plasmas are produced and sustained in steady state utilizing variable-energy neutral beams, expander divertors, end-bias electrodes, and an active plasma control system. With a rapid shot-pace and an extensive number of data channels, the amount of data generated necessitates automated signal processing. To this end, a machine learning algorithm consisting of a multi-layered neural network as well as other data processing software has been designed for signal feature identification, allowing for accurate and fast signal classification, anomalous condition detection, and providing for signal pre-processing. With a small set of training data, the neural network can be "bootstrapped" to provide a robust classification system while minimizing human oversight. An example using data from the theta pinch plasma formation pulsed power system is presented. With an overall accuracy of ∼97%-having classified more than 5 × 106 pulsed power signals-the classification scheme is more than sufficient to fine-tune machine set points. However, this technique can be used for near-real-time preprocessing of any plasma physics signal and has wide ranging application in fusion experiments for the varied data produced by plasma diagnostics.

Kinetic simulations of anomalous resistivity in high-temperature current carrying plasmas

Anomalous resistivity caused by lower hybrid drift instabilities (LHDIs) has been extensively studied in the literature, and has been invoked to explain the rates of magnetic diffusion and plasma profile evolution in the low-density plasma periphery of theta and screw pinch plasmas. This article presents a fully kinetic simulation study of LHDIs with a realistic mass ratio using the particle-in-cell code Chicago over a large range of drift speeds, 0.5<vdi2/vti2<14.5, plasma β, β≥2×10−5, and temperature ratios, 0.1<Te/Ti<10. The resistivity quantified from the simulations is compared with the analytic estimates from the literature and a generalized resistivity expression is presented, which can be applied over the entire range of plasma environments that were simulated in this study. This expression uses the amplitude of the LHDI fluctuations resulting from instability saturation by electron resonance broadening rather than plateau formation, current relaxation, or ion trapping. The generalized resistivity expression is appropriate for full-scale fluid simulations that encounter a large range of plasma conditions, and has better agreement with the resistivity quantified from kinetic simulations than previous expressions in the literature, particularly at low plasma β. Anomalous resistivity has also been considered as an explanation of the rapid magnetic field dissipation during magnetic reconnection events at large plasma β. At a large β, the magnetic field fluctuations of LHDIs can dominate the electrostatic fluctuations and this article investigates enhanced collisionality due to the nonlinear magnetic force, δJ×δB/c.

Study on a dense theta pinch plasma for ion beam stripping application for FAIR

Study on a dense theta pinch plasma for ion beam stripping application for FAIR

Estimation of the Current Sheath Dynamics and Magnetic Field for Theta Pinch by Snow Plow Model Simulation

The snowplow model and shock wave equation are used to simulate theta pinch plasma experiment. The model calculates the sheath velocity, radial position, plasma current, plasma temperature, and the induced magnetic field of the current sheath. The model uses two phases: the radial inward phase and the reflected shock phase. The governing equations are derived by using the current factor and the mass factor. The model calculates the induced magnetic field of the plasma by the deriving equation and the magnetic field of the coil. Also, the model is operated by using the Hydrogen, Deuterium and Tritium gases at the same of the mass density. The results illustrate that the temperature and the induced magnetic field are higher for tritium gas, which has a higher molecular weight, while the current sheath velocity of tritium is lower. Some results of the model were compared with Lee model and other paper.

Study on the Theta Pinch Plasmas for Applied as Ion Stripper

Study on the Theta Pinch Plasmas for Applied as Ion Stripper

Super-Alfvénic translation of a field-reversed configuration into a large-bore dielectric chamber.

An experimental device to demonstrate additional heating and control methods for a field-reversed configuration (FRC) has been developed. The newly developed device, named FRC Amplification via Translation (FAT), has a field-reversed theta-pinch plasma source and a low-elongation dielectric (transparent quartz) confinement chamber with quasi-static confinement field. In the initial experiments on the FAT device, FRC translation and trapping were successfully demonstrated. Although the typical elongation of the trapped FRC in the confinement region was roughly three, no disruptive global instability, such as tilt, was observed. The FAT device increases the latitude to perform translation-related experiments, such as those concerning inductive current drive, equivalent neutral beam injection effects, and wave applications.

Determination of Hydrogen Density by Swift Heavy Ions.

A novel method to determine the total hydrogen density and, accordingly, a precise plasma temperature in a lowly ionized hydrogen plasma is described. The key to the method is to analyze the energy loss of swift heavy ions interacting with the respective bound and free electrons of the plasma. A slowly developing and lowly ionized hydrogen theta-pinch plasma is prepared. A Boltzmann plot of the hydrogen Balmer series and the Stark broadening of the H_{β} line preliminarily defines the plasma with a free electron density of (1.9±0.1)×10^{16} cm^{-3} and a free electron temperature of 0.8-1.3eV. The temperature uncertainty results in a wide hydrogen density, ranging from 2.3×10^{16} to 7.8×10^{18} cm^{-3}. A 108MHz pulsed beam of ^{48}Ca^{10+} with a velocity of 3.652 MeV/u is used as a probe to measure the total energy loss of the beam ions. Subtracting the calculated energy loss due to free electrons, the energy loss due to bound electrons is obtained, which linearly depends on the bound electron density. The total hydrogen density is thus determined as (1.9±0.7)×10^{17} cm^{-3}, and the free electron temperature can be precisely derived as 1.01±0.04 eV. This method should prove useful in many studies, e.g., inertial confinement fusion or warm dense matter.

Determining the ion temperature and energy distribution in a lithium-plasma interaction test stand with a retarding field energy analyzer.

The ThermoElectric-driven Liquid-metal plasma-facing Structures (TELS) experiment at the University of Illinois is a gas-puff driven, theta-pinch plasma source that is used as a test stand for off-normal plasma events incident on materials in the edge and divertor regions of a tokamak. The ion temperatures and resulting energy distributions are crucial for understanding how well a TELS pulse can simulate an extreme event in a larger, magnetic confinement device. A retarding field energy analyzer (RFEA) has been constructed for use with such a transient plasma due to its inexpensive and robust nature. The innovation surrounding the use of a control analyzer in conjunction with an actively sampling analyzer is presented and the conditions of RFEA operation are discussed, with results presented demonstrating successful performance under extreme conditions. Such extreme conditions are defined by heat fluxes on the order of 0.8 GW m-2 and on time scales of nearly 200 μs. Measurements from the RFEA indicate two primary features for a typical TELS discharge, following closely with the pre-ionizing coaxial gun discharge characteristics. For the case using the pre-ionization pulse (PiP) and the theta pinch, the measured ion signal showed an ion temperature of 23.3 ± 6.6 eV for the first peak and 17.6 ± 1.9 eV for the second peak. For the case using only the PiP, the measured signal showed an ion temperature of 7.9 ± 1.1 eV for the first peak and 6.6 ± 0.8 eV for the second peak. These differences illustrate the effectiveness of the theta pinch for imparting energy on the ions. This information also highlights the importance of TELS as being one of the few linear pulsed plasma sources whereby moderately energetic ions will strike targets without the need for sample biasing.

Study of the Theta Pinch Plasma Numerically by Varying β Parameter and Applied Current

The implosion dynamics of theta pinch is studied by simulation technique through snow plow model with the help of MATHEMATICA software. The differential equation of motion for plasma implosion is derived and solved numerically by using this software. The variation effects of β parameter (plasma kinetic pressure to plasma magnetic pressure ratio) and applied current is examined on normalize radius of imploded plasma by variations in normalized time. The graphs between normalized radius and normalized time of imploded plasma indicates that theta pinching effect can be improved by decreasing β value and increasing in the magnitude of applied current.

Hydrogen plasma dynamics in the spherical theta pinch plasma target for heavy ion stripping

Due to the superior ability of dense and highly ionised plasmas to ionise penetrating heavy ion beams to degrees beyond those reachable by common gas strippers, many experiments have been performed to find suitable plasma generators for this application. In the field of gas discharges, mainly z-pinch devices have been investigated so far, which are known to be limited by the nonlinear focusing effects of the plasma columns sustaining current and by electrode erosion. The spherical theta pinch has therefore been proposed as a substitution for the z-pinch, promising progress by inductive rather than capacitive coupling and displacement of the outer magnetic field by the dense, diamagnetic discharge plasma. As yet mainly experiments with argon/hydrogen mixture gas have been performed, which is not suitable for the application as a plasma stripper, this paper describes the first detailed analysis of the plasma parameters and dynamics of a hydrogen plasma created by the spherical theta pinch. These include the time integrated and time resolved electron density, the dynamics of the plasma in the discharge vessel, the comparison with the argon dominated plasma, and an outlook to reachable characteristics with similar devices.

Single- and Repetitive-Pulse Conical Theta-Pinch Inductive Pulsed Plasma Thruster Performance

Impulse bits produced by conical theta-pinch inductive pulsed plasma thrusters possessing cone angles of 20°, 38°, and 60°, were quantified for 500-J/pulse operation by direct measurement using a hanging pendulum thrust stand. All three cone angles were tested in single-pulse mode, with the 38° model producing the highest impulse bits at roughly 1-mN-s operating on both argon and xenon propellants. A capacitor charging system, assembled to support repetitively pulsed thruster operation, permitted testing of the 38° thruster at a repetition rate of 5 Hz at power levels of 0.9, 1.6, and 2.5 kW. For similar conditions, the average thrust measured during repetitive-pulse operation exceeded the value obtained when the single-pulse impulse bit is multiplied by the repetition rate, suggesting that a greater impulse bit per pulse was produced when operating in the repetitive-pulse mode.

Review of the State-of-the-Art Development of the Spherical Theta Pinch Plasma Source

After the spherical theta pinch had been developed at the Institute of Applied Physics in Frankfurt, considerable progress in creating high efficiency and long lifetime plasma sources had been made. Several devices have been built to study the characteristics of the spherically confined plasma. Scaling rules and investigated setups are presented, showing devices with electron densities of up to some $10^{23}~{\rm m}^{-3}$ , electron temperatures of several eV and confinement times of ${\sim}{\rm 10}~\mu{\rm s}$ . Typical dimensions are: discharge vessel radius $R=10~{\rm cm}$ , radius of the pinched plasma $r=2~{\rm cm}$ , initial gas pressures of 1–200 Pa at currents of up to 35 kA and resonance frequencies of 10–40 kHz. During the harmonic discharge of the stored energy, the plasma periodically pinches and expands again as long as the current rise rate creates sufficient electrical field strengths for ionization, which allows for pinching sequences in the millisecond range. The principles, improvements, theoretical approaches, and applications including pulsed ion source, plasma ion stripping, and vacuum ultraviolet flash lighting are discussed.

Theoretical Study of Pinching Effect in the Theta Pinch Plasma by Variation in Kinetic Pressure

In this paper we have used the snow plow model of plasma pinching has been used for the study of pinching effect in the theta pinch plasma. One of the parameter, kinetic pressures of plasma has been varied and its effecton the pinching effect is examined by the solution of differential equation of motion of imploded plasma by using the MATHEMATICA software.

Pellet injectors developed at PELIN for JET, TAE and HL-2A

Three innovative repetitive and multi-barrel deuterium ice pellet injectors have been designed for plasma investigations in fusion facilities. The pellet injector for JET can inject small pellets (variable volumes within 1-2 mm 3) at frequency up to 67 Hz and velocity 80–200 m/s for ELM (edge localised modes) control and large pellets (volume within 35–70 mm 3) at frequency up to 15 Hz and velocity 100–300 m/s for plasma fueling. A new extrusion nozzle has been designed and tested to improve the extrusion stability and injection reliability. The pellet injector for HL-2A has a piston type extruder with a variable cross-section. It allows pushing out deuterium ice rod from the extruder at velocity up to 100 mm/s and injecting a limited number of pellets 1.3 mm diameter and variable length in the range 1.3–1.7 mm at frequency up to 30 Hz for plasma fueling. The pellet injector for fueling of a facility with theta pinch plasma is capable to inject one pellet with variable length from 5 to 15 mm and diameter 1 mm with extremely low velocity of about 40–50 m/s, followed by eleven pellets with variable length from 1 to 3 mm and diameter 0.9 mm at velocities 100–200 m/s. A novel gas valve with in-built punch mechanism has been developed to simplify the injector design. The design and distinct features of these three injectors are presented.

Characterization of a theta-pinch plasma using triple probe diagnostic

Plasma diagnostics were carried out in a theta-pinch device to investigate the applicability for plasma–material interaction under fusion-like conditions. A series of triple probe diagnostics show that the plasma is sustained for approximately 80μs at each pulse, with 3.0 (10)21m−3 plasma density and up to 40eV electron temperature when a 32μF main capacitor is discharged at 20kV. In order to increase plasma density and temperature, an RF antenna is installed near one end of a Pyrex tube and a 50μF preionization capacitor is connected to an electrode placed at the same end as the antenna. In this configuration, several time delays between the main and preionization capacitors are tested. When the preionization capacitor was triggered 45μs before the main bank discharge, it resulted in high energetic plasma being obtained with a few density spikes at 1022m−3 and electron temperature around 100eV.

LOW-ANGLE THOMSON SCATTERING EXPERIMENT FOR DETERMINATION OF PLASMA ELECTRON DENSITY AND TEMPERATURE

In this work we show results of measurements of the electron density ne and temperature Te by using small-angle Thomson scattering in a theta-pinch plasma. The method of measurement and analysis developed here is based on integration of only two ranges of the scattering profile. This method simplifies the experimental set up, because it needs only two detectors for simultaneous determination of ne and Te, instead of the commonly 5–7 detectors used in 90° experiments.

Experimental electron-impact excitation rate coefficients for beryllium-like Si XI

Absolute excitation rate coefficients for a large number of levels (up to n = 4) of the singlet and triplet system of Si XI have been experimentally determined in a well-diagnosed theta-pinch plasma with the plasma spectroscopic method. The population of the metastable triplet ground state has been derived from the intensity of the intercombination line. A theoretical population model has been developed to investigate whether collisional coupling of the levels and cascading affected the measurements. The model has also been used to calculate line intensity ratios of strongly density or temperature dependent line pairs which can be used for plasma diagnostic purposes. The measured effective excitation rate coefficients are accurate to about ±50%. All of them, apart from the value for the 2s4s 3S1 level, agreed within the error margins with existing theoretical data.

Experimental electron-impact excitation rate coefficients for lithium-like Si XII

Absolute excitation rate coefficients have been determined experimentally for all excited states up to 1s25f of the lithium-like ion Si XII using a well diagnosed theta-pinch plasma. A new method has been developed to determine the most crucial quantity of these measurements, the absolute concentration of the silicon ions in the plasma. For this we made use of the fact that sufficiently high lying levels are in partial local thermodynamic equilibrium (PLTE) with the ground state of the next higher ionization stage. A theoretical population model has been developed to study the influence of the different processes that might contribute to the population of the different levels at our plasma parameters. For most levels the theoretical excitation rate coefficients were found to be in fair agreement with the theoretical ones. The error of the experimentally determined excitation rate coefficients is of the order of ±50%.