High Temperature Plasma Physics Research Articles

Positive self-bias in a magnetized CCP discharge

Radio frequency (RF) plasmas are commonly used for surface treatments and plasma heating processes. Controlling the heat flux from the plasma to the RF electrode is a crucial issue for optimizing these processes and is, therefore, the subject of considerable research in the low- and high-temperature plasma physics communities. In an asymmetric capacitively coupled plasma discharge, the ions accelerated by the direct current (DC) self-bias are the prime factor of the wall heating process. In this work, investigations have been performed with the aim to act on the DC self-bias in a linear magnetized RF environment. The lateral side and one face of the electrode have been covered by ceramic in order to limit the electron flux toward these surfaces. The variations in DC self-bias voltage as a function of the gas pressure, coupled RF power, and tilt angle between the RF electrode and the axial magnetic field have been studied. A new regime was discovered at low pressures, higher magnetic fields, and grazing angles for which the self-bias is positive. An analytical model was developed, which is in agreement with the experimental results.

TRITIUM CONCENTRATION IN MONTHLY PRECIPITATION NEAR THE FUSION TEST FACILITY IN JAPAN BEFORE AND AFTER THE DEUTERIUM PLASMA EXPERIMENT.

In Japan, the deuterium plasma experiment using the Large Helical Device was started at the National Institute for Fusion Science (NIFS) in March 2017 to investigate high-temperature plasma physics and hydrogen isotope effects in research leading towards the realisation of fusion energy. The deuterium plasma experiment produces small amount of tritium by fusion reactions. To understand any impacts by the experiment to the surrounding environment, monthly precipitation samples have been collected at the NIFS site since November 2013 to assess the relationship between isotope composition and chemical species in precipitation including tritium. By comparing data before and after the deuterium plasma experiment start, it was found that tritium released from the main stack of the fusion test facility had no impact on the environment surrounding NIFS.

Участь у термоядерних дослідженнях «ЄВРАТОМ»: результати, досягнуті за програмою «Горизонт-2020», та перспективи в наступній програмі ЄС на 2021–2027 рр. (стенограма доповіді на засіданні Президії НАН України 17 лютого 2021 р.)

The report emphasizes the importance of developing thermonuclear research in the world, the need for further integration of Ukrainian research institutions into the European research area and increasing the participation of Ukrainian scientists in world-class research in plasma physics and controlled thermonuclear fusion. The urgency and complexity of the problem of controlled thermonuclear fusion, which covers not only various aspects of high-temperature plasma physics as the basis of energy of the future, but also problems of thermonuclear reactors, materials science, engineering aspects of thermonuclear energy, etc. are discussed.

The Physics of Accretion Onto Highly Magnetized Neutron Stars

Studying the physical processes occurring in the region just above the magnetic polesof strongly magnetized, accreting binary neutron stars is essential to our understanding of stellarand binary system evolution. Perhaps more importantly, it provides us with a natural laboratoryfor studying the physics of high temperature and density plasmas exposed to extreme radiation,gravitational, and magnetic fields. Observations over the past decade have shed new light on themanner in which plasma falling at near the speed of light onto a neutron star surface is halted. Recentadvances in modeling these processes have resulted in direct measurement of the magnetic fieldsand plasma properties. On the other hand, numerous physical processes have been identified thatchallenge our current picture of how the accretion process onto neutron stars works. Observationand theory are our essential tools in this regime because the extreme conditions cannot be duplicatedon Earth. This white paper gives an overview of the current theory, the outstanding theoreticaland observational challenges, and the importance of addressing them in contemporary astrophysicsresearch.

Isotope Composition and Chemical Species of Monthly Precipitation Collected at the Site of a Fusion Test Facility in Japan.

The deuterium plasma experiment was started using the Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS) in March 2017 to investigate high-temperature plasma physics and the hydrogen isotope effects towards the realization of fusion energy. In order to clarify any experimental impacts on precipitation, precipitation has been collected at the NIFS site since November 2013 as a means to assess the relationship between isotope composition and chemical species in precipitation containing tritium. The tritium concentration ranged from 0.10 to 0.61 Bq L−1 and was high in spring and low in summer. The stable isotope composition and the chemical species were unchanged before and after the deuterium plasma experiment. Additionally, the tritium concentration after starting the deuterium plasma experiment was within three sigma of the average tritium concentration before the deuterium plasma experiment. These results suggested that there was no impact by tritium on the environment surrounding the fusion test facility.

Equipment complex and method for aiming laser beams at specified target points

We describe a technique for automatic alignment and targeting of laser beams on a specific point in space using the simulated cubical targets typically used for focusing and targeting of laser light in a multiple beamline laser facility where the target is illuminated in cubical symmetry for high-temperature plasma physics research. We describe the targeting system and the steps for targeting and focusing laser beams at specific points in the target chamber, as well as a method for measuring the length of an arbitrary line segment on a distant object from an image on a display, and an experimental estimate of the accuracy of such measurements on a test object—the edge of the simulated cubical target.

Evaluation of a flat-field grazing incidence spectrometer for highly charged ion plasma emission in soft x-ray spectral region from 1 to 10 nm.

A flat-field grazing incidence spectrometer operating on the spectral region from 1 to 10 nm was built for research on physics of high temperature and high energy density plasmas. It consists of a flat-field grating with 2400 lines/mm as a dispersing element and an x-ray charged coupled device (CCD) camera as the detector. The diffraction efficiency of the grating and the sensitivity of the CCD camera were directly measured by use of synchrotron radiation at the BL-11D beamline of the Photon Factory (PF). The influence of contamination to the spectrometer also was characterized. This result enables us to evaluate the absolute number of photons in a wide range wavelength between 1 and 10 nm within an acquisition. We obtained absolutely calibrated spectra from highly charged ion plasmas of Gd, from which a maximum energy conversion efficiency of 0.26% was observed at a Nd:YAG laser intensity of 3 × 1012 W/cm2.

Applied and fundamental aspects of fusion science

Fusion research is driven by the applied goal of energy production from fusion reactions. There is, however, a wealth of fundamental physics to be discovered and studied along the way. This Commentary discusses selected developments in diagnostics and present-day research topics in high-temperature plasma physics.

A high time resolution x-ray diagnostic on the Madison Symmetric Torus.

A new high time resolution x-ray detector has been installed on the Madison Symmetric Torus (MST) to make measurements around sawtooth events. The detector system is comprised of a silicon avalanche photodiode, a 20 ns Gaussian shaping amplifier, and a 500 MHz digitizer with 14-bit sampling resolution. The fast shaping time diminishes the need to restrict the amount of x-ray flux reaching the detector, limiting the system dead-time. With a much higher time resolution than systems currently in use in high temperature plasma physics experiments, this new detector has the versatility to be used in a variety of discharges with varying flux and the ability to study dynamics on both slow and fast time scales. This paper discusses the new fast x-ray detector recently installed on MST and the improved time resolution capabilities compared to the existing soft and hard x-ray diagnostics. In addition to the detector hardware, improvements to the detector calibration and x-ray pulse identification software, such as additional fitting parameters and a more sophisticated fitting routine are discussed. Finally, initial data taken in both high confinement and standard reversed-field pinch plasma discharges are compared.

Determining the center of the interaction chamber of a multichannel laser installation

This paper describes a method and a precision optical sensor for remotely determining the coordinates of the center of a spherical interaction chamber (the target chamber) 10 m in diameter to within ±20 μm when the target of a multichannel laser installation intended for studies in high-temperature plasma physics is being automatically adjusted.

Multiparametric Data Processing on Fusion Device Stellarator L-2M

In studying the high-temperature plasma physics, the researcher is faced with the need to process large amounts of semi-structured data. Dozens of multi-channel diagnostics, the results of which are multi-million ensembles of time samples of plasma signals, are usually installed at the same time in thermonuclear devices. These samples contain information necessary to understand the mechanisms of relation between plasma macroparameters (plasma density and temperature, plasma current, magnetic field, auxiliary heating power, etc) and plasma microparameters determined by turbulence (energy, spectral composition, amplitude density distribution, etc). The methods of analysis of plasma signals based on the processing of a single time realization of turbulence cannot be used as a basis for comparison with macroparameters and identification of new patterns of plasma containment in a magnetic trap. The main objective of this work is to obtain information about the relationship between the macro- and microparameters of the plasma based on the structuring of the acquired data and its multi-parameter processing using the modern software and hardware.

Statistical model of radiation losses for heavy ions in plasmas

The new statistical approach for calculation of radiation processes with heavy multielectron ions in plasma is developed. The method consists in consideration of atomic structure as a condensed medium, characterized by the spectrum of elementary excitations with plasma frequency, determined by local atomic electron density. The radiation losses in this model are due to excitation of plasma type oscillations in atom under its collisions with plasma electrons and could be expressed in a universal statistical form for all sorts of multielectron ions. The calculations of radiation losses on tungsten ions are performed in the wide range of plasma temperature variation, typical for physics of high temperature plasma with magnetic confinement. It is shown that the universal statistical approach results are within the data scattering of current numerical codes. The proposed statistical method for description of collective excitations in complex atoms for calculations of plasma radiation losses is of general physical interest. It allows obtaining the necessary data faster with the lesser computational resources.

Editorial of the Special Historical Issue

This Special Historical Issue contains two papers of historical interest relating to the evolution of the US fusion program in the 1960s. The first, the 1966 AEC Policy and Action Paper on Controlled Thermonuclear Research, was prepared in response to a request from the US congressional (House-Senate) Joint Committee on Atomic Energy and the US House of Representatives Appropriations Committee. The second describes the history of a fusion concept unique to the US program, the Astron. It was first proposed in 1956, began construction in 1958, and ended in 1973. The 1966 policy paper was prepared within the Controlled Thermonuclear Research (CTR) Branch of the AEC’s Division of Research by Amasa S. Bishop and Stephen O. Dean, assisted by Richard F. Post of the Lawrence Livermore National Laboratory (then Lawrence Radiation Laboratory). The AEC was the original predecessor agency of the current US Department of Energy (DOE); its Research Division was the original predecessor of the current DOE Office of Science; and its CTR Branch was the original predecessor of the current DOE Office of Fusion Energy Sciences (OFES). Bishop was the first head of the US fusion program (1953–1956) and wrote the classic early history of fusion in the book Project Sherwood (Addison-Wesley Publishing Company 1958). He worked at the Princeton Plasma Physics Laboratory in the early 1960s, returning to once more head the US fusion program at AEC (1965–1970). He was a recipient of Fusion Power Associates Distinguished Career Award in 1992 and passed away in 1997. Dick Post is a pioneer of the US fusion program, dating back to the mid 1950s. He was the recipient of Fusion Power Associates first Distinguished Career Awards in 1987 and is still active in research. Dean was a member of the AEC CTR Branch (1962–1969) and again 1972–1979, after a 3-year stint at the US Naval Research Laboratory doing experimental research on laserproduced plasmas. He has been President of Fusion Power Associates since 1979. The Policy Paper was transmitted to Congress on July 11, 1966 by then AEC chairman Glenn Seaborg. A copy of the transmittal letter is contained in the paper. The transmittal letter states that the AEC’s General Advisory Committee (GAC) and the President’s Science Advisory Committee (now called PCAST), have reviewed the report and endorse ‘‘the importance of the program and the need for an intensified effort.’’ The report also includes, as appendices, a 1962 review of the CTR program by the AEC’s GAC and a review of the report by a special committee set up by the AEC in late 1965. The report concludes, ‘‘By any standard whatsoever, controlled thermonuclear research must be counted as one of the most challenging and potentially important efforts in the history of mankind.’’ It notes that ‘‘there are also compelling interests of a more basic nature for studying the behavior of plasmas’’ since ‘‘its bearing on other fields of knowledge and on the future activities of mankind may well be profound.’’ The central policy statement in the report reads as follows: ‘‘The AEC program will continue to be motivated by interest in eventually achieving controlled thermonuclear power. It is recognized, however, that there are many benefits which will accrue from an investigation of this vast and largely-unexplored field. As a result, the effort will emphasize not only a detailed understanding of the physics of high temperature plasmas and the means for confining S. O. Dean (&) Fusion Power Associates, Gaithersburg, MD 20879, USA e-mail: fusionpwrassoc@aol.com

The founder of experimental physics of high-temperature fusion plasma

The founder of experimental physics of high-temperature fusion plasma

Pulsed Power System for the Orion High Power Laser

This paper describes the design and testing of the pulsed power system for the Orion Laser, which will be used for high temperature and density plasma physics research. The system supplies 8 MJ of energy to laser amplifiers and Faraday rotators. It consists of 17 capacitor bank modules, each with up to sixteen 150 μF charge storage capacitors, 25 kV power supply and controls. The energy is delivered by spark-gap switches to give a pre-pulse to ionise the flashlamps and a main pulse of 490 μs. The system delivers 5 shots a day with less than 0.2% variation in charge voltage.

Design and initial operation of the LDX facility

The levitated dipole experiment (LDX) explores the physics of high-temperature plasmas confined by a dipole magnetic field. Stable high-beta plasma has been created and confined by the magnetic field of a superconducting coil. Discharges containing trapped electrons form when microwaves cause strong perpendicular heating at cyclotron resonance. To eliminate the losses to the supports, the magnetic dipole (a superconducting solenoid) will be magnetically levitated for several hours. The dipole magnetic field is generated by a Nb 3 Sn floating coil (F-coil), a maximum field of 5.3 T, operating for up to 2 h. A NbTi charging coil (C-coil) surrounds a portion of the vacuum chamber and induces the current in the floating coil. After the F-coil is lifted to the center of the chamber, the levitation coil (L-coil), made from high-temperature superconductor, magnetically supports it. In the first year of operation, the device has been operated in a supported mode of operation while experience has been gained in the cryogenic performance of the F-coil and the integration of the F-coil and C-coil. Current work focuses on the integration of the F and L coils in preparation for first levitation tests.

Technical Report: Absolute Radiometric Calibration at NSLS

For nearly 20 years the U3c and X8a beamlines at the National Synchrotron Light Source (NSLS) have been used for absolute calibration and X-ray characterization of detectors and optics. The motivation behind this capability has been the need of the U.S. Department of Energy's National Nuclear Security Agency (DOE-NNSA) programs at other U.S. national laboratories (Livermore, Sandia, and Los Alamos) to provide diagnostics capabilities for high-temperature plasma physics experiments. For example, specific applications of regularly NSLS-calibrated diagnostics are found at Livermore's National Ignition Facility (NIF) and Sandia's Z machine.

Proper performance prediction for ITER

In his letter in the February 2006 issue of Physics Today (page 10), David Montgomery expresses concern about the theoretical basis for predicting the performance of ITER, the planned international magnetic fusion energy experiment. He focuses specifically on the use of ideal (nondissipative) magnetohydrodynamics (MHD) as the zeroth-order description of equilibrium in magnetically confined plasmas. He correctly points out that such an ordering is not generally possible in the mechanics of neutral fluids.Although in principle that concern could be legitimate, in practice a wide range of experiments has confirmed that the ideal MHD model excellently describes the equilibrium of high-temperature magnetically confined plasmas. Global and detailed local measurements of plasma equilibrium show excellent agreement with ideal theory. Fast instabilities are also well described by ideal theory; slower, dissipative instabilities require more elaborate description, but under the usual operating conditions, the basic ordering that starts from ideal equilibrium is not violated. Ideal MHD calculations are routinely used in major magnetic confinement experiments to assemble and interpret measurements of magnetic fields, pressure gradients, and flow speeds, and to predict stability boundaries. Those MHD codes track experimental behavior consistently and in detail.Why does ideal MHD provide a legitimate zeroth-order description of magnetically confined plasmas? The answer is tied to researchers’ broad success in using magnetic fields to confine plasmas in relatively quiescent states free of large-scale MHD instability. By contrast, the evolution of globally turbulent neutral fluids depends on a dynamic balance between ideal forces, acceleration, and dissipation. In magnetically confined plasmas, the ideal equilibrium forces—j × B, ∇p, and to a lesser degree ρ(v · ∇)v—are measured to balance each other and dominate strongly over acceleration and nonideal dissipation terms, confirming experimentally the ordering procedure that Montgomery questions. Violation of that ordering corresponds to global acceleration on an ideal MHD time scale, and so to rapid plasma loss, which is observed only under circumstances in which ideal MHD stability is compromised. Even Montgomery’s model calculations support the use of an ordering that starts from ideal equilibrium MHD, since the results he displays show a very low flow speed, in the range of 1 m/s.One of the most exciting developments in fusion-energy research over the past decade has been the increasing agreement between magnetized plasma theory and confinement experiments. Recent progress in achieving predictive understanding of confined plasma behavior seems to us, an experimentalist and a theorist, truly remarkable. Theory—including MHD theory, more elaborate fluid models, and fully kinetic plasma descriptions—does not play a “decorative” role today in high-temperature plasma physics; such tools provide an essential guide in the design and execution of experiments. Theory and advanced computing, particularly through the US Department of Energy’s Scientific Discovery Through Advanced Computing and Leadership-Class Computing initiatives, are key elements of US preparations for ITER operation.© 2006 American Institute of Physics.

Parametric dependence of density limits in the Tokamak Experiment for Technology Oriented Research (TEXTOR): Comparison of thermal instability theory with experiment

The observed dependence of the TEXTOR [Tokamak Experiment for Technology Oriented Research: E. Hintz, P. Bogen, H. A. Claassen et al., Contributions to High Temperature Plasma Physics, edited by K. H. Spatschek and J. Uhlenbusch (Akademie Verlag, Berlin, 1994), p. 373] density limit on global parameters (I, B, P, etc.) and wall conditioning is compared with the predicted density limit parametric scaling of thermal instability theory. It is necessary first to relate the edge parameters of the thermal instability theory to n̄ and the other global parameters. The observed parametric dependence of the density limit in TEXTOR is generally consistent with the predicted density limit scaling of thermal instability theory. The observed wall conditioning dependence of the density limit can be reconciled with the theory in terms of the radiative emissivity temperature dependence of different impurities in the plasma edge. The thermal instability theory also provides an explanation of why symmetric detachment precedes radiative collapse for most low power shots, while a multifaceted asymmetric radiation from the edge MARFE precedes detachment for most high power shots.

Thermal instability theory analysis of multifaceted asymmetric radiation from the edge (MARFE) in Tokamak Experiment for Technology Oriented Research (TEXTOR)

The density limits for a series of shots in TEXTOR [Tokamak Experiment for Technology Oriented Research, E. Hintz, P. Bogen, H. A. Claaßen et al., in Contributions to High-Temperature Plasma Physics, edited by K. H. Spatschek and J. Uhlenbusch (Akademie Verlag, Berlin, 1994, p. 373)], over a range of heating powers, that ended in multifaceted asymmetric radiation from the edge (MARFE) have been analyzed within the context of thermal instability theory. The prediction of MARFE onset agrees with observation to within the experimental uncertainty.