Controlled Fusion Research Research Articles

Update on the Scientific Status of the Plasma Focus

This paper is a sequel to the 1998 review paper “Scientific status of the Dense Plasma Focus” with 16 authors belonging to 16 nations, whose initiative led to the establishment of the International Center for Dense Magnetized Plasmas (ICDMP) in the year 2000. Its focus is on understanding the principal defining characteristic features of the plasma focus in the light of the developments that have taken place in the last 20 years, in terms of new facilities, diagnostics, models, and insights. Although it is too soon to proclaim with certainty what the plasma focus phenomenon is, the results available to date conclusively indicate what it is demonstrably not. The review looks at the experimental data, cross-correlated across multiple diagnostics and multiple devices, to delineate the contours of an emerging narrative that is fascinatingly different from the standard narrative, which has guided the consensus in the plasma focus community for several decades, without invalidating it. It raises a question mark over the Fundamental Premise of Controlled Fusion Research, namely, that any fusion reaction having the character of a beam-target process must necessarily be more inefficient than a thermonuclear process with a confined thermal plasma at a suitably high temperature. Open questions that need attention of researchers are highlighted. A future course of action is suggested that individual plasma focus laboratories could adopt in order to positively influence the future growth of research in this field, to the general benefit of not only the controlled fusion research community but also the world at large.

IAEA activities in support of nuclear physics research and applications

Facilitation of development and promotion of nuclear applications for peaceful purposes and related capacity building are among the IAEA missions where Physics Section contributes most. The relevant activities fall under the IAEA’s program ‘nuclear science’ and cover four main thematic areas: applications of particle accelerators, sustainable utilization of research reactors, controlled fusion research and technology, and nuclear instrumentation. The Section also operates the Nuclear Science and Instrumentation Laboratory (NSIL) at Seibersdorf, located approximately 40 km south of Vienna. NSIL’s primary mission is to assist IAEA Member States to establish, operate and maintain various nuclear instrumentation and spectrometry-based techniques in support of a wide range of applications such as materials This paper will illustrate through a number of selected examples how the IAEA supports nuclear physics research and diverse applications in order to address key development priorities in many areas of societal importance and economic growth of the developing countries. In addition, some future plans on enhancing capabilities of the Nuclear Science and Instrumentation Laboratory as part of Physics Section will be highlighted, in particular by establishment of the neutron science facility and considerations for a compact ion beam accelerator.

Некоторые особенности модификации поверхности ионными пучками очень большого размера

Ion beam surface modification is typically carried out in a laboratory environment where the processing rate is not a big concern. Nevertheless, it is interesting to consider the possibilities that might allow substantial increase (orders of magnitude) in the processing rate, possibly for industrial application. Here we consider two possible approaches to very large scale ion beam surface modification – the use of very broad beam vacuum arc ion sources and the use of ion sources that have been developed to provide intense neutral beams as part of the world controlled fusion research effort. These sources can provide beam intensities in the tens of amperes range, and quasi-dc operation has been demonstrated. A third possibility is scale-up of the Plasma Immersion Ion Implantation approach, and some relevant work has been done along these lines. These approaches to giant ion beam surface modification systems, and the implied parameters and features of such systems, as well as the highly related concern of the necessary accompanying electrical systems, are briefly considered here.

Substantially enhanced deuteron-triton fusion probabilities in intense low-frequency laser fields

Deuteron-triton (DT) fusion is the primary fusion reaction used in controlled fusion research, mainly for its relatively high reaction cross sections compared to other fusion options. Even so, to attain appreciable reaction probabilities very high temperatures (on the order of 10-100 million kelvins) are required, which are extremely challenging to achieve and maintain. We show that intense low-frequency laser fields, such as those in the near-infrared regime for the majority of intense laser facilities around the world, are highly effective in transferring energy to the DT system and enhancing the DT fusion probabilities. The fusion probabilities are shown to be enhanced by at least an order of magnitude in 800-nm laser fields with intensities on the order of 10$^{21}$ W/cm$^2$. The demanding temperature requirement of controlled nuclear fusion may be relaxed if intense low-frequency lasers are exploited.

New analytic representation of the thermonuclear reaction rates

In the current note, new analytic representations of the Maxwellian reactivities for D(d, n)3He, D(d, p)T, D(3He, p)4He and D(t, n)4He reactions are obtained using a semi-empirical approach. These expressions for reaction rates are based on the tabulated data published in recent works. The analytical formulae for the reaction rates derived in our work agree very well with the tabulated data. These results can be easily used for further numerical simulations and quick estimation of the thermonuclear reactions rate in controlled fusion research and various models of the Big Bang nucleosynthesis.

Characteristics of a high-power RF source of negative hydrogen ions for neutral beam injection into controlled fusion devices

An injector of hydrogen atoms with an energy of 0.5–1 MeV and equivalent current of up to 1.5 A for purposes of controlled fusion research is currently under design at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences. Within this project, a multiple-aperture RF surface-plasma source of negative hydrogen ions is designed. The source design and results of experiments on the generation of a negative ion beam with a current of >1 A in the long-pulse mode are presented.

Kinetic description of rotating Tokamak plasmas with anisotropic temperatures in the collisionless regime

A largely unsolved theoretical issue in controlled fusion research is the consistent kinetic treatment of slowly-time varying plasma states occurring in collisionless and magnetized axisymmetric plasmas. The phenomenology may include finite pressure anisotropies as well as strong toroidal and poloidal differential rotation, characteristic of Tokamak plasmas. Despite the fact that physical phenomena occurring in fusion plasmas depend fundamentally on the microscopic particle phase-space dynamics, their consistent kinetic treatment remains still essentially unchallenged to date. The goal of this paper is to address the problem within the framework of Vlasov-Maxwell description. The gyrokinetic treatment of charged particles dynamics is adopted for the construction of asymptotic solutions for the quasi-stationary species kinetic distribution functions. These are expressed in terms of the particle exact and adiabatic invariants. The theory relies on a perturbative approach, which permits to construct asymptotic analytical solutions of the Vlasov-Maxwell system. In this way, both diamagnetic and energy corrections are included consistently into the theory. In particular, by imposing suitable kinetic constraints, the existence of generalized bi-Maxwellian asymptotic kinetic equilibria is pointed out. The theory applies for toroidal rotation velocity of the order of the ion thermal speed. These solutions satisfy identically also the constraints imposed by the Maxwell equations, i.e., quasi-neutrality and Ampere’s law. As a result, it is shown that, in the presence of nonuniform fluid and EM fields, these kinetic equilibria can sustain simultaneously toroidal differential rotation, quasi-stationary finite poloidal flows and temperature anisotropy.

10.1007/s11452-008-2001-9

Results from experimental studies on the injection of high-energy neutral hydrogen beams into the plasma of the Globus-M spherical tokamak are reviewed. In the Introduction, the importance of these studies for implementing the controlled fusion research program and constructing the ITER tokamak is proved. Some problems related to the use of neutral beam injection in small and low-aspect-ratio tokamaks is analyzed. Results are presented from numerical simulations of the experiment by using the ASTRA transport code. It is shown that the use of neutral beam injection in the Globus-M tokamak ensures efficient ion heating and increases the plasma stored energy. The greater part of the review is devoted to the survey of experiments on the injection of 22-to 30-keV hydrogen and deuterium beams with a power of 0.4–0.8 MW into the plasma of the Globus-M spherical tokamak in a wide range of plasma currents and densities. The experimental results are analyzed and compared with the results of numerical simulations. The achievement of top plasma parameters is highlighted.

A new diagnostic of the plasma density profile near the edge transport barrier from measurements of the radial profile of the Hα line intensity

An edge transport barrier is now one of the most important subjects of controlled fusion research. The edge transport barrier is located in the plasma region where hydrogen atoms readily penetrate, so the intensity of the H(alpha) (D(alpha)) line is high enough. A new diagnostic method uses the well-known property of hydrogen atoms that the ratio of the ionization rate S(i) to the excitation rate S(v) for the H(alpha) line is nearly constant over a wide range of plasma temperatures and densities. An expression has been derived that relates the radial profiles of the plasma density and H(alpha) intensity. The use of charge coupled device detectors makes it possible to measure the radial profile of H(alpha) line intensity with a resolution approximately 0.1 cm; a high intensity of the H(alpha) line ensures a high time resolution approximately 1 ms. A high resolution is thus achieved for the density profile calculated from the H(alpha) intensity profile. The method was tested when studying the plasma density profile in the region of edge transport barrier in the L-2M stellarator. It has been shown that the density gradient varies during the barrier formation and that a fine structure of the density profile correlates with a character of the plasma transport near resonance magnetic flux surfaces.

Electron-impact ionization ofB+

Electron-impact ionization cross sections are calculated for the ground and metastable states of ${\text{B}}^{+}$. Comparisons between perturbative distorted-wave and nonperturbative close-coupling calculations find reductions in the direct ionization cross sections due to long-range electron correlation effects of approximately 10% for the ground state and approximately 15% for the metastable state. Previous crossed-beams experiments, with a metastable to ground ratio of between 50% and 90%, are found to be in reasonable agreement with metastable state close-coupling results. New crossed-beams experiments, with a metastable to ground ratio of only 9%, are found to be in reasonable agreement with ground state close-coupling results. Combined with previous work on neutral B and ${\text{B}}^{2+}$, the nonperturbative close-coupling calculations provide accurate ionization cross sections for the study of edge plasmas in controlled fusion research.

Study of plasma heating in discharges with neutral beam injection in the Globus-M spherical tokamak

Results from experimental studies on the injection of high-energy neutral hydrogen beams into the plasma of the Globus-M spherical tokamak are reviewed. In the Introduction, the importance of these studies for implementing the controlled fusion research program and constructing the ITER tokamak is proved. Some problems related to the use of neutral beam injection in small and low-aspect-ratio tokamaks is analyzed. Results are presented from numerical simulations of the experiment by using the ASTRA transport code. It is shown that the use of neutral beam injection in the Globus-M tokamak ensures efficient ion heating and increases the plasma stored energy. The greater part of the review is devoted to the survey of experiments on the injection of 22-to 30-keV hydrogen and deuterium beams with a power of 0.4–0.8 MW into the plasma of the Globus-M spherical tokamak in a wide range of plasma currents and densities. The experimental results are analyzed and compared with the results of numerical simulations. The achievement of top plasma parameters is highlighted.

Current halo structures in high-current plasma experiments: θ-pinch

Experimental data elucidating mechanisms for halo formation in θ-pinch discharges are presented and discussed. The experiments were performed with different gases (H2, D2, He, and Ar) in a theta-pinch device with a porcelain vacuum chamber and an excitation coil 15 cm in diameter and 30 cm in length. The stored energy, the current in the excitation coil, and the current half-period were W = 10 kJ, I = 400 kA, and T/2 = 14 μs, respectively. It is found that the plasma rings (halos) surrounding the pinch core arise as a result of coaxial pinch stratification due to both the excitation of closed currents (inductons) inside the pinch and the radial convergence of the plasma current sheaths produced after the explosion of T-layers formed near the wall in the initial stage of the discharge. It is concluded that halo structures observed in pinches, tokamaks, and other high-current devices used in controlled fusion research have the same nature.

Application of MDSplus on EAST Tokamak

EAST is a fully superconducting Tokamak in China used for controlled fusion research. MDSplus, a special software package for fusion research, has been used successfully as a central repository for analysed data and PCS (Plasma Control System) data since the debugging experiment in the spring of 2006 [1]. In this paper, the reasons for choosing MDSplus as the analysis database and the way to use it are presented in detail, along with the solution to the problem that part of the MDSplus library does not work in the multithread mode. The experiment showed that the data system based on MDSplus operated stably and it could provide a better performance especially for remote users.

Progress toward fully noninductive, high beta conditions in DIII-D

The DIII-D Advanced Tokamak (AT) program in the DIII-D tokamak [J. L. Luxon, Plasma Physics and Controlled Fusion Research, 1986, Vol. I (International Atomic Energy Agency, Vienna, 1987), p. 159] is aimed at developing a scientific basis for steady-state, high-performance operation in future devices. This requires simultaneously achieving 100% noninductive operation with high self-driven bootstrap current fraction and toroidal beta. Recent progress in this area includes demonstration of 100% noninductive conditions with toroidal beta, βT=3.6%, normalized beta, βN=3.5, and confinement factor, H89=2.4 with the plasma current driven completely by bootstrap, neutral beam current drive, and electron cyclotron current drive (ECCD). The equilibrium reconstructions indicate that the noninductive current profile is well aligned, with little inductively driven current remaining anywhere in the plasma. The current balance calculation improved with beam ion redistribution that was supported by recent fast ion diagnostic measurements. The duration of this state is limited by pressure profile evolution, leading to magnetohydrodynamic (MHD) instabilities after about 1s or half of a current relaxation time (τCR). Stationary conditions are maintained in similar discharges (∼90% noninductive), limited only by the 2s duration (1τCR) of the present ECCD systems. By discussing parametric scans in a global parameter and profile databases, the need for low density and high beta are identified to achieve full noninductive operation and good current drive alignment. These experiments achieve the necessary fusion performance and bootstrap fraction to extrapolate to the fusion gain, Q=5 steady-state scenario in the International Thermonuclear Experimental Reactor (ITER) [R. Aymar et al., Fusion Energy Conference on Controlled Fusion and Plasma Physics, Sorrento, Italy (International Atomic Energy Agency, Vienna, 1987), paper IAEA-CN-77/OV-1]. The modeling tools that have been successfully employed to both plan and interpret the experiment are used to plan future DIII-D experiments with higher power and longer pulse ECCD and fast wave and co- and counterneutral beam injection in a pumped double-null configuration. The models predict our ability to control the current and pressure profiles to reach full noninductivity with increased beta, bootstrap fraction, and duration. The same modeling tools are applied to ITER, predicting favorable prospects for the success of the ITER steady-state scenario.

Possible gaps in ITER’s foundations

In news items about controlled fusion research (see, for example, Physics Today, August 2005, page 26), it has become commonplace to dwell on the political and financial aspects but to act as if the important scientific questions have been answered and only engineering details remain. However, there are scientific gaps in the conceptual foundations of ITER, the proposed international prototype fusion energy reactor—or any other magnetic confinement device—that are worth further consideration from the whole physics community.The most awkward gap is the lack of a manageable mathematical framework for calculating the state of the plasma in a confinement device in a way that relates the calculation globally and convincingly to what actually happens in the machine. This becomes apparent when theoretical claims are set alongside the experiments being contemplated. For half a century, the textbook theoretical starting point for such a global calculation has been the use of ideal magnetohydrodynamics to provide a zeroth-order description of the state of the confined plasma. “Ideal” MHD here means a fluid-mechanical formulation from which the dissipative terms (viscous and resistive) have been dropped. Much more elaborate and less restrictive descriptions are often used to identify instabilities that might make the confined plasma less tractable than the initial ideal MHD equilibria would suggest. In such perturbative instability or “turbulent transport” calculations, which sometimes claim to include microscopic turbulence, dissipative terms are often reintroduced, despite their omission from the calculated ideal MHD configuration that is being “perturbed.”This perspective, which slides freely back and forth between ideal and non-ideal descriptions, is not supportable in fluid mechanics, nor is there reason to expect it to be in plasma physics. 1 1. See, for example, L. P. J. Kamp, D. C. Montgomery, J. Plasma Phys. 70, 113 (2004); https://doi.org/10.1017/S0022377803002629 Phys. Plasmas 10, 157 (2003) https://doi.org/10.1063/1.1524629. In both cases the introduction of the dissipative terms changes the mathematical character of the steady states that can be sustained and made to satisfy boundary conditions. One cannot generally get close to a non-ideal steady state with an ideal one, nor can one treat the stability of a non-ideal steady state with anything like the systematic mathematics that the ideal case permits. This awkwardness is not the subject of debate; rather, it is simply ignored. So is the fact that the extent of MHD turbulence is often best predicted by dimensionless numbers, such as Reynolds or Hartmann numbers, that have dissipation coefficients in their denominators and diverge as these coefficients approach zero.It follows that in any project as ambitious as ITER (or for that matter, the Joint European Tokamak or the Tokamak Fusion Test Reactor, or other similar devices), what one is doing is carrying out experiments—mostly just trying things—with theory largely having a decorative role, or at best providing suggestions for something to try next. Such a secondary role for theory is not necessarily fatal. Many important discoveries have been made in the absence of a proper theoretical framework to predict them. But two points should be noted here. First, the conceptual gap between the theory and device building does not need to be as great as it becomes when discussions of it are essentially not taking place. And second, all the consensus, all the money, publicity, and sophisticated organization in the world will not necessarily satisfy the “aim to get 10 times more power out than goes in,” as the August Physics Today, item reports. That is still just wishful thinking.REFERENCESection:ChooseTop of pageREFERENCE <<1. See, for example, L. P. J. Kamp, D. C. Montgomery, J. Plasma Phys. 70, 113 (2004); https://doi.org/10.1017/S0022377803002629 , Google ScholarCrossref, ISI Phys. Plasmas 10, 157 (2003) https://doi.org/10.1063/1.1524629. , Google ScholarCrossref, ISI© 2006 American Institute of Physics.

Rendel Sebastian Pease

Rendel Sebastian Pease

Control of electron internal transport barriers in TCV

Current profile tailoring has been performed by application of electron cyclotron heating (ECH) and electron cyclotron current drive, leading to improved energy confinement in the plasma core of the TCV tokamak. The improved confinement is characterized by a substantial enhancement (H-factor) of the global electron energy confinement time relative to the prediction of the RLW scaling law (Rebut P H et al 1989 Proc. 12th Int. Conf. of Plasma Physics and Controlled Fusion Research (Nice, 1988) vol 2 (Vienna: IAEA) p 191), which predicts well Ohmic and standard ECH discharges on TCV. The improved confinement is attributed to a hollow current density profile producing a reversed shear profile creating an electron internal transport barrier. We relate the strength of the barrier to the depth of the hollow current density profile and the volume enclosed by the radial location of the peak current density. The (Tresset G et al 2002 Nucl. Fusion 42 520) criterion is used to evaluate the performance of the barrier relative to changes in the ECH parameters or the addition of Ohmic current, which aid in identifying the control parameters available for improving either the strength or volume of the barrier for enhanced performance. A figure of merit for the global scaling factor is used that scales the confinement enhancement as the product of the barrier volume and strength.

The interaction of error fields and resistive wall modes

An investigation is presented of the effect of an error field on the growth of a resistive wall mode (RWM). This issue is of importance to many toroidal confinement devices, but particularly to the proposed International Thermonuclear Experimental Reactor (ITER) [ITER Physics Basis, Nucl. Fusion 39, 2175 (1999)], where if advanced tokamak performance is required, then the RWM instability is especially vulnerable. The paper includes a discussion of error field amplification when RWM marginal stability is approached, and it is pointed out that error field current is the fixed quantity in this calculation, not the error field flux, which is essentially an eigenvalue of the calculation. The key issue concerns the effect of plasma rotation on the RWM. The distinction is made between the resistive wall tearing mode (RWTM), which is stabilized by modest plasma rotation, and the ideal RWM, which is not. The importance of the joint torques that are present when an error field and an RWM of the same helicity coexist is investigated. Using torque balance a simulation is presented of the effect of reducing error field amplitude on the duration of a stable discharge. This shows features in accord with experimental observations from the DIII-D device [J. L. Luxon et al., Plasma Physics and Controlled Fusion Research (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p.159].

Low to high confinement transition theory of finite-beta drift-wave driven shear flow and its comparison with data from DIII-D

Shear flow stabilization of edge turbulence in tokamaks has been the accepted paradigm for the improvement in confinement observed in high (H) confinement mode plasmas. Results on the generation of zonal flow and fields in finite β plasmas are presented. This theory yields a criterion for bifurcation from low to high (L–H) confinement mode, proportional to Te/Ln, where Te is the electron temperature and Ln is the density scale-length at the steepest part of the density gradient. When this parameter exceeds a critical value (mostly determined by the strength of the toroidal magnetic field), the transition occurs. The predicted threshold based on this parameter shows good agreement with edge measurements on discharges undergoing L–H transitions in DIII-D [J. L. Luxon, R. Anderson, F. Batty et al., in Proceedings of the 11th Conference on Plasma Physics and Controlled Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159]. The observed differences in the transitions with the reversal of the toroidal magnetic field are reconciled in terms of this critical parameter due to the differences in the density gradient scale-lengths in the edge. The theory also provides a possible explanation for lowered threshold power, pellet injection H modes in DIII-D, thereby providing a unified picture of the varied observations on the L–H transition.

The L-5 stellarator: A compact torsatron with a controlled structure of the magnetic configuration

The project of an L-5 stellarator is discussed. This multipurpose device of the compact torsatron type is considered as a possible prototype of the principal experimental device of the Plasma Physics Department (the General Physics Institute, the Russian Academy of Sciences) involved in the stellarator program of controlled fusion research. This device is intended to take place of the L-2 (L-2M) stellarator, which is now in operation. A peculiar feature of this project is the extremely low aspect ratio of a toroidal plasma configuration and, at the same time, the experimental flexibility allowing creation of a wide range of magnetic configurations. This allows scanning of main plasma parameters, determining plasma confinement within substantial limits. The latter circumstance makes it possible to carry out studies of magnetic plasma confinement with the purpose to investigate effects of different theoretical limitations on plasma stability and equilibrium, and also to study efficiency of some methods prop...