Critical Physics Issues Research Articles

Millimeter-wave imaging of magnetic fusion plasmas: technology innovations advancing physics understanding

Electron cyclotron emission (ECE) imaging is a passive radiometric technique that measures electron temperature fluctuations; and microwave imaging reflectometry (MIR) is an active radar imaging technique that measures electron density fluctuations. Microwave imaging diagnostic instruments employing these techniques have made important contributions to fusion science and have been adopted at major fusion facilities worldwide including DIII-D, EAST, ASDEX Upgrade, HL-2A, KSTAR, LHD, and J-TEXT. In this paper, we describe the development status of three major technological advancements: custom mm-wave integrated circuits (ICs), digital beamforming (DBF), and synthetic diagnostic modeling (SDM). These have the potential to greatly advance microwave fusion plasma imaging, enabling compact and low-noise transceiver systems with real-time, fast tracking ability to address critical fusion physics issues, including ELM suppression and disruptions in the ITER baseline scenario, naturally ELM-free states such as QH-mode, and energetic particle confinement (i.e. Alfvén eigenmode stability) in high-performance regimes that include steady-state and advanced tokamak scenarios. Furthermore, these systems are fully compatible with today’s most challenging non-inductive heating and current drive systems and capable of operating in harsh environments, making them the ideal approach for diagnosing long-pulse and steady-state tokamaks.

In-vessel saddle coils for MHD control in ASDEX Upgrade

A set of 24 in-vessel saddle coils is planned for MHD control experiments in ASDEX Upgrade. These coils can produce static and alternating error fields for suppression of Edge Localised Modes, locked mode rotation control and, together with additional conducting wall elements, resistive wall mode excitation and feedback stabilisation experiments. All of these applications address critical physics issues for the operation of ITER. This extension is implemented in several stages, starting with two poloidally separated rings of eight toroidally distributed saddle coils above and below the outer midplane. In stages 2 and 3, eight midplane coils around the large vessel access ports and 12 AC power converters are added, respectively. Finally (stage 4), the existing passive stabilising loop (PSL), a passive conductor for vertical growth rate reduction, will be complemented by wall elements that allow helical current patterns to reduce the RWM growth rate for active control within the accessible bandwidth. The system is capable of producing error fields with toroidal mode number n = 4 for plasma edge ergodisation with core island width well below the neo-classical tearing mode seed island width even without rotational shielding. Phase variation between the three toroidal coil rings allows to create or avoid resonances with the plasma safety factor profile, in order to test the importance of resonances for ELM suppression.

Technical Survey of Simply Connected Compact Tori (CTs): Spheromaks, FRCs and Compression Schemes

A possible means for reducing core complexity and size could lie with research into simply connected compact tori. Much progress has been made in the last 20 years, and now tokamak-like confinement is being reported, with work focusing on understanding beta-limits, transport and novel means of generating magnetic fields both in sustained and pulsed scenarios. Compact torus research is maturing, with many experiments integrated into a national program to resolve well defined critical physics issues. This article summarizes the work from the last 20 years both as a historical overview and an outline of the present status.

Progress in Electromagnetic Launch Science and Technology

Electromagnetic (EM) launch science and technology in the United States continues to advance at a significant pace. The computational and experimental tools for understanding the critical physics issues are sufficiently mature that they are being utilized to provide insight and resolution of the remaining major technical challenges. For example, the primary computational electrodynamics code, EMAP3D, is now implemented in a hybrid finite element-boundary element configuration with moving conductors, providing faster computation and increased understanding of these complex systems. An increased focus on railgun tribology and experiments to understand the interface between railgun armatures and the rail surface is providing important insight for improved railgun lifetimes. Control algorithms for counter-rotating pulsed alternators have been developed and validated experimentally with physical scale simulators. Solid-state switches are replacing the more traditional vacuum switches for high-current and high-voltage railgun power sources. Silicon is the material of choice, but silicon carbide shows great promise for reduced size and increased electrical performance. As a consequence of these and other advances, the U.S. Army has initiated an effort to build a power source with a matched pair of counter-rotating pulsed alternators and a cantilevered railgun to accelerate projectiles to 2-MJ kinetic energy. The U.S. Navy has initiated a new effort to launch large projectiles to hypervelocity. Because of the large naval platform, the size and weight of the power source are not significant issues for the U.S. Navy, whose primary interest is in providing precision fire support to ranges in excess of 200nmi (500 km). The critical technical issues in implementing this application for the Navy are railgun bore lifetime and high-acceleration-tolerant guidance and control. This paper highlights the recent major achievements in hypervelocity physics, pulsed power, and railgun lifetimes that provide the critical science and technology underpinnings for the Army and Navy EM gun programs

Internal transport barriers: critical physics issues?* *Invited paper.

Plasmas regimes with improved core energy confinement properties, i.e. with internal transport barriers (ITB), provide a possible route towards simultaneous high fusion performance and continuous tokamak reactor operation in a non-inductive current drive state. High core confinement regimes should be made compatible with a dominant fraction of the plasma current self-generated (pressure-driven) by the bootstrap effect while operating at high normalized pressure and moderate current. Furthermore, ITB regimes with ‘non-stiff’ plasma core pressure break the link observed in standard inductive operation between fusion performances and plasma pressure at the edge, thus offering a new degree of freedom in the tokamak operational space. Prospects and critical issues for using plasmas with enhanced thermal core insulation as a basis for steady tokamak reactor operation are reviewed in the light of the encouraging experimental and modelling results obtained recently (typically in the last two years). An extensive set of data from experiments carried out worldwide has been gathered on ITB regimes covering a wide range of parameters (q-profile, T i/T e, gradient length, shaping, normalized toroidal Larmor radius, collisionality, Mach number, etc). In the light of the progress made recently, the following critical physics issues relevant to the extrapolation of ITB regimes to next-step experiments, such as ITER, are addressed:(i) conditions for ITB formation and existence of a power threshold,(ii) ITB sustainment at T i ∼ T e, with low toroidal torque injection, low central particle fuelling but at high density and low impurity concentration,(iii) control of confinement for sustaining wide ITBs that encompass a large volume at high βN,(iv) real time profile control (q and pressure) with high bootstrap current and large fraction of alpha-heating and(v) compatibility of core with edge transport barriers or with external core perturbations (such as frozen hydrogen isotope pellets injection).It is shown that the present experimental results provide some valuable and promising answers to these critical issues.

Spheromak Reactor: Physics Opportunities and Issues

The spheromak is a magnetic confinement device with a more attractive fusion reactor potential than the leading geometry, the tokamak. This results in large part from the absence of a toroidal field coil and other structures linking the plasma along the geometric axis. However, because of the lack of a strong external magnetic field, the physics is more complex so that considerable research is required to learn how to achieve the reactor potential. Several critical physics issues am considered here, including stability to low mode number magnetohydrodynamic (MHD) modes, energy confinement, helicity injection and current drive, the magnetic turbulence associated with this dynamo, and the beta (ratio of plasma and magnetic pressures) which can be supported in the geometry.

The Polywell™: A Spherically Convergent Ion Focus Concept

The Polywell™ spherically convergent ion focus concept for controlled thermonuclear fusion is described. The device magnetically confines electrons by a quasi-spherical-cusp magnetic field, forming a potential well. Ions are electrostatically confined by this well, converging to a dense focus in the center of the spherical potential, where the fusion rate is large because of the high local density of transient energetic ions. Power balance and critical physics issues are outlined, along with current experimental and theoretical work. The potential of the device for D-3He operation is described by the derivation of scaling laws for energy gain.

The scientific and technical merit of deuterium-tritium operation in TFTR

Understanding the properties of burning plasmas is one of the key technical issues for the magnetic fusion program as stated in the Magnetic Fusion Program Plan. The planned first step toward gaining this understanding has been the TFTR program whose primary objectives are the following: (1) achieve reactor-level temperatures (10 keV) and densities (1020 m -3) in 2.5 MA plasmas, (2) achieve approximate energy breakeven (Q 1) at reactorlevel fusion power densities (106 W m-3), and (3) gain experience in the solution of engineering problems of a large D-T fusion system. The present TFTR program plan is to fully explore critical physics issues in deuterium plasmas in 1987-89 and then proceed with a D-T program consisting of 100 full-power D-T pulses in 1990. Because of the decision to include the initiation of the Compact Ignition Tokamak (CIT) construction project at PPPL in the FY 1988 Presidential Budget Request, it is important that the Department examine the transition from TFTR to CIT and make appropriate decisions regarding the overall burning plasma physics program.

Panel 2: Critical physics issues of aneutronic migma reactor

This panel examines the critical issues of equilibrium, containment, and stability for the migma reactor configuration.

Fractional Power Operation of Tokamak Reactors: Issues and Prospects

The capability of a power plant to operate at a wide range of output power is essential for initial commissioning and normal maintenance. Critical physics issues related to operating a tokamak fusion reactor at fractions of its rated power are explored, and methods for power control are identified. Analysis is carried out with a steady-state, profile-dependent, zero-dimensional power balance model of the plasma, in which several empirical transport scalings appropriate to tokamaks are used. It is found that reactor operation depends strongly on the confinement model, the plasma beta limit, and the effect of alpha power on transport. Parametric calculations indicate that density, auxiliary heating power, and an effective external confinement control mechanism are the key control elements, and burn control is required in most cases. Transition between power plateaus is facilitated by operating in the hybrid transformer mode. In general, the impact of fractional power operation on full-power reactor designs appears to be small.

Field-reversed configuration research at Los Alamos

Exploratory field-reversed-configuration (FRC) experiments, initiated at Los Alamos in the midseventies, demonstrated FRC lifetimes substantially longer than predicted from MHD stability theory. Subsequent experimental and theoretical advances have provided considerable understanding of FRC stability physics, the characteristics of the configuration loss processes, and the particle confinement scaling with size. The critical FRC physics issues, which directly relate to the development of an FRC fusion reactor and need to be addressed in a new generation of experiments, have been clearly identified.