17th IAEA Fusion Energy Conference Research Articles

Self-assembling of tubular skeletons from electric current filaments composed of magnetized thin rods

Abstract Formation of a skeleton composed of a fractal condensed matter was suggested [A.B. Kukushkin, V.A. Rantsev-Kartinov, in: Proceedings of the 17th IAEA Fusion Energy Conference, vol. 3, Yokohama, Japan, 1998, pp. 1131–1134, http://www.iaea.org/programmes/ripc/physics/pdf/ifp_17.pdf ] to explain unexpected longevity of filamentary structures observed in laboratory electric discharges. A simple 3D model [A.B. Kukushkin, K.V. Cherepanov, physics/0512234 ] of many-body system of magnetized, electrically conducting thin rods (1D magnetic dipoles) managed to describe the integrity of a hypothetical, “manually-assembled” tubular skeleton under the action of external forces. Here we demonstrate the possibility of electrodynamic self-assembling of coaxial tubular skeleton in a system of ∼500 magnetic dipoles, which are initially arranged as 25–50 linear electric current filaments with a fraction of the dipoles with uncompensated magnetic flux.

Electron Bernstein wave-bootstrap current synergy in the National Spherical Torus Experiment

Current driven by electron Bernstein waves (EBW) and by the electron bootstrap effect are calculated separately and concurrently with a kinetic code to determine the degree of synergy between them. A target β=40% NSTX [M. Ono, S. Kaye, M. Peng et al., Proceedings of the 17th IAEA Fusion Energy Conference, edited by M. Spak (IAEA, Vienna, Austria, 1999), Vol. 3, p. 1135] plasma is examined. A simple bootstrap model in the collisional-quasilinear CQL3D Fokker–Planck code (National Technical Information Service document No. DE93002962) is used in these studies: the transiting electron distributions are connected in velocity space at the trapped-passing boundary to trapped-electron distributions that are displaced radially by a half-banana-width outwards/inwards for the co-passing/counter-passing regions. This model agrees well with standard bootstrap current calculations over the outer 60% of the plasma radius. Relatively small synergy net bootstrap current is obtained for EBW power up to 4 MW. Locally, bootstrap current density increases in proportion to increased plasma pressure, and this effect can significantly affect the radial profile of driven current.

Efficient coupling of thermal electron Bernstein waves to the ordinary electromagnetic mode on the National Spherical Torus Experiment

Efficient coupling of thermal electron Bernstein waves (EBW) to ordinary-mode (O-mode) electromagnetic radiation has been measured in plasmas heated by energetic neutral beams and high harmonic fast waves in the National Spherical Torus Experiment (NSTX) [M. Ono, S. Kaye, M. Peng et al., Proceedings of the 17th IAEA Fusion Energy Conference, edited by S. Spak (IAEA, Vienna, Austria, 1999), Vol. 3, p. 1135]. The EBW to electromagnetic mode coupling efficiency was measured to be 0.8±0.2, compared to a numerical EBW modeling prediction of 0.65. The observation of efficient EBW coupling to O mode, in relatively good agreement with numerical modeling, is a necessary prerequisite for implementing a proposed high power EBW current drive system on NSTX.

Enhanced conversion of thermal electron Bernstein waves to the extraordinary electromagnetic mode on the National Spherical Torus Experiment

A fourfold increase in the conversion of thermal electron Bernstein waves (EBW) to the extraordinary mode (X mode) was measured when the density scale length (Ln) was progressively shortened by a local boron nitride limiter in the scrape-off of an Ohmically heated National Spherical Torus Experiment plasma [M. Ono, S. Kaye, M. Peng et al., Proceedings of the 17th IAEA Fusion Energy Conference (IAEA, Vienna, 1999), Vol. 3, p. 1135]. The maximum conversion efficiency approached 50% when Ln was reduced to 0.7 cm, in agreement with theoretical predictions that used locally measured Ln. Calculations indicate that it is possible to establish Ln<0.3 cm with a local limiter, a value predicted to attain ∼100% EBW conversion to the X mode in support of proposed EBW heating and current drive scenarios.

Enhancement of mode-converted electron Bernstein wave emission during National Spherical Torus Experiment H-mode plasmas

A sudden, threefold increase in emission from fundamental electrostatic electron Bernstein waves (EBW) which mode convert and tunnel to the electromagnetic X-mode has been observed during high energy and particle confinement (H-mode) transitions in the National Spherical Torus Experiment (NSTX) plasma [M. Ono, S. Kaye, M. Peng et al., in Proceedings of the 17th IAEA Fusion Energy Conference (IAEA, Vienna, Austria, 1999), Vol. 3, p. 1135]. The mode-converted EBW emission viewed normal to the magnetic field on the plasma midplane increases when the density profile steepens in the vicinity of the mode conversion layer, which is located in the plasma scrape off. The measured conversion efficiency during the H-mode is consistent with the calculated EBW to X-mode conversion efficiency derived using edge density data. Calculations indicate that there may also be a small residual contribution to the measured X-mode electromagnetic radiation from polarization-scrambled, O-mode emission, converted from EBWs.

Kinetic stability of electrostatic plasma modes in a dipolar magnetic field

An axially symmetric plasma immersed in a poloidal magnetic field with closed lines is considered. Low-frequency electrostatic modes are studied kinetically for an “intermediate collisionality” ordering, in which the particle collision frequency is much smaller than the transit or bounce frequency, but much larger than the mode, magnetic drift, and diamagnetic drift frequencies. This ordering is appropriate for the Levitated Dipole Experiment (LDX) [J. Kesner et al., 17th IAEA Fusion Energy Conference, Yokahama, Japan (IAEA, Vienna, 1999)] and some other closed field line devices. “High-frequency” magnetohydrodynamic-like and “low-frequency” entropy modes are found and stability boundaries are determined. Collisional effects are considered and the corresponding ion gyro-relaxation effects are evaluated. These effects introduce dissipation (or inverse dissipation) and are shown to modify the stability picture considerably, while leaving large stability regions in the d, η parametric space, where η is the ratio of the gradients of temperature and density and d is the ratio of the diamagnetic and magnetic drift frequencies.

A 170-kA DC circuit breaker for the quench protection of the central solenoid coil of the ITER tokamak

High current and high voltage DC circuit breakers have already been used in the field of fusion technology, in the Joint European Torus (JET) (P. Dokopulos and K. Kriechbaum, DC Circuit Breaker Arrangement 73 kA, 24 kV for the Joint European Torus, Elektrotechnische Zeitschrift A, 1976; P.L. Mondino, The JET ohmic heating circuit, 9th Symposium on Engineering Problems of Fusion Research, Chicago, US, Vol. 1, October 1981, pp. 386–389) and JT60 (R. Shimada et al., JT 60 power supplies, Fus. Eng. Des. 5 (1987) 47–68; R. Shimada et al., Development of high current DC circuit breaker for large Tokamak device, Fusion Technol. 1 (1978) 401–406) Tokamaks, with pulsed currents up to 90 kA or more, but were not able to simultaneously carry steady state currents of 60 kA and to interrupt current under high voltage. Therefore, R&D on high current (60 kA) and high voltage (20 kV) switches, especially on DC circuit breakers, has been undertaken in order to ensure the protection of the superconducting coils of the ITER Tokamak (Y. Shimomura et al., ITER Overview, 17th IAEA Fusion Energy Conference, Yokohama, Japan, October 1998; M. Huguet, The Integrated Design of the ITER Magnets and Their Auxiliary Systems, 17th IAEA Fusion Energy Conference, Yokohama, Japan, October 1998) in case of a quench (Y. Shimomura et al., ITER Overview, 17th IAEA Fusion Energy Conference, Yokohama, Japan, October 1998). For this purpose, the coil magnetic energy (in the range of several GJ) is discharged in a large resistor connected in parallel with the DC circuit breaker. Most of the coil circuits operate at 60 kA maximum DC current. Nevertheless, the Central Solenoid (CS) coil (Y. Shimomura et al., ITER Overview, 17th IAEA Fusion Energy Conference, Yokohama, Japan, October 1998) of the ITER Tokamak was designed, till the middle of 1998, to be operated at 168 kA. The assessment of such a circuit breaker concept, using parallel operation of 60 kA rated mechanical switches, will be developed in this paper. This theoretical analysis assumes the use of pure mechanical switches connected in parallel without any use of semi-conductor devices.

Zonal flow generation by parametric instability in magnetized plasmas and geostrophic fluids

Two-dimensional magnetized plasmas and geostrophic fluids exhibit a common nonlinearity due to the advection of vorticity. It is shown here that due to this nonlinearity, the propagation of small scale wave packets is accompanied by instability of a low frequency, long wavelength component. This instability is the coherent hydrodynamic generalization of the resonant type mean flow instability identified recently [P. H. Diamond, M. N. Rosenbluth, F. L. Hinton, M. Malkov, J. Fleischer, and A. Smolyakov, 17th IAEA Fusion Energy Conference, IAEA-CN-69/TH3/1, Yokohama, 1998 (to be published, International Atomic Energy Agency, Vienna)]. The mechanism discussed here, along with the resonant type, constitutes the “hydrodynamic” and “kinetic” regimes of the same process, similar to the case of plasma-beam instabilities. It is suggested that this generic mechanism is responsible for the generation of mean flow in atmospheres of rotating planets and magnetized plasmas.

Numerical simulation of magnetic compression on a field-reversed configuration plasma

A two-dimensional magnetohydrodynamic (MHD) simulation of an axial magnetic compression on a field-reversed configuration (FRC) plasma is carried out for the parameter range of a corresponding experiment conducted on the FRC Injection Experiment (FIX) [S. Okada et al., 17th IAEA Fusion Energy Conference 1998 (International Atomic Energy Agency, Vienna) (in press)]. The simulation results show that during the initial stage of the magnetic compression the front part of the FRC plasma is mainly compressed radially, and that after this stage, the compression is primarily axial. Of particular interest is expected that the closed magnetic flux surfaces of the FRC can be retained without any degradation during the magnetic compression process. Further, it is observed in the simulation that the axial magnetic compression enables a transition of the MHD equilibrium from a long and thin to a short and fat FRC. The effects of this magnetic compression on FRC plasmas are discussed.