IAEA Technical Committee Meeting Research Articles

Numerical simulation of the hot-tail runaway electron production mechanism using CQL3D and comparison with Smith–Verwichte analytical model

The hot-tail mechanism of runaway electron (RE) production (Harvey et al 2000 Phys. Plasmas 7 4590) is the primary source of RE in the case of rapidly cooling tokamak plasma. Quantifying this mechanism is very important as it can provide most of the post-thermal-quench (TQ) current, or a seed current for the secondary source of RE through the avalanche mechanism. An analytic model which omits pitch-angle scattering is often used in literature for estimating the hot-tail RE density (Smith and Verwichte 2008 Phys. Plasmas 15 072502). In the present study, we use the CQL3D bounce-averaged Fokker–Planck code (Harvey and McCoy 1992 Proc. IAEA Technical Committee Meeting on Advances in Simulation and Modeling of Thermonuclear Plasmas p 527) to test the limits of validity of the model. In particular, we examine the cases of Z= 1 and Z= 18 ions, for sets of different initial temperature, density, electric field and the characteristic time of temperature decay. We show that for Z = 1 plasma, the ratio of RE density computed by CQL3D to that estimated from the model is within 0.6–6.0 in studied cases. For the Z = 18 case, this factor is systematically a much smaller number, typically 0.02–0.6. We suggest a simple correction to the model that narrows down the range of this ratio to 0.3–3.8 in all of the cases, including Z = 1 and Z = 18 plasmas.

Numerical study of the synergy effects of electron cyclotron wave and two lower-hybrid waves in the current drive process

In recent experiments on the experimental advanced superconducting tokamak, the electron cyclotron wave and the two lower-hybrid waves at different frequencies, i.e., 4.6 GHz and 2.45 GHz, are applied simultaneously to sustain and control the plasma current. To investigate the synergy effects of the three waves, the Fokker-Planck equation with the quasi-linear diffusions induced by the three waves is solved numerically with the CQL3D code [R. W. Harvey and M. G. McCoy, in Proceedings of IAEA Technical Committee Meeting on Advances in Simulation and Modeling of Thermonuclear Plasmas, Montreal, Canada (1992)]. It is found that there might be strong synergy effects between the three waves. The electrons in the low velocity region in the velocity space can be accelerated perpendicularly by the electron cyclotron wave, and their parallel velocities can be increased due to scattering and fall into the resonance regions of the lower-hybrid waves. Therefore, such processes may bring more electrons to resonate with the lower-hybrid waves and enhance the current drive of the lower-hybrid waves. The synergy effects strongly depend on the distance between the resonance regions in the velocity space of the three waves.

Synergy effects during current drive by two lower-hybrid waves

In recent lower-hybrid current drive experiments on the experimental advanced superconducting tokamak, two lower-hybrid waves are launched simultaneously from different locations with different phase velocities to drive the plasma current. To understand the synergy effects of the two LH waves, the analytical expression for the electron velocity distribution is obtained based on Fuchs' model [Fuchs et al., Phys. Fluids 28(12), 3619–3628 (1985)], which is in good agreement with that obtained by solving the quasi-linear equation numerically via the CQL3D code [R. W. Harvey and M. G. McCoy, in Proceedings of IAEA Technical Committee Meeting on Advances in Simulation and Modeling of Thermonuclear Plasmas, Montreal, Canada (1992)]. The synergy factor is also obtained analytically. It is found that the existence of two resonant regions may bring more resonant electrons interacting with each wave and the perpendicular dynamics can further enhance the synergy effect by increasing the effective electron temperature, which in turn increases the number of electrons in the resonance with each wave.

Plasma isotopic effect on the damping rate of toroidal Alfvén eigenmodes with intermediate toroidal mode numbers**This paper follows up the contribution presented at the 12th IAEA Technical Committee Meeting on Energetic Particles in Magnetic Confinement Systems (Austin, TX, 8–10 September 2011).

This paper reports on the results of recent experiments performed on the JET tokamak on toroidal Alfvén eigenmodes (TAEs) with toroidal mode number (n) in the range up to |n| = 7. The stability properties of these medium-n TAEs are investigated experimentally using a set of compact in-vessel antennas, providing a direct and real-time measurement of the frequency, damping rate and amplitude for each individual toroidal mode number. The measurements, reported here, of the damping rate (γ/ω) for these medium-n modes were obtained during a deuterium to helium to hydrogen changeover experimental campaign, and are used to infer the effect of the plasma effective isotopic composition (AEFF) on the stability properties of these medium-n TAEs. We find that the damping rate of n = 1 TAEs decreases approximately as γ/ω ∼ 1/AEFF as reported previously, but only for modes whose frequency is close to the centre of the n = 1 toroidal gap and for density and current profiles giving an open gap structure. Conversely, for n ⩾ 3 TAEs we find that their damping rate approximately increases as γ/ω ∼ AEFF.

The dependence of the damping rate of medium-n toroidal Alfvén eigenmodes on the edge plasma elongation in JET**This paper is an extended version of a contribution presented at the 11th IAEA Technical Committee Meeting on Energetic Particles (Kyiv, Ukraine, 21–23 September 2009).

This paper reports the first quantitative analysis of the measurements of the damping rate (γ/ω) for stable Alfvén eigenmodes (AEs) with toroidal mode number (n) in the range |n| = 3–15 as a function of the edge plasma elongation (κ95). We find that the damping rate γ/ω versus κ95 for medium-n toroidal AEs (TAEs), with n = 3 and n = 7, increases for increasing elongation, i.e. its scaling versus κ95 follows the same trend previously measured and explained theoretically for the n = 1 and n = 2 TAEs. Theoretical analysis of the measurements for the n = 3 TAEs has been performed using the LEMan code. The results are in good agreement (within a factor of two) for all the magnetic configurations where there is only a very minor up/down asymmetry in the poloidal cross-section of the plasma. These experimental results further confirm the possibility of using the edge shape parameters as a real-time actuator for control of the stability of alpha-particle driven AEs in burning plasma experiments, such as ITER.

Fast ion redistribution and losses in JET advanced tokamak scenario**This paper is an extended version of a contribution presented at the 11th IAEA Technical Committee Meeting on Energetic Particles, Kyiv (Ukraine), 21–23 September 2009.

The influence of Alfvén eigenmodes (AEs) (core-localized toroidal Alfvén eigenmode (TAE) and high-m global TAE) on redistribution and loss of fast ions in the advanced tokamak scenario has been investigated in JET dedicated experiments. Temporal evolution of the spatial structure of AEs present in the plasma during q-profile evolution has been calculated with the MISHKA code. On the other hand, fast ion losses with resolution in energy and pitch angle were measured with a scintillator probe, while the internal redistribution of the fast ions was analysed by measuring the temporal evolution of the radial profiles of γ-ray emission coming from the fast ions colliding with the main impurities, Be and C. Correlating the measurements above, it is possible to assess the effect of AEs on redistribution and loss of fast ions. It is found that the TAE localized in the core of the plasma causes a severe redistribution of the fast ions in the plasma centre, leading to a flattening of the vertical profile of γ-ray emission. In addition, the core-localized TAE also causes enhanced fast ion losses. As the core-localized TAE moves radially outwards, a transition to high-m global TAE occurs. In the transformation phase, a coupled mode with components both in the core and in the edge of the plasma exists. The maximum number of losses is normally measured when this coupled mode with a large radial extent is present in the plasma.

Report of IAEA Technical Committee Meeting on Revision of the Basic Safety Standards (BSS): Newest Recommendations on Health Effects from Radon -The Impact of Regulatory Requirements-

Report of IAEA Technical Committee Meeting on Revision of the Basic Safety Standards (BSS): Newest Recommendations on Health Effects from Radon -The Impact of Regulatory Requirements-

An assessment of full wave effects on the propagation and absorption of lower hybrid waves

Lower hybrid (LH) waves (Ωci⪡ω⪡Ωce, where Ωi,e≡Zi,eeB/mi,ec) have the attractive property of damping strongly via electron Landau resonance on relatively fast tail electrons and consequently are well-suited to driving current. Established modeling techniques use Wentzel–Kramers–Brillouin (WKB) expansions with self-consistent non-Maxwellian distributions. Higher order WKB expansions have shown some effects on the parallel wave number evolution and consequently on the damping due to diffraction [G. Pereverzev, Nucl. Fusion 32, 1091 (1991)]. A massively parallel version of the TORIC full wave electromagnetic field solver valid in the LH range of frequencies has been developed [J. C. Wright et al., Comm. Comp. Phys. 4, 545 (2008)] and coupled to an electron Fokker–Planck solver CQL3D [R. W. Harvey and M. G. McCoy, in Proceedings of the IAEA Technical Committee Meeting, Montreal, 1992 (IAEA Institute of Physics Publishing, Vienna, 1993), USDOC/NTIS Document No. DE93002962, pp. 489–526] in order to self-consistently evolve nonthermal electron distributions characteristic of LH current drive (LHCD) experiments in devices such as Alcator C-Mod and ITER (B0≈5 T, ne0≈1×1020 m−3). These simulations represent the first ever self-consistent simulations of LHCD utilizing both a full wave and Fokker–Planck calculation in toroidal geometry.

Turbulence behaviour in the presence of transport barriers**Invited paper.

There is general agreement that the creation of transport barriers in magnetized plasmas is associated with the reduction in turbulence-driven transport. The fundamental physics involved in barrier formation is the effect of equilibrium E × B shear and zonal flows on turbulence and transport. This paper focuses on three major issues in turbulence and transport barriers that were discussed at the Tenth IAEA Technical Committee Meeting on H-Mode Physics and Transport Barriers: (1) zonal flows and their effects on turbulence, (2) spatial spreading of turbulence from regions of instability to regions of stability and (3) the effects of short wavelength turbulence. This work gives a short summary of experimental work bearing on each of the themes and raises fundamental questions to motivate future research in each of these areas.

4th IAEA Technical Committee Meeting on Control, Data Acquisition, and Remote Participation for Fusion Research

4th IAEA Technical Committee Meeting on Control, Data Acquisition, and Remote Participation for Fusion Research

Conference Summary: Summary of the 14th IAEA Technical Committee Meeting on Research UsingSmall Fusion Devices

Report on the 14th IAEA Technical Committee Meeting on Research Using Small FusionDevices held on 25--27 June 2001 at Sao Paulo University, Brazil.

Conference Summary: Energetic particles in magnetic confinement systems

Report on the 7th IAEA Technical Committee Meeting held at the Scandic Hotel Opalen, Gothenburg, Sweden, 8-11 October 2001

Fusion Reactor Safety

Report on the 7th IAEA Technical Committee Meeting held at Cannes, France, 13-16 June 2000.

Fusion safety studies in Russia from 1996 to 2000

The paper is a review of results of some fusion safety studies performed in Russia during 1996–2000 (after the 6th IAEA Technical Committee Meeting on ‘Developments in Fusion Safety’). In particular, changes of hydrogen isotope inventory in co-deposited layers on tungsten and graphite under simulation of normal tokamak operation and plasma disruptions are reported. Size distribution of graphite and W-Be-dust under the experiments at issue revealed two maximums. Erosion products were collected within tokamak T-10. Their microstructure and chemical composition were studied and are reported in the paper. Chemical interaction of beryllium powder with steam (in temperature range of 400–600°C) and air (at 500–1000°C) was investigated. Mathematical models describing it were developed. Chemical interaction of Be-powder with carbon was studied too. A dependence of beryllium carbide layer thickness growth versus time at 950°C was found on the basis of the experiments.

Overview of the Japanese fusion reactor studies programme

Fusion reactor design studies conducted in Japan since the fifth IAEA Technical Committee Meeting and Workshop on Fusion Reactor Design and Technology in 1993 are introduced. During the 5 years since the last IAEA-TCM, the fusion reactor studies in Japan became more focused concentrating on tokamaks, helical devices and laser fusion. There are four tokamak reactor studies, namely, A-SSTR, CREST, DREAM and IDLT. There are helical reactor studies such as FFHR and one laser fusion reactor study on KOYO. Compared with the situation of 5 years ago having many types of confinement schemes being studied, the confinement schemes are now limited to three and each of the effort is larger involving greater number of scientists. The cooperation and exchanges between the design groups have increased through various committees, meetings and symposia.

Energetic particles in magnetic confinement systems

Report on the 6th IAEA Technical Committee Meeting held at Naka-machi, Naka-gun, Ibaraki, Japan 12-14 October 1999

Steady-state operation of magnetic fusion devices - plasma control and plasma facing components

Report on the IAEA Technical Committee Meeting held at Fukuoka, 25-29 October 1999.

First principles based transport theory

Report on the IAEA Technical Committee Meeting held at Kloster Seeon, Germany, 21-23 June 1999

Neoclassical transport coefficients for general axisymmetric equilibria in the banana regime

Using the standard approach of neoclassical theory, a set of relatively simple kinetic equations has been obtained, suited for an implementation in a numerical code to compute a related set of distribution functions. The transport coefficients are then expressed by simple integrals of these functions and they can be easily computed numerically. The code CQL3D [R. W. Harvey and M. G. McCoy, in Proceedings of IAEA Technical Committee Meeting on Advances in Simulation and Modeling of Thermonuclear Plasmas, Montreal, 1992 (International Atomic Energy Agency, Vienna, 1993), pp. 489–526], which uses the full collision operator and considers the realistic axisymmetric configuration of the magnetic surfaces, has been modified to solve the bounce-averaged version of these equations. The coefficients have then been computed for a wide variety of equilibrium parameters, high-lighting interesting features of the influence of geometry at small aspect ratio. Differences with the most recent formulas for the ion neoclassical heat conductivity are pointed out. A set of formulas, which fit the code results, is obtained to easily evaluate all the neoclassical transport coefficients in the banana regime, at all aspect ratios, in general axisymmetric equilibria. This work extends to all the other transport coefficients, at least in the banana regime, the work of Sauter et al. [O. Sauter, C. Angioni, and Y. R. Lin-Liu, Phys. Plasmas 6, 2834 (1999)] which evaluates the neoclassical conductivity and all the bootstrap current coefficients. Formulas for arbitrary collisionality regime are proposed, obtained combining our results for the banana regime with the results of Hinton and Hazeltine [F. L. Hinton and R. D. Hazeltine, Rev. Mod. Phys. 48, 239 (1976)], adapted for small aspect ratio.

Steady state operation of tokamaks

Report on the IAEA Technical Committee Meeting held at Hefei, China 13-15 October 1998.