Dynamic modelling of lower hybrid current drive

Injection of lower hybrid current drive into the current ramp-up phase of Joint European Torus (JET) discharges has been observed to produce an annular current distribution with a core region of essentially zero current density [Hawkes, et al., Phys. Rev. Lett. 87, 115001 (2001)]. Similar "current holes" have been observed in Japan Atomic Energy Research Institute (JAERI) Tokamak 60 Upgrade (JT-60U) discharges with off-axis current drive supplied by bootstrap current [T. Fujita, et al., Phys. Rev. Lett. 87, 245001 (2001)]. In both cases, the central current does not go negative although current diffusion calculations indicate that there is sufficient non-inductive current drive for this to occur. This is explained by Multi-level 3D code (M3D) nonlinear 2D and 3D resistive magnetohydrodynamic (MHD) simulations in toroidal geometry, which predict that these discharges undergo n = 0 reconnection events -"axisymmetric sawteeth" -that redistribute the current to hold its core density near zero. Unlike conventional sawteeth, these events retain the symmetry of the equilibrium, and thus are best viewed as a transient loss of equilibrium caused when an ι = 0 rational surface enters the plasma. If the current density profile has a central minimum, this surface will enter on axis; otherwise it will enter off-axis. In the first case, the reconnection is limited to a small region around the axis and clamps the core current at zero. In the second case, more typical of the JET experiments, the core current takes on a finite negative value before the ι = 0 surface appears, resulting in discrete periodic axisymmetric sawtooth events with a finite minor radius. Interpretation of the simulation results is given in terms of analytic equilibrium theory, and the relation to conventional sawteeth and to a recent reduced-MHD analysis of this phenomenon in cylindrical geometry [Huysmans, et al., Phys. Rev. Lett. 87, 245002 (2001)] is discussed.

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“…13 The response of the current profile to the sudden application of an off-axis source is shown in Fig. 1.…”

Section: Predictions Of the 1d Modelmentioning

confidence: 99%

Simulation studies of the role of reconnection in the “current hole” experiments in the Joint European Torus

2003

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“…This is modeled using the LSC code [23], using an adjoint method [24] for solving the Boltzmann equation. The spectrum of n || is assumed to peak between 1.45 and 2.45.…”

Section: External Current Drive and Heatingmentioning

confidence: 99%

Current control in ITER steady state plasmas with neutral beam steering

Budny

2010

Physics of Plasmas

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Predictions of quasi steady state DT plasmas in ITER are generated using the PTRANSP code. The plasma temperatures, densities, boundary shape, and total current (9 -10 MA) anticipated for ITER steady state plasmas are specified. Current drive by negative ion neutral beam injection, lower-hybrid, and electron cyclotron resonance are calculated. Four modes of operation with different combinations of current drive are studied. For each mode, scans with the NNBI aimed at differing heights in the plasma are performed to study their effects on current control on the q profile. The time-evolution of the currents and q are calculated, and long-duration transients (up to ≃ 1500 s) are predicted. Effects of the beam and alpha ion pressures on the MHD equilibrium are predicted to significantly alter the bootstrap current. The TEQ equilibrium solver is found to be much more accurate than the VMEC solver. Quasi steady state, strongly reversed q profiles are predicted for some beam injection angles when the current drive and bootstrap currents are sufficiently large.

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“…The Tokamak Simulation Code El61 (TSC) coupled t o the Lower Hybrid Simulation Code [17] (LSC) was used to simulate the LHCD results on PBX-M. As documented in reference (17), the program assumes an axisymmetric toroidal geometry, and uses ray tracing to determine the lower hybrid wave p r o p agation. The influence of the local electric field on the electron velocity distribution function is neglected in the calculation of the absorbed radio frequency (rf) power.…”

Section: Tsc/lsc Without Diffusionmentioning

confidence: 99%

“…It is possible to write the Fokker-Planck equation for the electron distribution function as [17] where f is the electron distribution function, and the terms on the right-hand side represent the quasilinear rf source, collisional spreading in velocity, collisional slowing, and spatial diffusion. Suppose Df,, is independent of velocity, or that the range of velocities of interest is small if Df,, is velocity-dependent.…”

Section: Heuristic Current Diflusion Estimatementioning

confidence: 99%

Spreading of wave-driven currents in a tokamak

Ignat¹,

Kaita²,

Jardin³

et al. 1996

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ABSTRACT. Lower hybrid current drive (LHCD) in the tokamak Princeton Beta Experiment-Modification (PBX-M) is computed with a dynamic model in order to understand an actual discharge aimed at raising the central q above unity. Such configurations offer advantages for steady-state operation and plasma stability. For the particular parameters of this PBX-M experiment, the calculation found singular profiles of plasma current density J and safety factor q developing soon after LHCD begins. Smoothing the lower hybrid-driven current and power using a diffusion-like equation and a velocity-independent diffusivity for fast-electron current brought the model into reasonable agreement with the measurements if DfaJt rs 1.0 mZ/s. Such a value for Dfast is in the range suggested by other work.

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Dynamic modelling of lower hybrid current drive

Cited by 94 publications

References 50 publications

Simulation studies of the role of reconnection in the “current hole” experiments in the Joint European Torus

Simulation studies of the role of reconnection in the “current hole” experiments in the Joint European Torus

Current control in ITER steady state plasmas with neutral beam steering

Spreading of wave-driven currents in a tokamak

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