Lower hybrid resonance: field structure and numerical modeling

The behavior the electromagnetic fields in the vicinity of the lower hybrid resonance point is studied in case of 1D plasma non-uniformity. The first of two found solutions of Maxwell’s equations is singular and describes the wave travelling to the lower hybrid resonance layer. This wave if fully absorbed without reflections. Another solution which is regular describes the standing wave. To extend the range of validity of the solutions found, they are matched to the WKB solutions. Three possibilities for numerical solving the wave propagation problem in presence of the lower hybrid resonance zone are discussed in the paper.Поведение электромагнитных полей в непосредственной близости от нижнего гибридного резонанса изучено в случае одномерной неоднородности плазмы. Первое из двух найденных решений уравнений Максвелла сингулярно и описывает волну, падающую на слой нижнего гибридного резонанса. Эта волна полностью поглощается без отражений. Другое решение, которое регулярно, описывает стоячую волну. Чтобы расширить область применимости найденных решений, они приведены к виду ВКБ-решений. Обсуждаются три возможности для численного решения задачи распространения волн в присутствии зоны нижнего гибридного резонанса.Поведінка електромагнітних полів в безпосередній близькості від нижнього гібридного резонансу вивчено в разі одновимірної неоднорідності плазми. Перше з двох знайдених рішень рівнянь Максвелла сингулярно і описує хвилю, що падає на шар нижнього гібридного резонансу. Ця хвиля повністю поглинається без відбиття. Інше рішення, яке регулярно, описує стоячу хвилю. Щоб розширити область застосовності знайдених рішень, вони приведені до вигляду ВКБ-рішень. Обговорюються три можливості для чисельного рішення задачі поширення хвиль у присутності зони нижнього гібридного резонансу

Monte Carlo simulation of ICRF discharge initiation in ITER

Discharges produced and sustained by ion cyclotron range of frequency (ICRF) waves in absence of plasma current will be used on ITER for (ion cyclotron-) wall conditioning (ICWC). The here presented simulations aim at ensuring that the ITER ICRH& CD system can be safely employed for ICWC and at finding optimal parameters to initiate the plasma. The 1D Monte Carlo code RFdinity1D3V was developed to simulate ICRF discharge initiation. The code traces the electron motion along one toroidal magnetic field line, accelerated by the RF field in front of the ICRF antenna. Electron collisions in the calculations are handled by a Monte Carlo procedure taking into account their energies and the related electron collision cross sections for collisions with H-2, H-2(+) and H+. The code also includes Coulomb collisions between electrons and ions (e - e; e - H-2(+); e - H+). We study the electron multiplication rate as a function of the RF discharge parameters (i) antenna input power (0.1-5MW), and (ii) the neutral pressure (H-2) for two antenna phasing (monopole [0000]-phasing and small dipole [0 pi 0 pi]-phasing). Furthermore, we investigate the electron multiplication rate dependency on the distance from the antenna straps. This radial dependency results from the decreasing electric amplitude and field smoothening with increasing distance from the antenna straps. The numerical plasma breakdown definition used in the code corresponds to the moment when a critical electron density nec for the low hybrid resonance (omega = omega(LHR)) is reached. This numerical definition was previously found in qualitative agreement with experimental breakdown times obtained from the literature and from experiments on the ASDEX Upgrade and TEXTOR

Ion cyclotron wall conditioning experiments on Tore Supra in presence of the toroidal magnetic field

Wall conditioning techniques applicable in the presence of the high toroidal magnetic field will be required for the operation of ITER for tritium removal, isotopic ratio control and recovery to normal operation after disruptions. Recently ion cyclotron wall conditioning (ICWC) experiments have been carried out on Tore Supra in order to assess the efficiency of this technique in ITER relevant conditions. The ICRF discharges were operated in He/H-2 Mixtures at the Tore Supra nominal field (3.8 T) and a RF frequency of 48 MHz, i.e. within the ITER operational space. RF pulses of 60 s (max.) were applied using a standard Tore Supra two-strap resonant double loop antenna in ICWC mode, operated either in pi or 0-phasing with a noticeable improvement of the RF coupling in the latter case. In order to assess the efficiency of the technique for the control of isotopic ratio the wall was first preloaded using a D-2 glow discharge. After 15 minutes of ICWC in He/H-2 gas mixtures the isotopic ratio was altered from 4% to 50% at the price of an important H implantation into the walls. An overall analysis comparing plasma production and the conditioning efficiency as a function of discharge parameters is given

investigation of probe surfaces after ion cyclotron wall conditioning in asdex upgrade

For the first time, material analysis techniques have been applied to study the effect of ion cyclotron wall conditioning (ICWC) on probe surfaces in a metal-wall machine. ICWC is a technique envisaged to contribute to the removal of fuel and impurities from the first wall of ITER. The objective of this work was to assess impurity migration under ICWC operation. Tungsten probes were exposed in ASDEX Upgrade to discharges in helium. After wall conditioning, the probes were covered with a co-deposited layer containing D, B, C, N, O and relatively high amount of He. The concentration ratio He/C+B was 0.7. The formation of the co-deposited layer indicates that a fraction of the impurities desorbed from the wall under ICWC operation are transported by plasma and deposited away from their original location. Keywords: ICWC, Erosion-deposition, Fuel removal, Ion beam analysis, ASDEX Upgrad

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