Ergodic divertor experiments on the route to steady state operation of Tore Supra

Abstract

Ergodic divertor operation on Tore Supra is characterized by good performance in terms of divertor physics. Control of particle recirculation and impurity screening are related to the symmetry, both poloidally and toroidally, of the shell of open field lines and to its radial extent, ∆r ≈ 0.16 m. Feedback control of the divertor plasma temperature has led to controlled radiative divertor experiments. In particular, good performance is obtained when the plasma is controlled to be at a temperature comparable to the energy involved in the atomic processes (15-20 eV). For standard discharges with 5 MW total power and ICRH heating, the low parallel energy flux ≈10 MW m −2 is reduced to ≈3 MW m −2 with nitrogen injection. This is achieved at a modest cost in core dilution, ∆Z eff ≈ 0.3. Despite the large volume of open field lines (≈36%), the ergodic divertor does not reduce the possible current in the discharge since stable discharges are achieved with qsep ≈ 2. It is shown that the reorganization of the current profile in conjunction with a transport barrier in the electron temperature on the separatrix stabilizes the (2,1) tearing mode. Confinement follows the standard L mode confinement. In a few cases at high density and with no gas injection (wall fuelled discharges), 'RI-like' modes are reported with a modest increase in confinement (≈40%). Despite the lack of core fuelling on Tore Supra, high densities during ICRH pulses can be achieved with Greenwald fractions fG ≈ 1. Compatibility with both ICRH and LH is demonstrated. In particular, long pulse operation with a flat-top in excess of 20 s is achieved with LHCD.

Numerical analysis of particle recycling in the TEXTOR helical divertor

“…The TEXTOR helical divertor is one such configuration, which continues earlier RMP related research, e.g. at Tore Supra with its ergodic divertor [1][2][3][4][5]. As a result of RMP application at TEXTOR, a 3D open chaotic magnetic field structure with helical strike lines is generated at the plasma edge [6][7][8], similar to the magnetic structure of divertors in stellarators such as LHD [9,10].…”

“…Following the definition of detachment in [3] for the ergodic divertor at Tore Supra: "departure from the high-recycling regime", this latter regime at n sepx ≥ 3.5 can be regarded as detachment.…”

“…A result of the present simulations for D ⊥ = 1.0 and 1.4 is that t increases almost linearly with n sepx up to n sepx ≈ 3-3.5, after which t begins to saturate. Following the definition of detachment in [3] for the ergodic divertor at Tore Supra: 'departure from the high-recycling regime', this latter regime at n sepx 3.5 can be regarded as detachment.…”

Escape patterns of chaotic magnetic field lines in a tokamak with reversed magnetic shear and an ergodic limiter

“…20 Originally, the ergodic limiter was proposed as a procedure to obtain a uniform distribution of the exhausted power on the wall. 10,[21][22][23] The field line ergodization resulting from the resonant perturbation created by such a divertor would lead to a broadening of the scrape-off layer and a widening of the contact zone between plasma and the wall. 24 Devices based on this concept have been tested on several machines.…”

“…24 Devices based on this concept have been tested on several machines. 19,[21][22][23][25][26][27] However, further experiments have revealed that the heat and particle deposition patterns are not actually uniform as previously assumed, but are rather strongly structured, presenting a self-similarity that suggests an underlying fractal structure. [28][29][30][31][32][33][34][35] These structured patterns of the heat and particle deposition turn out to be undesirable from the point of view of controlling plasma-wall interactions, and have been investigated in a series of experiments with the dynamic ergodic divertor.…”