P. Moreau

Experimental study of disruption mitigation using massive injection of noble gases on Tore Supra

Disruptions are a major threat for future tokamaks, including ITER. Disruption-generated heat loads, electromagnetic forces and runaway electrons will not be tolerable for next-generation devices. Massive noble gas injection is foreseen as a standard mitigation system for these tokamaks. Disruption mitigation experiments have been carried out on Tore Supra to study various injection scenarios and to investigate gas jet penetration and mixing. Comparisons of different gases (He, Ne, Ar, He/Ar mixture) and amounts (from 5 to 500 Pa m3) were made, showing that light gases are more efficient regarding runaway electron suppression than heavier gases. Eddy currents in the limiter are moderately reduced by all the gases, and may be more dependent on the time constants of the structures than on the gas species. The density rise induced by the massive injection before the thermal quench is higher and faster with light gases. Gas jet penetration in the cooling phase is observed to be shallow and independent of the gas nature and amount. The gas cold front is stopped along the q = 2 surface where it triggers MHD instabilities, expelling thermal energy from the plasma core.

Validation of the ITER-relevant passive-active-multijunction LHCD launcher on long pulses in Tore Supra

A new ITER-relevant lower hybrid current drive (LHCD) launcher, based on the passive-active-multijunction (PAM) concept, was brought into operation on the Tore Supra tokamak in autumn 2009. The PAM launcher concept was designed in view of ITER to allow efficient cooling of the waveguides, as required for long pulse operation. In addition, it offers low power reflection close to the cut-off density, which is very attractive for ITER, where the large distance between the plasma and the wall may bring the density in front of the launcher to low values. The first experimental campaign on Tore Supra has shown extremely encouraging results in terms of reflected power level and power handling. Power reflection coefficient <2% is obtained at low density in front of the launcher, i.e. close to the cut-off density, and very good agreement between the experimental results and the coupling code predictions is obtained. Long pulse operation at ITER-relevant power density has been demonstrated. The maximum power and energy reached so far is 2.7 MW during 78 s, corresponding to a power density of 25 MW m −2 , i.e. its design value at f = 3.7 GHz. In addition, 2.7 MW has been coupled at a plasma–launcher distance of 10 cm, with a power reflection coefficient <2%. Finally, full non-inductive discharges have been sustained for 50 s with the PAM.

WEST Physics Basis

With WEST (Tungsten Environment in Steady State Tokamak) (Bucalossi et al 2014 Fusion Eng. Des. 89 907-12), the Tore Supra facility and team expertise (Dumont et al 2014 Plasma Phys. Control. Fusion 56 075020) is used to pave the way towards ITER divertor procurement and operation. It consists in implementing a divertor configuration and installing ITER-like actively cooled tungsten monoblocks in the Tore Supra tokamak, taking full benefit of its unique long-pulse capability. WEST is a user facility platform, open to all ITER partners. This paper describes the physics basis of WEST: the estimated heat flux on the divertor target, the planned heating schemes, the expected behaviour of the L-H threshold and of the pedestal and the potential W sources. A series of operating scenarios has been modelled, showing that ITER-relevant heat fluxes on the divertor can be achieved in WEST long pulse H-mode plasmas.

An analog integrator for thousand second long pulses in Tore Supra

In the year 2000, a new version of the analog integrator used in Tore Supra for magnetic measurements has been designed. Although based on the same principles, this new integrator was improved in order to satisfy the new long duration of pulses, up to 1000 s, expected in near future on Tore Supra. After recalling the basic principles of the analog integrators and recent improvements, this article explains the main characteristics of the new integrators, tests and obtained results. The conclusion underlines the most important results and, in particular, the capability of the new integrator to answer to the International thermonuclear experimental reactor (ITER) requirements.

Operational limits during high power long pulses with radiofrequency heating in Tore Supra

Issues related to the limitations and optimization of long pulse operation at high radiofrequency (RF) power levels in the Tore Supra tokamak are presented. An increasing operational limitation was encountered during the experimental campaigns in 2006?2007, affecting the high power and long pulse performance. This limitation was characterized by the sudden appearance of a multifaceted asymmetric radiation from the edge (MARFE), often followed by a disruption. The analyses revealed that the limitation could be linked to over-heating and flaking of the carbon re-deposition layers on the main plasma facing components (PFCs). The carbon deposits on all PFCs were therefore completely removed during the winter shutdown 2007?2008. Following this, a remarkable improvement in the injected power capability was observed, resulting in almost 12?MW of injected power during 10?s, without any of the previous signs of limitation (MARFE, disruption).Furthermore, the RF antennas are subject to localized heat loads due to RF sheath effects and interaction by fast particles, effects which need to be minimized in particular for long pulse operation. Experimental results concerning the heat load on the antennas, caused by fast ion losses in the presence of magnetic ripple, are presented in this paper.

Temperature oscillating regimes in Tore Supra diagnosed by MHD activity

This paper describes what we can learn on the regimes of spontaneous electron temperature oscillations discovered in Tore Supra from the analysis of MHD activity. Since the first observations of this oscillating behaviour of plasma equilibrium, and its interpretation as a predator-prey system involving lower hybrid waves power deposition and electron confinement, analysis of MHD modes has confirmed the reality of safety factor profile oscillations. This points towards the importance of rational values of the safety factor in the transition to transport barriers in reversed magnetic shear plasmas.

Experimental evaluation of stable long term operation of semiconductor magnetic sensors at ITER relevant environment

The paper deals with radiation resistant sensors and their associated measuring instrumentation developed in the course of R and D activities carried out in the framework of an international collaboration. The first trial tests of three-dimensional (3D) probes with Hall sensors have been performed in European tokamaks TORE SUPRA (2004) and JET (2005). Later in 2009 six sets of 3D probes were installed in JET and now continue to operate. The statistical analysis performed in 2014 on the basis of the JET database have demonstrated stable long term operation of all 18 sensors of 3D probes. The results of measurements conducted at the neutron fluxes of nuclear reactors have demonstrated the operability of the sensors up to high neutron fluences of F > 1018n cm−2 that exceeds the maximum one for the locations of steady state sensors in ITER over its total lifetime.

MHD stability of fully non-inductive discharges in Tore Supra

Fully non-inductive discharges with lower hybrid (LH) current drive have been run successfully in Tore Supra, allowing for stationary plasma conditions for more than 6?min. The operational domain has been explored in terms of total plasma current (Ip), magnetic field (Bvac), and parallel refractive index (n?), which characterize the deposition profile of LH waves. MHD activity has been encountered in a number of cases, either at a low saturated level, or at a high level in the so-called MHD regime (Maget P. et al 2004 Nucl. Fusion 44 443). This work first addresses the linear MHD stability properties of the operational space (Bvac, Ip, n?). Although most regions are found to be linearly unstable, the condition for triggering the deleterious MHD regime is more restrictive, and is shown to result from the full reconnection of the double-tearing mode. A simple criterion based on the helical flux ?*, namely ?* = 0 at the outer resonant surface, is used for the identification of the region where the MHD regime is triggered.

From MHD regime to quiescent non-inductive discharges in Tore Supra: experimental observations and MHD modelling

Attempts to run non-inductive plasma discharges on Tore Supra sometimes fail due to the triggering of magneto-hydro-dynamic (MHD) instabilities that saturate at a large amplitude, producing degraded confinement and loss of wave driven fast electrons (the so-called MHD regime (Maget et al 2005 Nucl. Fusion 45 69–80)). In this paper we investigate the transition to this soft (in the sense of non-disruptive) MHD limit from experimental observations, and compare it with non-linear code predictions. Such a comparison suggests that different non-linear regimes, with periodic relaxations or saturation, are correctly understood. However, successful non-inductive discharges without detectable magnetic island at q = 2 cannot be reproduced if realistic transport coefficients are used in the computation. Additional physics seems mandatory for explaining these discharges, such as diamagnetic effects, that could also justify cases of abrupt transition to the MHD regime.

MHD stability of (2,1) tearing mode: an issue for the preforming phase of Tore Supra non-inductive discharges

The early phase of a tokamak plasma discharge can have a dramatic impact on the main heating phase. This has been a persistent problem for the development of the steady state, fully non-inductive scenario using lower hybrid current drive (LHCD) on Tore Supra. The present paper reports on recent experimental and numerical investigations showing that a tearing mode coupled to the internal kink grows on q = 2 in the ohmic phase when the total current is too low, due to the weakening of field line curvature stabilization. Then, the application of LHCD drives the island to a larger size and undermines the development of the non-inductive phase. Decreasing the edge safety factor or increasing the Lundquist number S is found to be beneficial in both the linear and non-linear MHD analyses. The experimental database, which allows covering the edge safety factor dependence, supports this interpretation.