Tore Supra Tokamak Research Articles

Electric Probes in Tokamaks: Experience in Tore Supra

AbstractEfforts to provide regular, automatic, and reliable reciprocating probe measurements in the scrape‐off layer of the Tore Supra tokamak are described. Real time feedback on fast magnetic reconstruction is used to control the probe position relative to the last closed flux surface, while analysis of the current‐voltage characteristics is used to adapt the voltage waveform applied to the probe, improving the dynamic range of the measurement. A large probe head is employed to ensure operation in the strongly magnetized plasma regime. Concave collectors embedded in the probe head, so‐called “tunnel probes”, are shown to be immune to sheath expansion effects, providing absolutely calibrated measurements of parallel ion current density (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fuel retention study in fusion reactor walls by micro-NRA deuterium mapping

Nuclear Reaction Analysis (NRA) with a 3He ion beam is a powerful analytical technique for analysis of light elements in thin films. The main motivation for 3He focused beam applications is lateral mapping of deuterium using the nuclear reaction D(3He,p)4He in surfaces exposed to a tokamak plasma, where a lateral resolution in the μm-range provides unique information for fuel retention studies.At the microprobe at the Jožef Stefan Institute typical helium ion currents of 300pA and beam dimensions of 4×4μm2 can be obtained. This work is focused on micro-NRA studies of plasma-facing materials using a set-up consisting of a silicon partially depleted charge particle detector for NRA spectroscopy applied in parallel with a permanently installed X-ray detector, an RBS detector and a beam chopper for ion dose monitoring. A method for absolute deuterium quantification is described. In addition, plasma-deposited amorphous deuterated carbon thin films (a-C:D) with known D content were used as a reference.The method was used to study deuterium fuel retention in carbon fibre composite materials exposed to a deuterium plasma in the Tore Supra and TEXTOR tokamaks. The high lateral resolution of micro-NRA allowed us to make a detailed study of the influence of topography on the fuel retention process. We demonstrated that the surface topography plays a dominant role in the retention of deuterium. The deep surfaces inside the castellation gaps showed approximately two orders of magnitude lower deuterium concentrations than in areas close to the exposed surface.

Laser-based and thermal methods for fuel removal and cleaning of plasma-facing components

The efficiency of two methods for in situ fuel removal has been tested on carbon and tungsten limiters retrieved from the TEXTOR and Tore Supra tokamaks: laser-induced ablation of co-deposits and annealing in vacuum at elevated temperature. The analyses of gas phase and surfaces performed with thermal desorption spectrometry, optical spectroscopy, ion beam analysis, surface profilometry and microscopy methods have shown: (i) the ablation leads to the generation of dust particles of 50nm – 2μm; (ii) volatile products of ablation undergo condensation on surrounding surfaces; (iii) D/C ratio in such condensate is in the range 0.02–0.03; (iv) long-term annealing of 623K for 70h results in release of not more than ∼10% of deuterium accumulated in plasma-facing components; (v) effective removal is reached by heating to 900–1300K.

Characterization and damaging law of CFC for high heat flux actively cooled plasma facing components

The carbon fiber reinforced carbon composite (CFC) Sepcarb N11 has been used in the Tore Supra (TS) tokamak (Cadarache, France) as armour material for the plasma facing components. For the fabrication of the Wendelstein 7-X (W7-X) divertor (Greifswald, Germany), the NB31 material was chosen. For the fabrication of the ITER divertor, two potential CFC candidates are the NB31 and NB41 materials. In the case of Tore Supra, defects such as microcracks or debonding were found at the interface between CFC tile and copper heat sink. A mechanical characterization of the behaviour of N11 and NB31 was undertaken, allowing the identification of a damage model and finite element calculations both for flat tiles (TS and W7-X) and monoblock (ITER) armours. The mechanical responses of these CFC materials were found almost linear under on-axis tensile tests but highly nonlinear under shear tests or off-axis tensile tests. As a consequence, damage develops within the high shear-stress zones.

Test of fiber optic based current sensors on the Tore Supra tokamak

Fiber optic current sensors have been recently considered for current measurements in ITER. These sensors exploit Ampere's law by measuring the change of light polarization along an optical fiber subjected to a magnetic field aligned along the fiber axis. Through collaboration between CEA and SCK·CEN, such fiber optic current sensors have been installed on Tore Supra tokamak as a first demonstration for possible future applications in ITER. This paper aims at reporting the results obtained on Tore Supra. It discusses the technique used to separate the effects of the intrinsic linear birefringence and the magneto-induced circular birefringence, the latter enabling the determination of the current flowing through the area defined by the optic fiber loop. Three optical fibers have been installed on Tore Supra for assessment of their linearity response on a wide range of plasma current from 0.3 to 1.4 MA as well as their long term stability. The extrapolation of the results and the issues related to a future application on ITER are finally discussed.

Coupling characteristics of the ITER-relevant lower hybrid antenna in Tore Supra: experiments and modelling

A new concept of lower hybrid antenna for current drive has been proposed for ITER (Bibet et al 1995 Nucl. Fusion 35 1213–23): the passive active multijunction (PAM) antenna that relies on a periodic combination of active and passive waveguides. An actively cooled PAM antenna at 3.7 GHz has recently been installed on the tokamak Tore Supra. This paper summarizes the comprehensive experimental characterization of the coupling properties of the PAM antenna to the Tore Supra plasmas. In this paper, the electromagnetic properties of the antenna are measured at a reduced power (<1 MW) to allow a systematic comparison with linear wave coupling theory and the associated modelling based on the linear ALOHA code. In a wide range of edge electron densities at the antenna aperture (spanning a factor 20 from 0.5 × nc to 10 × nc where nc is the slow wave density cut-off, nc = 1.7 × 1017 m−3 at 3.7 GHz) and antenna phasing, the ALOHA simulations reproduce the experimental results observed on Tore Supra. In addition, reduced power reflection coefficients (<5%) are measured at a low edge density, close to nc, i.e. in the range 0.5–3 × nc. Measurement and analysis with ALOHA of the antenna–plasma scattering matrices provide explanation of the good coupling properties of the PAM antenna close to nc by highlighting the crucial role of the slow wave intercoupling between active and passive waveguides through the plasma edge. This detailed validation of the coupling modelling is an important step towards the validation of the PAM concept in view of further optimizing the electromagnetic properties of the future ITER antenna.

Control and Verification of the Safety-Factor Profile in Tokamaks Using Sum-of-Squares Polynomials.

AbstractIn this paper, we propose a method of using the sum-of-squares methodology to synthesize controllers for plasma stabilization in Tokamak reactors. We use a partial differential model of the poloidal magnetic flux gradient and attempt to stabilize a reference safety-factor profile. Our methods utilize full-state feedback control and are based on solving a dual version of the Lyapunov operator inequality. In addition, we implement the controller in-silico using experimental conditions inferred from the Tore Supra tokamak.

Diagnostics carried by a light multipurpose deployer for vacuum vessel interventions

ITER will greatly rely on remote-handling operations to accomplish its scientific missions. Robotic systems will also be required to operate inside vacuum vessels in order to limit or replace human access, to intervene quickly between experimental sessions for in-vessel inspections and measurements, and to preserve the machine conditioning and thus improve machine availability. In this prospect, a multipurpose carrier prototype called Articulated Inspection Arm (AIA) was developed by CEA laboratories within the European work program. With an embedded camera, it successfully demonstrated close inspection feasibility inside Tore Supra tokamak. The AIA robot was designed for mini-invasive operations with interchangeable diagnostics to be plugged at its head. This covers various applications for the safety, the operation and the scientific mission (in-vessel inspection, plasma diagnostics calibrations or inner components analysis and treatments). This paper presents recent analysis and results obtain with diagnostics developed by CEA for in-vessel remote-handling intervention.

Feasibility study of carbon particles injection by laser ablation to calibrate spectroscopic erosion measurements in Tore Supra

To check if spectroscopic measurements can be used for erosion determination, we propose to inject in the line of sight of the diagnostic, during plasma operation, a known carbon particle source, produced by laser ablation. A first assessment of this technique will be presented in the context of the Tore Supra tokamak.

Similarities and differences between dust produced in laboratory plasmas and in the MAST and Tore Supra tokamaks

It is shown that among the dust produced from plasma–surface interaction in tokamaks, nanoparticles growing in gas phase can be produced in large quantities in MAST and Tore Supra, two tokamaks of carbon-based plasma-facing components. Laboratory plasma experiments and models are also presented showing that when carbonaceous species are present, complex chemical pathways which depend strongly on plasma conditions lead to various molecular precursors. Despite this fact, the final solid particles produced in the laboratory plasmas and tokamaks considered here, present similarities giving information on their growth and transport in the plasma.

Ion Temperature Measurements in the Tore Supra Scrape‐Off Layer Using a Retarding Field Analyzer

AbstractThe retarding field analyzer (RFA) is one of the only widely accepted diagnostics for measuring the ion tem‐perature Ti in the tokamak scrape‐off layer. An overview of the outstanding RFA performance over ten years of operation in Tore Supra tokamak is given and the validation of Ti measurements is addressed. The RFA measurements in Tore Supra are found to be well reproducible. The ion‐to‐electron temperature ratio is higher than one at low‐to‐moderate ion‐electron collisionality regime and converges to unity at high collisionality regime (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Development of in situ diagnostics and tools handled by a light multipurpose carrier for tokamak in-vessel interventions

A multipurpose carrier prototype called Articulated Inspection Arm (AIA) has been developed and demonstrated by CEA laboratories to operate embedded diagnostics or tools into a tokamak vacuum-vessel. The AIA robot with a vision diagnostic has been deployed inside the vacuum-vessel of Tore Supra tokamak and has realized closed inspections of plasma facing components’ under relevant conditions (high vacuum and temperature). CEA is engaged in development of interchangeable diagnostics or tools to be plugged on the front head of the arm. This covers applications such as in-vessel inspections, water leak localization, Plasma Facing Components (PFCs) characterization and treatment. This paper describes R&D activities performed in CEA laboratories to develop diagnostics and tools for in-vessel interventions. The AIA multipurpose carrier to carry such devices is also presented. The first demonstration of a quick intervention for plasma facing components performed in the Tore Supra Facility is described as well.

Particle transport in low core turbulence Tore-Supra plasmas

Electron and impurity transport has been studied in sawtoothing plasmas in the Tore-Supra tokamak. High time and space resolution measurements of the electron density reveal the existence of a flat profile region encompassing the q = 1 surface, on which is superimposed a density peak building up between sawtooth relaxations. For the first time in this regime, we have determined the underlying transport of both nickel and electrons independently of the effect of sawteeth in the central part of the plasma. Electron transport is consistent with the neoclassical expectations only in the close vicinity of the magnetic axis. Further out, it exceeds the neoclassical values as calculated with the NCLASS code, although the turbulence level is very low in the whole central region region. In contrast, nickel transport is in good agreement with the neoclassical calculations in the same region. The neoclassical effect on trapped particles of a persisting mode due to incomplete reconnection of the magnetic surfaces is consistent with these observations.

Field Stabilization of the Iseult/Inumac Magnet Operating in Driven Mode

A neuroscience research center with very high field MRI equipments was opened in November 2006 by the CEA life sciences division. Three MRI systems operating at 3, 7 and 17 T have been already installed. One of the imaging systems will require a 11.75 T magnet with a 900 mm warm bore. The large aperture and high field strength of this magnet provide a substantial engineering challenge compared to the largest MRI systems ever built. This magnet is being developed within an ambitious R&D program, Iseult, whose focus is high field MRI. Traditional MRI magnet design principles are not readily applicable and thus concepts taken from high energy physics or fusion experiments, namely the Tore Supra tokamak magnet system, will be used. The coil will be made of a niobium-titanium conductor cooled by a He II bath at 1.8 K, permanently connected to a cryoplant. Due to its design the magnet will be operated in a non-persistent mode. As the field stability needed for MRI imaging requires a field drift of less than 0.05 ppm/h, it is hardly feasible to directly transpose these requirements in the power supply specification. Two existing solutions developed for other applications have been selected: one using a semi-persistent mode, and the other using a short-circuited superconducting coil in the inner bore. In order to make a decision on experimental basis, an ambitious R&D field stability program has been set-up based on magnet prototypes, high field test facility (Seht, a 44 H and 8 T magnet with a warm bore to 600 mm). We will present development and experimental results of the two stabilization solutions. In conclusion, the stability solution selected for the Iseult magnet is given.

The Iseult/Inumac Whole Body 11.7 T MRI Magnet R&amp;D Program

A neuroscience research center with very high field MRI equipments has been opened in November 2006 by the CEA life science division. One of the imaging systems will require a 11.75 T magnet with a 900 mm warm bore. Regarding the large aperture and field strength, this magnet is a real challenge when compared to the largest MRI systems ever built, it is being developed within an ambitious R&D program, Iseult, focused on high field MRI. The conservative MRI magnet design principles are not readily applicable, other concepts taken from high energy physics or fusion experiments, namely the Tore Supra tokamak magnet system, will be used. The coil will thus be made of a niobium-titanium conductor cooled by a He II bath at 1.8 K, permanently connected to a cryoplant. Due to the high level of stored energy, about 340 MJ, and a relatively high nominal current, about 1500 A, the magnet will be operated in a non-persistent mode with a conveniently stabilized power supply. In order to take advantage of superfluid helium properties and regarding the high electromagnetic stresses on the conductors, the winding will be made of wetted double pancakes meeting the Stekly criterion for cryostability. The magnet will be actively shielded to fulfill the specifications regarding the stray field. In order to develop the magnet design on an experimental basis, an ambitious R&D program has been set-up based on magnet prototypes, high field test facility (Seht) and stability experiments. The main results from these experiments and their impact on the Iseult magnet design will be discussed.

Hardening Inspection Devices to Ultra-High Vacuum, Temperature and High Magnetic Field

In order to demonstrate the feasibility of a mini-invasive inspection arm in ITER, the next step in fusion energy research machines, and to match the need for such a device in the French tokamak Tore Supra, a hardened long-reach robot has been developed by the CEA-LIST robotics team. The AIA (Articulated Inspection Arm), a 8 m long, highly articulated dexterous carrier with a 10 kg payload has been developed to be deployed anywhere in the Tore Supra vacuum vessel from one single O250 mm entry point, while the vessel is kept at the operating temperature (120) and pressure . Since its successful demonstration in Tore Supra in September 2008, the robot has been identified by different teams of scientists working on the tokamak for various potential tasks, from vision to diagnostic calibration or component analysis. In order to make such a device even more available, the CEALIST team is now working on the magnetic field constraint on robots. The goal being to demonstrate capabilities of intervention in ITER conditions, the considered field is 4 to 8 T. Technologies, command schemes and design strategies available for an inspection robot tolerant to high magnetic field have been studied. This paper first details the different technologies chosen for the AIA. How such an inspection device has been integrated on a multidisciplinary research machine such as Tore Supra is then detailed in a second part. The paper finally presents various concepts and experimentations to be used in a magnetic field tolerant robot for inspection of a large superconductor machine.

Analysis of the JET FIR interferometer beam phase changes during plasmas and application for fast fringe jump corrections by electronics

The interferometer diagnostics using Far Infrared (FIR) laser beams provide, by phase measurement techniques, precise line-electron density measurements of magnetic fusion plasmas. But the FIR beams still suffer the effect of refraction when traversing the plasma. This can cause, when the density gradients are strong, temporary losses of signal that induce the so called fringe jumps on the estimated phase. On the CEA Tore Supra tokamak, a Field Programmable Get Array (FPGA) electronics has been developed using a time delay method to calculate the phase and correct the fringe jumps at the frequency rate of the probe sinusoidal signal (100 KHz). To test the efficiency of the algorithm on various plasma scenarios, a prototype of the CEA electronics has been installed on the JET tokamak and the data have been compared with those issued from the present JET electronics, which calculates the phase by a different method. Statistical comparisons between the two methods on more than 1500 JET shots are reported in this article and show that the two methods give similar results but that none of them is 100% reliable as still some fringe jumps remain, in particular when Edge Localised Modes (ELMs), pellet injections or disruptions occur. To understand this phenomenon, an analysis of the fast changes of the 100 KHz raw input signals during ELMs and pellet injections has been done with a 1MHz numerical acquisition. The typical durations of signal losses have been found to be few hundreds of micro-seconds. Meanwhile, the line density can increase and then return to its original value. Simulations show that an algorithm that would block the phase calculation during a longer time (i.e. 500 μs) than the disturbed period would help to avoid fringe jumps.

Comparison of scrape-off layer profiles in outboard-versus inboard-limited plasmas in Tore Supra

In the Tore Supra tokamak, the scrape-off layer profiles of discharges limited either on the inboard or on the outboard side are compared. Inboard-limited discharges are characterized by substantially longer SOL e-folding lengths of ion and electron temperatures and electron density measured near the top of the plasma, providing strong evidence that the ion and electron energy transport across the SOL is enhanced on the outboard side, similar to the particle transport. The parallel heat flux density extrapolated to the last closed flux surface is found to be inversely proportional to its e-folding length in the SOL. The outboard limiters thus receive higher and more peaked heat loads than the inboard limiters. These results are important for the optimization of the plasma start-up scenario and design of the first wall in ITER.

Influence of the low order rational q surfaces on the pellet deposition profile

Pellet injection is planned to be the main fuelling method on ITER. The high temperature of the plasma during a fusion burn will limit the penetration of the pellet to the outer third of the minor radius. This limited penetration is expected to be compensated by a polarization drift, which will deposit the particles deeper in the plasma for the pellets injected from the high field side. In order to evaluate the expected depth of the fuelling on ITER, a good understanding of this drift effect is important. Experimental data acquired on the DIII-D (USA) and Tore Supra (France) tokamaks show that the polarization drift is influenced by the low order rational q surfaces. These surfaces appear to attenuate the polarization mechanism as the drifting particles cross them. In this paper, a correlation between the maximum of the pellet mass deposition profile and the positions of the q = 2 and q = 3 surfaces on DIII-D and Tore Supra is shown for high field side and low field side injection. A model is proposed to explain this effect and compared with the experimental results. To conclude, the possible consequences of this phenomenon on the fuelling in ITER are described.

Cherenkov-type diamond detectors for measurements of fast electrons in the TORE-SUPRA tokamak

The paper presents a schematic design and tests of a system applicable for measurements of fast electron pulses emitted from high-temperature plasma generated inside magnetic confinement fusion machines, and particularly in the TORE-SUPRA facility. The diagnostic system based on the registration of the Cherenkov radiation induced by fast electrons within selected solid radiators is considered, and electron low-energy thresholds for different radiators are given. There are some estimates of high thermal loads, which might be deposited by intense electron beams upon parts of the diagnostic equipment within the TORE-SUPRA device. There are some proposed measures to overcome this difficulty by the selection of appropriate absorption filters and Cherenkov radiators, and particularly by the application of a fast-moving reciprocating probe. The paper describes the measuring system, its tests, as well as some results of the preliminary measurements of fast electrons within TORE-SUPRA facility.