Toroidal Pump Limiter Research Articles

Power deposition and margins on the Tore Supra pump limiter and fabrication of the high heat flux components

A complete toroidal pump limiter (TPL) is being built at Tore Supra. This limiter will cover 7.5 m 2 of the bottom part in the vacuum inner vessel. It will be composed of 574 elementary high heat flux components facing the plasma, named fingers. Below the limiter in the pumping throat, 48 other fingers are positioned as neutraliser elements. All the fingers, made of CuCrZr alloy, are actively cooled by pressurised water and covered with carbon fiber composite (CFC). The limiter is designed to remove continuously (1000 s) a convective power of 15 MW and to exhaust a particle flow up to 4 Pa m 3/s. The power deposition on all the elements of this limiter was predicted using the TOKAFLU code developed by CEA and taking into account the self-shadowing of the limiter. The heat flux on the standard finger remains below 10 MW/m 2 whereas it reaches up to a peaked value of 15 MW/m 2 for some neutraliser fingers. Margins with regard to critical heat flux (CHF), surface temperature and interface temperature between CFC and copper are studied. The fingers are now in fabrication. Before launching the manufacture, scale-one elements were fatigue tested and industrialisation processes were optimised in order to increase the manufacturing efficiency. As non-destructive controls are used all along the manufacturing process, a good reliability of the components is expected. Specific high temperature helium leak testing and infrared testing are used as a global acceptance test of each element. Up to now 100 fingers have been delivered.

Modelling of particle flows in a scrape-off layer with limiter biasing

The superconducting tokamak Tore Supra will be equipped with an actively cooled toroidal pump limiter (TPL), in the framework of the CIEL (Composants Internes Et Limiteurs) project, dedicated to plasma facing component design for steady state operation. The TPL is equipped with throats, located only on the high field side, for particle collection allowing the control of plasma density which is essential for long plasma discharges. The present design work of the CIEL includes a biasing system in order to enhance the particle pumping. A fluid model, based on the classical fluid equation, is used to estimate the effects of the electric field on the particle flows in the Scrape-Off Layer (SOL). The modifications of the density, the particle flow (toroidal and poloidal) and the position of the stagnation point are discussed as a function of the bias voltage. The model clearly illustrates the different resulting effects on particle pumping for a divertor and a limiter configuration which are designed respectively for poloidal or parallel particle collection.

Simulation of different pumping configurations for the CIEL project on Tore Supra

In the framework of the CIEL project, allowing to improve Tore Supra heat and particle exhaust capability, a toroidal pump limiter will be implemented in the bottom of the machine. This paper presents the study of the pumping system associated with this limiter, in terms of pumping efficiency and extracted flux. Simulations have been performed for different configurations, using the EIRENE neutral transport code, and show that the pumping system should provide efficient density control for the scenarios contemplated for CIEL.

Heat flux deposition on plasma-facing components using a convective model with ripple and Shafranov shift

A heat flux deposition code is a very useful tool for the design of plasma-facing components. The classical cosine model representative of the convective parallel heat flux was coded within the standard finite element code of the CEA Castem 2000. Two perturbations to the idealised magnetic surfaces were taken into account: the Shafranov shift and the ripple of the toroidal magnetic field. The calculation scheme – named Tokaflu – was confronted to previous computations on bottom modular limiters and to experiments on the inner first wall. The code was used to optimise the shape of future plasma-facing components in Tore Supra designed for the CIEL project, namely the toroidal pumped limiter and its neutralisers. Developments are under way to include the shadowing of the components, other power deposition schemes and possibly later plasma sections other than circular.

Thermal load distribution near the tips of the ALT-II limiter roof on TEXTOR-94

The aim of these investigations is the description of the power flux on roof-like limiters near the LCFS. Here, the specific subject is the toroidal pump limiter ALT-II of TEXTOR-94. Similar to other plasma facing objects, the surface of ALT-II is shaped to enhance the plasma wetted area; this is achieved by reducing the angle of incidence of the magnetic field to less than 1° for the first 10 mm of the SOL. This small angle of incidence enhances all effects of toroidal non-uniformity as given, for example, by magnetic field ripple. Extensive modelling explains well the observed heating pattern on the limiter surface due to the ripple effect. In contrast to the expectations from density and temperature distributions in the SOL and at the edge of the confined region, an excessive power density is deposited on the first few millimetres near the tip of the limiter roof. Physical effects which could cause this phenomenon are discussed.

Design, fabrication and testing of an improved high heat flux element, experience feedback on steady state plasma facing components in Tore Supra

Actively cooled plasma facing components (PFC) have been developed and used in Tore Supra since 1985. One of the main technological problem is due to the expansion mismatch between graphite armour and metallic heat sink material. A first technology used graphite tiles with or without a reinforcement and a compliant layer, brazed with titanium copper–silver (TiCuAg) alloy. The next technology used carbon fiber material (CFC) tiles with a 2 mm pure copper compliant layer, since the good mechanical strength of the CFC allowed the reinforcement layer to be suppressed. No destructive inspection during the manufacturing procedure was found to be essential to insure a good reliability of the elements. A recent technology was developed for the new actively cooled toroidal pump limiter of Tore Supra (designed to remove up to 15 MW during 1000 s with incident heat flux up to 10 MW m −2). This new technology uses an active metal casting (AMC) of copper onto CFC tiles whose surface is prepared by a laser treatment. Copper compliant layer is then electron-beam welded to the heat sink. This technology is silver free and could be ITER relevant. The first bonding is X-ray controlled and the second one ultrasonic (US) tested. Finally all the elements are controlled by an infrared imaging technique during a hot water test.

Background, motivation, concept and scientific aims for building a dynamic ergodic divertor

The dynamic ergodic divertor is a new concept for distributing the power flux from the plasma to a large area target plate while keeping or even improving the particle removal capabilities of the toroidal pump limiter ALT-II. The transport in the ergodized boundary magnetic field shields the impurities from penetrating into the central plasma while the option of a radiative mantle should become even more efficient. To achieve the ergodization, a set of perturbation coils is oriented parallel to the field lines of a resonant layer (q=3 layer). A four phase perturbation current (up to 15 kA) is selected to impose the option of field rotation; the available frequencies are DC, 50 Hz and the band between 1 and 10 kHz. The low frequency option is chosen for smearing out the divertor strike point while the rotation high frequency option may give rise to a `temporal' ergodization in addition to the spatial one and may improve the plasma confinement and increase the disruption limit.

Toward Long-Pulse, High-Performance Discharges in Tore Supra: Experimental Knowledge and Technological Developments for Heat Exhaust

In view of high-power, long pulse steady-state operation, Tore Supra has incorporated in its design the active control of heat and particles in a realistic environment. A new inner-wall technology was developed in 1994 and is to be tested in 1995-1996 with a totally rebuilt 40-deg toroidal sector. A carbon-fiber-reinforced carbon-metal compound is based on the newest brazing technology and regorous quality control. Components such as the toroidal pump limiter and the guard limiters of plasma-heating antennas are being developed in the same way. For structures where brazing is difficult, boron carbide-coated components have been developed and installed in Tore Supra. For lower heat fluxes, a bolted concept has been designed and tested. The influence of inner-first-wall misalignment in Tore Supra on the power exhaust limitation of brazed components has been studied. Results from the technological development for the different power exhaust systems and the associated experimental knowledge obtained during plasma operation in Tore Supra are presented. 63 refs., 34 figs., 2 tabs.

Study of the power exhaust and the role of impurities in the Torus Experiment for Technological Oriented Research (TEXTOR)

Controlled application of radiating impurities in the boundary layer can help to solve the problem of power exhaust in a fusion reactor. Experiments in the Torus Experiment for Technological Oriented Research (TEXTOR) [J. Nucl. Mater. 145–147, 3 (1987)] are presented, which show that impurities with sufficiently high atomic number (≥10) are well suited for this purpose. Injection of neon, a gas recycled at the wall, enabled the establishment of a quasistationary radiating boundary layer, from which more than 90% of the input power was emitted. The required neon density was established by means of a feedback control for the neon influx, which was made possible by the toroidal pump limiter Advanced Limiter Technology (ALT-II) [J. Nucl. Mater. 162–164, 115 (1989)]. Alternatively, or in addition silicon was introduced as a condensing element—either by surface reactions from siliconized walls or by silane [SiH(D)4] injection—which revealed self-controlling mechanisms effective with changing plasma parameters. In neither case was a significant increase in central impurity concentration observed and good energy confinement time was maintained up to the highest plasma densities. Based on the information from various refined edge diagnostics, the underlying mechanisms for the buildup of a radiating plasma mantle and the interdependences of neon and silicon on other impurities are discussed.

Measurements of the edge power flow to the ALT-II limiter in TEXTOR

Thermographic measurements at the ALT-II pump limiter on TEXTOR have been performed with the following results. (1) The design of the toroidal pump limiter provides a rather uniform temperature distribution and prevents an overheating of the graphite cover tiles. (2) The local measurements of the convective energy flux to the limiter (IR scanner) and of the radiated power (bolometer) show normally a good power balance; this indicates that the power is leaving the plasma toroidally symmetrically. (3) From the analysis of the temperature distribution in poloidal and toroidal direction along the ALT-II tiles a power decay length of 5–7 mm is derived. (4) The ion temperature at the last closed flux surface is obtained from the energy flux along the magnetic field lines. At low plasma densities, the ion temperature is up to a factor of 2 higher than the electron temperature and at high densities both temperatures converge to the same value.

Toroidal pump limiter experiments in the TEXTOR tokamak

The Advanced Limiter Test-II (ALT-II) is a large area toroidal pump limiter in the TEXTOR tokamak. Discrete target plates located in channels at eight toroidal positions behind the main limiter surface neutralize a portion of the plasma efflux from the core. The resulting gas is exhausted by eight external pumps. The primary experimental goals of ALT-II are aimed at power loading studies and plasma density control during long pulse (4 s), high power (6 MW) tokamak discharges. It is found that both the power and the plasma flow to the limiter are asymmetric and depend on line density. Peak neutral pressures of 0.8 m torr and removal rates of up to 0.15 torr⋅L/s per pump station are achieved in the Ohmic phase. The projected exhaust efficiency of ALT-II with full pumping is 5-10%. During ion cyclotron resonance frequency heating, the particle removal rate exceeds 0.4 torr⋅L/s per blade, and the exhaust efficiency is 4-5% for power levels up to 2.6 MW.

Particle exhaust during neutral injection by the ALT-II pump limiter in TEXTOR

The ALT-II (Advanced Limiter Test) toroidal belt pump limiter has been used for particle exhaust during discharges heated by neutral beam injection (NI) in the TEXTOR tokamak. Pumping is now available at all eight blades, whereas previously only two blades have been pumped. Results from discharges with co- and with counter-injection are presented in this paper. Large rises (~ 3 × over ohmic conditions) in collected flux are observed in the collection scoops with NI, but the electron temperature rise is modest. The plasma density near the neutralizes is substantially higher than at the scoop entrances. Total collected fluxes at the neutralizers have reached 16 × 10 20 ions/s (260 A), and removal rates of nearly 7 × 10 20 atoms/s (15 mbarl/s) have been measured, compared to 1.6 × 10 20 atoms/s injected in a neutral beam.

The toroidal belt pump limiter, ALT-II, in the TEXTOR tokamak

A toroidal belt pump limiter has been designed, constructed, installed and operated in the TEXTOR tokamak. The Advanced Limiter TEST-II, or ALT-II, is carried out as a partnership under a joint agreement between the European Community, Japan and the United States. ALT-II is designed to withstand the high heat fluxes (greater than 500 W/cm 2) intrinsic to the boundary region of tokamak plasmas while simultaneously removing 5% to 10% of the core plasma efflux. The belt consists of eight segments, each 28 cm wide, 150 cm long, and 1.2 cm thick, and each is covered with shaped graphite tiles. Mounted behind each blade is a “scoop” system which captures plasma flowing past the leading edges and behind the blades. This plasma neutralizes on a collector plate and the resulting neutral gas is pumped. During ohmic discharges, 50 to 60 A of current is collected in the scoops and removal rates up to 0.25 Torr-l/s are achieved. The blades are instrumented with Langmuir probes and thermo couples, and other diagnostics include infrared cameras, H α-monitors, pressure gauges, helium charge exchange spectroscopy, and helium gas diagnostics. The plasma edge and the operation of the pump limiter system are described.

Neutral particle transport in a tokamak toroidal belt pump limiter

The Advanced Limiter Test‐II (ALT‐II) is an experimental toroidal belt pump limiter in the TEXTOR tokamak. This configuration is designed to study power and particle removal capabilities of the toroidal limiter during the long pulse (4 s), high power (7 MW) tokamak discharges. Probe measurements of plasma parameters in the limiter shadow are used to calibrate the incident ion flux. Neutral gas pressure and particle throughput to the pumps are measured with Bayard–Alpert gauges. In the ohmic phase, the plasma density in the limiter scoops is 3 to 8×1011 cm−3, electron temperatures are 8 to 12 eV, and the neutral pressure can reach 0.5 mTorr. Numerical calculations show that the background plasma influences atomic and molecular gas transport in the scoops, and the estimated particle removal rates agree with experimental measurements under ohmic (0.1–0.2 Torr l/s) and high flux conditions with neutral beam injection (NBI) (>1.0 Torr l/s).

Invited paper: ICRF/Edge physics research on textor

Extensive investigations of ICRF-induced effects on the edge plasma and on plasma-wall interaction were conducted on TEXTOR under different wall- and limiter as well as plasma- and heating conditions. Several strong effects of ICRF on the edge parameters were observed on TEXTOR, such as density rise, instantaneous electron heating, modification of SOL profiles, influx of light and/or heavy impurities, increased heat flux to the limiters, and production of energetic ions in the SOL. The fast response time of some of the changes and the observation of a maximum in the SOL profile of electron temperature, heat flux and metal sputtering clearly demonstrated that RF power is directly absorbed in the SOL. Estimates of this power amount to several percent of the total RF power launched into the plasma. Plasma-wall interaction during ICRF was substantially reduced by an appropriate choice of the wall conditioning procedures (wall carbonization with liner at 400° C or, above all, boronization). As a result record low values of the radiated power fraction were achieved during ICRF and long pulse, high power, low impurity operation was possible. Further improvement was obtained by ICRF antenna phasing. When ICRF power is coupled to the plasma, several effects on the core and edge plasma influence the operation of the toroidal pump limiter ALT-II. Experimental and theoretical studies were performed to elucidate the mechanisms responsible for the ICRF-induced effects, including the propagation of plasma waves in the edge plasma and nonlinear phenomena such as parametric decay, important changes in the DC current between the antenna structure and the liner due to the sheath effect at the antennas, and the generation of waves at harmonics of the RF generator frequency. Radial profiles of the DC radial and poloidal electric fields as well as a localized RF electric field structure were measured in the SOL using a fast scanning probe.

International collaboration on TEXTOR and toroidal pump limiter ALT-II.

TEXTOR tokamak experiments were operated since 1983 and a toroidal pump limiter ALT-II (Advanced LimiterTest II) was installed in 1986. Main topics of the international collaboration on TEXTOR and present status of TEXTOR are reviewed for the period of 1983-1990.

MOdelling of Impurity Generation and Transport in Scrape‐Off Layers

AbstractThe scrape‐off layer plasma is simulated with a 2‐dimensional fluid plasma code (SLIM) and a Monte Carlo neutral transport code (DEGAS) where the two codes are made runs iteratively until almost stable results are obtained. The geometry of the scrape‐off layer is referred to a toroidal pump limiter system. In the scrape‐off plasma thus obtained, ionization of neutral impurity particles ejected from the first wall, drift motion of charged impurities along the magnetic field lines and random diffusion across the magnetic field lines are simulated by a Monte Carlo method using atomic reaction rates of relevant processes and transport coefficients in the scrape‐off layer. From the statistics on calculations of destination of many impurity particles, the screening efficiency of the scrape‐off layer for impurity penetration into the core plasma, the impurity redeposition rate on the first wall, the exhaust efficiency of impurities are quantitatively analyzed.

ALT-II toroidal belt pump limiter performance in TEXTOR

The Advanced Limiter Test (ALT-II) is a toroidal belt pump limiter in the TEXTOR tokamak. ALT-II is composed of 8 blade segments which form an axisymmetric toroidal belt of 3.4 m 2 exposed surface area, located on the outside of the torus at 45° below the horizontal midplane. Ohmic plasma operation with ALT-II as the main limiter is characterized by a line-averaged density range of 5 × 10 12 to 5.5 × 10 13 cm −3 at B T = 2 T and I P = 340 kA , Z eff = 1.1 to 2 and typically 40 to 95% of the power radiated depending on the plasma density. ICRH heating of the plasma with up to 2.6 MW of incident power has been achieved, which modifies the scrape-off layer (SOL) and the pump limiter performance. The recycling coefficient in TEXTOR is normally close to one, but helium RG conditioning and baking of the limiter at 400 ° C is found to lower the recycling coefficient to 0.8 for the order of 10 shots. Measurements by arrays of probes in the SOL and thermocouples in the limiter tiles indicate the flow to the limiter is toroidally symmetric (taking field ripple into account) and poloidally asymmetric. The asymmetries result in different power and particle fluxes to the ion and electron drift sides of the limiter. The density and power scrape-off lengths are on the order of 1 cm and significantly longer on the outside of the torus (electron drift side). In spite of the flow asymmetry favoring the ion drift side near the tangency point, the longer e-folding lengths on the electron side in the SOL result in equal or higher particle collection by the electron side. The probe arrays indicate that during ohmic heating a total of 15 to 20% of the core efflux is incident on the neutralizer plates located in scoops beneath the blades. More particles are collected during ICRH auxiliary heating due to changes in the SOL profiles and shorter particle confinement times. Based on particle re moval experiments with pumping on one blade, the total exhaust efficiency of the limiter if pumped at all eight blades is 5 to 10%.

Observations on international cooperation

The motives for international cooperation and some reasons for engaging in such activities are considered. The author presents some thoughts on the European Community and on JET. Some cooperative programmes within EURATOM or in International Energy Agency are described. The TEXTOR toroidal pump limiter project is discussed in some detail. Some future possibilities are briefly examined.

The High-Power (3-MW) Long-Pulse (3-s) Radio-Frequency System for Ion Cyclotron Resonance Heating Experiments on TEXTOR

A multimegajoule ion cyclotron resonance heating (ICRH) experiment was installed on the Torus Experiment for Technology-Oriented Research (TEXTOR) tokamak. The system consists of two independent power lines each designed to generate and launch 1.5 MW of radio-frequency (rf) power into the machine during a 3-s period in the 25- to 29-MHz frequency range. Each power line consists of the following items: a 1.5-MW transmitter, a transmission line system, and an interface linking the transmission line to the antenna of the shielded strip-line type placed along the tokamak's hot liner. Details of the line and antenna diagnostics and data acquisition system together with the subsequent impedance characteristic calculations are given. The rf radiation shielding for the ICRH experiment is explained. The control of the rf setup as a TEXTOR sub-system and the generator pulse control and operation modes are outlined. The antenna loading and power limitation in the presence of plasma and the conditioning procedure are discussed. Finally, the new rf system compatible with the toroidal pump limiter Advanced Limiter Test-II is presented.