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A new regime of enhanced confinement (I-mode) is found in plasmas with circular cross-sections in the pump limiter tokamak TEXTOR with boronized walls. This regime is obtained with three types of auxiliary heating, namely NBI co-injection, NBI counter-injection+ICRH and NBI co-injection+NBI counter-injection, and has many similarities with the H-mode regime obtained in divertor tokamaks. The energy confinement times obtained in these discharges scale as favourably as those in stationary H-mode discharges with edge localized modes. A detailed analysis of the scaling of the confinement time with plasma current, heating power and plasma density is presented. Characteristic electron density and temperature profiles are observed, with large central values and well developed edge pedestals. They are compared with those found in H-mode discharges and supershots. A poloidal beta limit of 1.6 is found in the I-mode discharges of TEXTOR. The maximum toroidal beta values obtained reach nearly 1%, i.e. 0.7 times the Troyon limit in TEXTOR. I-mode confinement is always linked with low recycling and absence of MHD activity. If these conditions are not met. L-mode scaling is retrieved. MHD activity, which is more likely to occur at low plasma densities and currents, can cause a sudden drop to L-mode scaling. So far, no transition from the L-mode scaling to the I-mode scaling has been observed

To enable the investigation of plasma-wall interactions the TEXTOR-Tokamak [l] is equipped with a replaceable first wall or liner which is supported inside the toroidal vacuum vessel (fig. 1). The vacuum vessel is an all-welded metal torus consisting of four large and four small rigid sections separated by eight bellows to provide the required electrical resistance of 4 milliohms in toroidal direction. The rigid sections are individually supported by the stainless steel frames of the toroidal field coils [2]. The liner consists of a 1 mm thick Inconel shell in the shape of a torus. Like the vacuum vessel its electrical resistance in toroidal direction is 4 milliohms. Liner replacement is accomplished during a machine opening by first dividing the liner in half at two opposite toroidal locations, then separating the vacuum vessel into two half tori, one movable and one fixed. The movable torus part, enclosed by 7 toroidal field coils, can be moved 3.5 m horizontally to permit removal and installation of liner halves. After the new liner halves are installed and the vacuum vessel is moved together and welded the liner halves are connected to become a full toroidal structure inside the vacuum vessel [3]. ALT-II takes advantage of TEXTOR design features (fig. 2) through the use of a new liner specially adapted for installation of the toroidal belt limiter [4,5]. Besides facilitating an easier integration of the pump limiter, the new liner also minimized TEXTOR down-time because operation with the previous liner could be continued while trial installation of limiter modules was done with the new liner outside TEXTOR before interrupting TEXTOR operations (fig. 2).

A description is given of the engineering design concept of the Advanced Limiter Test-II.

Neutral beam injection (NBI-co and NBI-counter), ICRH and their combinations have been compared on TEXTOR for what concerns the heating and their effect on the energy anisotropy and the confinement. In most of the cases, stationary heated plasma conditions have been obtained with boronized wall. The main results are: (i) Production of a hot ion mode with NBI-co. It is characterized by a large energy anisotropy, by a large noninductively driven toroidal current and by the stabilization of the sawteeth (of the monster type) due to the hot ion tail. (ii) Significant enhancement of these effects when ICRH is added to NBI-co. The neutron yield is also increased by the addition of ICRH. Peak electron and ion temperature around 3 keV have been obtained and 70% of the total current has been non inductively driven. (iii) Large excess of the total energy content (up to 2.3) with respect to the L-mode scaling predictions has been obtained with the combinations NBI-co+ICRH or NBI-co+NBI-counter. (iv) Up to 6 MW of additional power has been coupled to the plasma leading to beta p=1.5 and beta t equal to 70% of the Troyon limit.

The heat removal from the leading edge of limiter blades is a critical issue for the technical feasibility of the pump limiter concept. The aim of the present work was to investigate the capability of heat pipes to remove concentrated local heat fluxes. Tubular and flat heat pipes were subjected to local surface heat loads in the JUDITH electron beam facility. The heat pipes were tested until failure or until the operational limit of the component was reached. The absorbed heat fluxes at this point were of the order of several hundred W/cm 2 .

The transport of impurities in the core of a Tokamak shall be studied by a pellet of light elements injected into the plasma. Emission of light by these elements in the vicinity of the pellet and to some extent within the plume of ablated material serves for diagnostic purposes. Pellets of tens of milligrams at a velocity of a 1.5 km/s and more are necessary to reach the plasma core. A sabot technique is be used to accelerate pellets of materials as lithium deuteride or boron carbide. This paper describes the application of an electrothermally driven mass accelerator of simple design as a pellet injector. Pellets of up to 100 mg mass can be brought to velocities of 2.0 km/s. The accelerator barrel has a caliber of 5 mm and a length of 250 mm. The pellet, diameter 3 mm, is contained in a sabot which fits the barrel. The sabot closes the barrel after the release of the pellet so that the unavoidable driving gas is kept back nearly completely and only the pellet reaches the plasma. The results of acceleration experiments with different sabot materials are given and the design of the complete injector is described.

A nine barrel pellet injector has been built and tested in Grenoble with 1.5 mm D2 pellets in the range of 400 m/s to 1200 m/s by using one stage gas guns. The pellets are formed simultaneously in the barrels by “in situ” condensation technique in the 1-6 mm3range (6 × 1019to 3.6 × 1020atoms D2). The firing of the pellets provides a large degree of freedom because some barrels can be grouped and the delay between the individual guns can be chosen between a millisecond and ten seconds or even more. This choice allows an injection at any time of the TEXTOR plasma pulse. The barrels end in a system for the separation of the propellant gas from the pellets reducing the amount of gas to 10−3to 10−4of its initial value. The system which was built in Julich consists of three sets of differentially pumped volumes combined with fast closing valves. In addition this unit is equipped with diagnostics for the pellet mass and the pellet velocity. The barrel unit and the gas separation unit have been combined and commonly operated. The paper describes in detail the commissioning test results of the nine shot pellet injector.

For TEXTOR a multiple pellet injector system is in preparation which consists of a) nine freezing cells for the pellet formation accommodated in a common cryostat and nine gas guns being built at CEA, Service des Basses Temperatures in Grenoble, b) a system for the propellant-gas handling together with the pellet diagnostics being built at KFA Julich. The pellets have a cylindrical shape with a size of 1.3 mm in diameter and a length of 1.5 mm. The guns are operated with a propellant gas pressure of up to 200 bar and the pellets are accelerated up to velocities of 1.4 km/s. The propellant gas valve of each gun is optimized to release as little gas as possible; nevertheless the gas release amounts to 0.3 bar·l. This amount of gas (typically hydrogen) has a mass more than a factor of hundred larger than that of one pellet, but only an amount of one percent of the pellet mass can be tolerated. The conventional way of handling the propellant gas is either to feed the pellets through a capillary guiding tube or by using two or three differential pumping chambers. The first scheme may cause pellet ruptures at velocities higher than 1 km/s and the second one requires large expansion volumes and strong pumping. In the TEXTOR pellet injector we are using a combination of a muzzle brake, three small expansion volumes and two fast closing valves for each gun. The muzzle brake diffuses the stream of the propellant gas immediately after the end of the barrel. Most of the gas is directed into two expansion chambers which have volumes of only 0.5 l each. Within 1 ms after the passage of the pellet, a newly developed valve closes the first two expansion volumes from TEXTOR, thus reducing the residual propellant gas to less than 0.5 mbar·l. A second combination of an expansion volume of 1 l and a fast valve reduces the propellant gas further down to about 10−2 mbar·l, which is less than 1 % of the pellet mass (typically 4 mbar·l). Three IR-light barriers are integrated in each pellet path; their photodiodes provide signals for the pellet speed determination and trigger signals for the fast closing valves. They also give the open/closed status of the valves. The mass of the individual pellet is measured by a microwave system consisting of a μW-cavity, a wave guide, a gunnplexer oscillator and a detection circuit operating at a frequency of 10.3 GHz.

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 m2 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 × 1012 to 5.5 × 1013cm−3 at BT = 2 T and IP = 340 kA, Zeff = 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%.

Graphite limiters in the KFA tokamak TEXTOR have been hit by run-away electrons, which “burned” craters into the material surface. The craters have been formed by material erosion and/or sublimation and by chipping of thin laminates from the surface and additionally by redeposition of graphitic material. Moreover, thin metallic layers have been detected which may have been redeposited independently from the run-away electron event, and which could be found also outside the run-away electron crater. The redeposited carbon showed primarily two types of structure, namely oriented pyrocarbon and carbon-black-like material. The crater has been investigated by metallography, optical microscopy, secondary electron microscopy combined with energy-dispersive X-ray analysis, and automatic quantitative image analysis. The examined parameters are crater geometry, cracking behaviour and type and structure of redeposited material.

Graphite is the preferred material for plasma facing components in today's fusion experiments and it is a prime candidate material also for coming fusion devices. At least in the existing tokamaks it cannot be excluded that the graphite surface will become contaminated, e.g. with metals, which leads to undesired effects on the plasma. Instead of an expensive exchange for new graphite parts, an appropriate cleaning method should be applied. This paper describes comparative tests of different mechanical cleaning methods applied to metal contaminated graphite tiles of the ALT-II toroidal belt pump limiter in the TEXTOR tokamak. Scanning electron microscope techniques have been used to characterize the cleaning efficiency. Best results have been obtained by sandblasting the graphite surface with boron carbide grains, which proves to be a very effective, inexpensive and fast cleaning method. After reassembly of the tiles, in-situ tests by exposure to tokamak discharges showed undetectably low (≤10−5) metal concentrations in the TEXTOR plasma.

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.

The mechanical structure of the tokamak TEXTOR has been designed according to certain specific requirements of the machine, i.e. ample radial and tangential access for diagnostics, neutral beamlines, divertors and pumping ports, and the necessity of splitting the complete device into two parts in order to exchange the liner. The toroidal shell structure, holding all the magnets, can be divided into two halves, one of which can be rolled off 3.5 meters away from the other. During the commissioning of the machine the full loads - forces and torques due to magnetic fields and induced currents and forces due to aerostatic pressure - have been applied several hundred times. Measurements and displacements at various parts of the structure are described as well as the procedure of dividing and rejoining the shell structure and poloidal field coils in respect of the accuracy with which the mechanical and electrical joints come together.

The vacuum vessel of TEXTOR is designed for a baking temperature of 350°C and to house a liner at an average temperature up to 600°C. The liner is ohmically heated by a current of 14 kA d.c. and the temperature of the vacuum vessel is controlled by a heat transfer fluid flowing through tubes distributed on the outer surfaces of the vessel sections. The fluid consists of a eutectic mixture of biphenyl and biphenyl - oxide with a maximum working temperature of 400°C at a pressure of 11 bars. The circuit consists of an electrical heater and an air cooler with a power of 235 kW and 185 kW, respectively. The heat transfer fluid serves as a heating or cooling agent depending on the actual temperature of the liner.

The new ICRH system for TEXTOR, presented at the previous SOFT conference, has been tested and installed during spring and summer of 1987. Pulses of up to 2.8 MW have been achieved representing a power density at the antenna of about 3.1 MW/m 2 and over 90% of the installed RF power. Taking into account the already achieved voltages in the system one could extrapolate that a power density of 10 MW/m 2 with an transmission efficiency well over 90% would be technically feasible.

An ICR heating experiment on TEXTOR at the 3 MW rf power level with maximum pulse lengths of 3 s will start in 1983. The transmitter set-up consists of two independent 1.5 MW systems tunable in the frequency range 25-29 MHz. Each system is constituted by the following items: 1°) The low power excitation which provides appropriately shaped rf pulses 2°) The power amplifier whose construction and test procedure are described in a companion paper. 3°) The transmission line and antenna two-stub tuning system made of pressurized 6″ 1/8 and 9″ rigid coaxial line components. 4°) The antenna of the shielded strip-line type to be installed along the machine's hot liner (up to 500°). The antenna is coupled to the pressurized transmission line by means of an interface which includes: a) A coaxial line section under high vacuum with ceramic vacuum feedthroughs at both ends and a fixed stub through which cooling is introduced and which also will lower the VSWR of the line. b) A flexible line section which takes up the thermal expansion of the liner and constitutes the heat barrier between the antenna and the water cooled line section.

At the 13th SOFT a report was given /1/ on KFA tests with different types of metallic seals for large circular and noncircular ports of the toroidal vacuum vessel of TEXTOR (Tokamak Experiment for Technology Oriented Research, Fig. 1). The vessel is built for bake-out temperatures of up to 350°C and for a base pressure of 10 -8 mbar.

The new TEXTOR ICRH system compatible with the toroidal pump-limiter ALT-II, which will be installed beginning 1987, is outlined. The system includes two pairs of new low-field-side retractable antennae of the “plug-in” type, four new interfaces between transmission line and antennae, and upgraded generators (2 × 2MW, 3s). Tuning problems resulting from possible cross-talk between antennae and from the phasing needed for k-shaping and possibly current drive are discussed. For the existing ICRH system, the updated generator waveform control system is briefly described.

This publication comprises papers from the PISCES and ALT-II Programs at UCLA which were presented at the International Plasma Surface Interactions Meeting held in Juelich, FRG, on May 2-6, 1988. A list of publications from the PISCES and ALT-II contained in this report are: Deuterium pumping and erosion behavior of selected graphite materials under high flux plasma bombardment in PISCES; Erosion and redeposition behavior of selected NET-candidate materials under high-flux hydrogen, deuterium plasma bombardment in PISCES; Presheath profiles in simulated tokamak edge plasmas; Boundary asymmetries and plasma flow to the ALT-II toroidal belt pump limiter; ALT-II toroidal belt pump limiter performance in TEXTOR; and An in-situ spectroscopic erosion yield measurement with applications to sputtering and surface morphology alterations.

The Advanced Limiter Test-II (ALT-II) /1, 2/ is a toroidal pump limiter experiment designed to be installed into the TEXTOR-Tokamak /3/ of the Institute for Plasmaphysics, KFA Julich.

A grazing incidence spectrometer for the 10-600-A wavelength range equipped with a 1-m concave grating of 600 lines/mm or 3600 lines/mm is described. The angle of incidence can be continuously varied between 89 degrees and 80 degrees This makes it possible to shift the angle of incidence to where the reflected intensity of the wavelength to be measured and the intensity ratio of this wavelength to interfering wavelengths of higher orders are maximized. Spectra of the C V (lambda = 40.73 A) and O VII (lambda = 21.8 A second order) lines show that in fact the second-order spectra can be reduced considerably. For photoelectric registration, twregistration,two independent detectors can be moved along the Rowland circle. The arrangement allows the simultaneous measurement of closely adjacent lines, and it is used to observe the time dependenceof N VI (lambda = 29.08 A) and N VII (lambda = 24.78 A) lines emitted by a thetapinch. The resolution of the instrument, lambda/Deltalambda has been determined to be better than 1600 at 66 A.

Highly reproducible light sources using sliding sparks through polyethylene capillaries are described. They emit an essentially continuous spectrum which can be used for absorption measurements or as a secondary intensity standard. The light pulses (reproducible within ±4%) can be triggered with jitter times shorter than ±5 nsec and with a typical half-duration of 60 nsec. In consequence of the high electron density (2.5×1019 cm−3) achieved, the visible continuum intensity (a typical value being 1.0×1011 W cm−3 sr−1 at 4340 A) follows approximately the blackbody radiation law. The corresponding temperature (450 000°K) is in agreement with the observation of lines and free-bound continua of highly-ionized carbon ions (C v and C vi) in the soft x-ray region.