G. Telesca

High confinement and high density with stationary plasma energy and strong edge radiation cooling in the upgraded Torus experiment for technology oriented research (TEXTOR-94)

An overview of the results obtained so far for the radiative I-mode regime on the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94) [Proceedings of the 16th IEEE Symposium on Fusion Engineering (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995), Vol. 1, p. 470] is given. This regime is obtained under quasistationary conditions with edge neon seeding in a pumped limiter tokamak with circular cross section. It combines high confinement and high β (up to a normalized beta, βn=2) with low edge q values (down to qa=2.8) and high density even above the Greenwald limit together with dominant edge radiative heat exhaust, and therefore shows promise for the future of fusion research. Bulk and edge properties of these discharges are described, and a detailed account is given of the energy and particle confinement and their scaling. Energy confinement scales linearly with density as for the nonsaturated Ohmic Neo-Alcator scaling, but the usual degradation with total power remains. No deleterious effects of the neon seeding on fusion reactivity and plasma stability have been observed.An overview of the results obtained so far for the radiative I-mode regime on the upgraded Torus Experiment for Technology Oriented Research (TEXTOR-94) [Proceedings of the 16th IEEE Symposium on Fusion Engineering (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1995), Vol. 1, p. 470] is given. This regime is obtained under quasistationary conditions with edge neon seeding in a pumped limiter tokamak with circular cross section. It combines high confinement and high β (up to a normalized beta, βn=2) with low edge q values (down to qa=2.8) and high density even above the Greenwald limit together with dominant edge radiative heat exhaust, and therefore shows promise for the future of fusion research. Bulk and edge properties of these discharges are described, and a detailed account is given of the energy and particle confinement and their scaling. Energy confinement scales linearly with density as for the nonsaturated Ohmic Neo-Alcator scaling, but the usual degradation with total power ...

Comparison of the performance of ICRF antennas with and without Faraday shield on TEXTOR

After the first demonstration of successful operation with ion cyclotron resonance heating (ICRH) on TEXTOR using an antenna without Faraday shield (FS), a further comparison of an antenna with FS and an antenna without FS has been made over a wide range of plasma conditions (including the improved confinement regime on TEXTOR), in zero phase and pi phase operation, using various heating scenarios. No restriction in the operational conditions was found to exist for the screenless antenna. A theoretical analysis shows that antenna side limiters are sufficient to protect the antenna conductor from the plasma and that the plasma in front of the antenna can take over the role of the FS without introducing additional losses. The impact of the operation with both antennas on plasma temperature, density, energy, impurities, etc., is discussed in detail

Analysis and modelling of the magnetic and plasma profiles during PPCD experiments in RFX

In this paper, we analyse the main features of the pulsed poloidal current drive (PPCD) technique, used in the reversed field pinch configuration to achieve improved confinement conditions. In the RFX experiment, PPCD corresponds to a decrease of the magnetic fluctuations, to a peaking of the temperature profile, and to a reduced transport and plasma–wall interaction. A three-dimensional MHD nonlinear code and one-dimensional time-dependent transport models have been applied to study the effect of PPCD on the magnetic and plasma profiles. The three-dimensional MHD simulations show that the external inductive drive pinches and peaks the current profile driving the configuration through a transient phase, where the spontaneous turbulent dynamo action is quenched. The one-dimensional transport codes indicate that the experimental profile modifications associated with PPCD are consistent with a reduction of the stochastic transport.

Ion cyclotron resonance heating of a tokamak plasma using an antenna without a Faraday shield

An experiment was performed on the TEXTOR tokamak which proves that the plasma can be heated by ion cyclotron resonance using antennas without a Faraday shield. The heating performance of the unshielded antenna was found to be similar to that of a shielded antenna. Although the radiated power was somewhat higher with the unshielded antenna, no impurity problems were encountered. These results are explained on the basis of radiofrequency sheath theory and by the almost perfect shielding of parallel electric fields by the plasma. Implementation criteria for unshielded antennas were obtained.

Recent progress in reversed field pinch research in the RFX experiment

The article presents an overview of recent experimental results obtained on the RFX device. The authors obtained and studied a reversed field pinch plasma with a plasma current of up to 1 MA, negligible radiation losses and low effective charge. The local power and particle balance shows that in standard operation the plasma core is dominated by magnetic turbulence and that the global confinement is mainly provided by the edge region, where a strongly sheared radial electric field is present. With poloidal current drive the amplitude of magnetic fluctuations and the thermal conductivity of the plasma core are reduced, leading to improved confinement. Reduced heat transport is also observed when the width of the n spectrum of magnetic fluctuations is reduced.

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. ...

Effects of auxiliary heating on dc electric fields in the TEXTOR boundary

Abstract The generation of d.c. electric fields in the TEXTOR edge (inside the last closed flux surface) and scrape-off layer (SOL) during auxiliary heating has been studied using electric probes and spectroscopy. Peak radial and poloidal electric fields in the edge during neutral injection are 5 kV/m and 3 kV/m respectively while little effect is seen in the SOL. ICRH alters the radial electric field in the edge, and generates large (8 kV/m) fields in the SOL. These modifications depend sensitively on the limiter-antenna configuration. Large poloidal electric fields of 2 kV/m are also generated in the SOL, but nearly vanish in the edge. Spectroscopic measurements of changes in poloidal rotation in the edge are in agreement with the probe measurements in neutral beam heated discharges. Profile modifications due to the electric fields are observed. The radial particle velocities corresponding to the measured poloidal electric fields exceed by many times the diffusive velocity, particularly during ICRH, suggesting that convective flows may be important.

Studies of fuelling rates of CO, CH4 and oxygen in the TEXTOR tokamak

Impurity gases have been introduced into TEXTOR at well defined rates to study the fuelling process and the effect they have on the overall energy balance. It is found that carbon and oxygen, introduced as CO and CH 4 gases have radically different behaviour. Oxygen has a high recycling coefficient ∼-0.99 whereas carbon has a low recycling coefficient ∼-0.3. The radial position of the gas source was varied from the wall to the limiter radius but this made little difference to the effect on the global plasma parameters. For very high impurity influxes of both impurities drastic changes in the edge plasma were observed and disruptions could be induced.

Neon radiation efficiency for different confinement regimes in TEXTOR-94

For neon seeded discharges the quantity [?Prad/?Zeff(0)]/e02, which depends linearly on the effective neon cooling rates and which can be taken as a measure of the radiation efficiency of neon, is not a constant for a given machine. When, according to the mode of operation, [?Prad/?Zeff(0)]/e02 is high, radiative power exhaust can be efficiently achieved at low Zeff. The value of this ratio, which we refer to as the quality of neon cooling, is found to be correlated not only with the plasma edge temperature but also, and even more strongly, with the confinement properties of the discharges. Two different high confinement radiative regimes in TEXTOR-94 have been compared: radiative improved (RI) mode (highest confinement) and gas puff/pure radiofrequency mode (slightly lower confinement). Although the electron temperature profile is similar in the two regimes, due to feedback control of the input power level, it has been found experimentally that the quality of neon cooling is lower for the RI mode than for the gas puff/pure radiofrequency one. Similar behaviour has been observed for the quality of cooling of the residual intrinsic carbon, determined with independent diagnostics and methods. Simulations with the self-consistent RITM code confirm that particle transport, in particular the perpendicular diffusivity at the edge, is a key parameter to determine the level of the power radiated at the edge for a given impurity concentration in the plasma core.

Demonstration of real-time control of impurity seeding plus outboard strike-point sweeping in JET ELMy H-mode plasmas

Strike-point sweeping and real-time-controlled (RTC) impurity seeding are both expected to be needed on JET following its upgrade to an all-metal wall with enhanced neutral-beam heating, thereby anticipating exhaust-control requirements plus the materials planned for ITER. Preliminary trials in the previous carbon device have combined these techniques in high-triangularity type I H-mode plasmas, using a VUV spectroscopic signal for feedback control of nitrogen injection. Compared with earlier unswept feedforward counterparts, similar strong mitigation of divertor heat load between ELMs was achieved in swept RTC cases for less than half the integrated nitrogen input and correspondingly less adverse effect upon other properties. Both sweeping and RT control contributed to this improvement. Time-average normalized energy confinement H98yt ~ 1, Greenwald density fraction fGwdt ~ 0.9 and particularly purity denoted by effective ionic charge Zefft ≈ 1.7, all remained closer to good reference levels. Transient effluxes in ELMs were also less affected, however, and would require separate active control.