Search

The Tore Supra lower hybrid (LH) control system has been in operation for 10 years. Although its control system has given excellent service, it is an ageing system, and incompatible with many modern components and techniques. Hence, in order to improve the LH system's reliability, efficiency, compatibility and visibility, a new design of its control system was undertaken. A solution based on modern programmable logic controller coupled with a fast logic hardware ‘state machine’ has been selected as a replacement. This can handle the ‘state diagram’ and associated safety interlock functions for each klystron with the required time response. This paper presents the association of this state machine for fast control with the PLC for slow control, a PC based human interface, and all the control and process diagnostic functions which complete this Tore Supra LH control and security system.

A new LHCD launcher, designed to inject 4 MW during 1000 s, has been installed on Tore Supra and successfully tested. A power of 3 MW with 7% power reflection coefficient has been reached. The coupling has been studied for various plasma parameters and positions, in order to study the robustness of the mode converter used in the antenna to divide the power into three in the poloidal direction. Due to the reduced electric field and the very efficiently water-cooled guard limiter, the temperature increase induced by the acceleration of electrons in the near field has been drastically reduced.

The suprathermal electron cyclotron radiation is used to investigate fundamental phenomena governing electron tail formation during lower hybrid current drive (LHCD). To this end, an approximate analytical solution of the 2-D time-dependent Fokker-Planck equation is developed. Electron cyclotron emission (ECE) is analysed with this model. The pitch-angle scattering time, the characteristic time of the quasi-linear diffusion process, the perpendicular temperature and the quasi-linear diffusion coefficient are measured by fitting the temporal evolution of the ECE si,signal. (C) 2001 Elsevier Science B.V. All rights reserved.

A new actively cooled advanced launcher is being built for Tore Supra LHCD to inject 4 MW during 1000 s at 3.7 GHz, at a power density of 25 MW/m 2 (a conservative value observed in Tore Supra experiments). It is made from an array of 6×48 active and 6×9 passive waveguides. The design uses technologies which are relevant for a next step machine such that it can: (i) withstand a plasma radiated flux of 0.15 MW/m 2 ; (ii) radiate power with spectra having peak N// values of 2.02±0.35; (iii) withstand a total torque of 8.6 10 4 N m during disruptions; (iv) allow an antenna 20 cm radial stroke adjustable in real time, (v) withstand a convected power flux of 10 MW/m 2 on its guard limiter made of CFC tiles. A prototype of each new component of this antenna has been tested successfully at the nominal power with a pulse length of 1000 s.

As a result of experimental observations of localized heat flux on components magnetically connected to radiating waveguides in Tore Supra and in TdeV, the acceleration of electrons near lower hybrid (LH) antennas has been investigated. A simple analytical model has been developed to compute the dynamics of the particles in the near field approximation. Landau damping of the very high N|| (20 < N|| < 100) component of the launched spectrum on the thermal electrons of the scrape-off layer (SOL) is found to occur. Simulation of a typical LH pulse in Tore Supra indicates that the electrons can be accelerated up to 2-3 keV. Modelling of the interaction of this fast electron population with the edge plasma allows a calculation of the heat flux on plasma facing components that are magnetically connected to the antenna. Model results and the results of experiments in Tore Supra and TdeV are compared. The calculated heat fluxes are found to be fairly consistent when the variation of convective heat flux at the grill aperture is taken into account. From this analysis, it is concluded that, for an LH power density of 25 MW/m2, the resulting heat flux along the field lines (3.5 MW/m2) is manageable for the components connected to the antenna, provided that good coupling can be maintained at a low density in front of the grill.

The full lower hybrid current drive (LHCD) system on the Joint European Torus (JET) has been operational since the start of the pumped divertor phase of JET in 1994. By a campaign of vacuum conditioning in the beginning of operation, the power transmission and conditioning of the fuli launcher was speeded up, as compared with the prototype launcher used in 1990/91. After vents of the tokamak, the conditioning can be recovered by -300 vacuum pulses over 3-4 days. Even after extensive conditioning and with efficient pumping, the power handling of the fuli LHCD system on plasma is limited by the maximum electric field in the waveguides, the limit being 400-500 kV/m.

Recent results on feedback control of the global shape of the current density profile in discharges with lower hybrid current drive (LHCD) on Tore Supra are presented. The global shape of the current density profile is characterized by its internal inductance li, and feedback controlled through the LH power or the launched LH wave spectrum. Feedback control of the flux on the plasma boundary has allowed for exploration of regimes with partial current drive (constant loop voltage) and full current drive (zero loop voltage). In stationary, steady state discharges at 0.8 MA (qa=7), the current profile shape is characterized by an li between 1.5 and 1.7 and a safety factor on-axis (q0) between 1 and 1.4. The energy confinement is 1.3 to 1.6 times higher than the value predicted by the Rebut-Lallia-Watkins scaling law

Strong toroidal asymmetries in power deposition during lower hybrid current drive experiments have been observed on TdeV and Tore Supra. These asymmetries are characterized by high heat loads on components magnetically connected to the LHCD antenna. The experimental results indicate that fast electrons generated by the rf fields in the scrape-off layer in front of the antenna are responsible for the localized heat loads. The rf power lost in the edge though this channel increases with both the edge density and the rf power level. In TdeV, the fraction of the injected power lost can exceed 10% at high power and high density, while it stays under 2% in Tore Supra. A simple model describing the trajectories of electrons in front of the antenna suggests that the high N| components of the near field spectrum are responsible for the acceleration of the scrape-off layer electrons.

The perpendicular x-ray emission up a to few MeV of runaway electrons has been measured in JET low-density ohmic discharges by means of the fast electron bremsstrahlung profile monitor. A diffusion model simulating the temporal evolution of the line-integrated x-ray signals is used to determine the runaway radial transport coefficient in the central region of the plasma ( for r/a

Stable and stationary states with hollow current density profiles have been achieved in Tore Supra with lower hybrid current drive (LHCD) during reduced toroidal magnetic field operation and in weak LH absorption regimes. For these plasma conditions, off-axis LH power deposition profiles are obtained in a reproducible manner when the internal LH caustics prevent central absorption of the waves. In the multipass LH wave propagation regime, the validity of the statistical treatment of stochastic wave diffusion is shown both theoretically and experimentally. When a large fraction of the plasma current (above 50%) is non-inductively sustained by the LH waves, the magnetic shear is reversed in the plasma core, i.e. inside a normalized plasma radius of the order of 0.4. The resulting hollow current density profiles have led to an enhancement of the total electron thermal energy content, up to a factor of 1.6 compared with L-mode discharges. The confinement improvement is attributed to a strong reduction of the electron thermal diffusivity in the central reversed shear region, nearly down to its neoclassical level.

A new code has been developed and benchmarked with data from lower hybrid current drive (LHCD) experiments in JET. The model includes stochastic radial diffusion of electrons and scattering of lower hybrid (LH) waves. The code is validated by comparing calculated and experimental current drive efficiencies as well as calculated and measured hard X ray bremsstrahlung emission radial profiles. Reasonable agreement has been found for full LH current driven cases and LH plus moderate DC electric field

The JET neutral beam injection system has proved to be both effective and reliable as a plasma heating device. The ion heating and plasma fuelling characteristics of the system are ideally suited to the production of high fusion performance plasmas while the flexibility in the choice of beam species (H, D, T, 3 He or 4 He) and the ability to inject into almost any JET plasma configuration allows a wide variety of related physics experiments to be carried out. The capability to inject (for the first time) tritium beams was essential to the successful execution of the first tritium experiments in which 1.7 MW of power from DT fusion reactions was generated.

An account is given of the high performance plasmas established by development of the H-mode regime in JET in the experimental campaigns up to 1992. High performance in this case is measured in terms of the confinement enhancement achieved over the L-mode scaling as measured using the plasma diamagnetism. Three JET H-mode regimes have achieved enhancement factors (H G DIA ) over Goldston L-mode scaling of 2.5 < H G DIA < 4.0. These are the Pellet Enhanced Performance (PEP) H-MODE, the high bootstrap fraction (high β POL ) H-mode and the Hot Ion (HI) H-mode. The phenomenology of these three regimes is reviewed and contrasts and common threads are elucidated

The information provided by different diagnostics has been combined in order to characterize the fast electron distribution function in lower hybrid current drive experiments on JET. In particular, X ray and electron cyclotron emission data are complementary in defining the fast electron energy contents in the directions parallel and perpendicular to the magnetic field. A numerical analysis chain has been developed which identifies the main moments of the distribution function of the current carrying fast electrons and allows simulations based on these moments to be compared with X ray emission, electron cyclotron emission and magnetic data. The method of analysis and the associated diagnostics are described, and results are presented which have been obtained during the lower hybrid current drive campaign on JET.

A detailed investigation is presented of the X rays that emerge from a tokamak inner wall after a random walk in which Compton scattering is the dominant process. The ability to measure accurately plasma emission above 50 keV along lines of sight facing the inner wall is assessed using a Monte Carlo code, the standard solution for radiation transport in a bounded medium. For peaked emission profiles, the scattered contribution can be large compared with the direct plasma emission for lines of sight probing the border of the plasma. Good quantitative agreement is found between the predicted and the measured hard X ray data for this case

The design of a Lower Hybrid Waves (LHW) system for ITER EDA is based on the robust concept of the passive active multijunction (PAM) which associates a good coupling of the slow wave with an efficient cooling of the grill mouth. The PAM is fed by an oversized section (hyperguide) exited by several mode converters which ensure the poloidal sharing of the RP power. Specific components, such as the mode converter and hyperguide have now been successfully tested at high power and long pulse duration (up to 1000 s on test bed). The 50 MW required to cover the envisioned scenarios, can be coupled by two launchers located in two ports of the machine. The working incident power density is 23 MW/m/sup 2/ which is routinely achieved in present experiments at 3.7 GHz.

The new Tore Supra objective is to produce steady state operation up to 1000 seconds. The Lower Hybrid test bed of Tore Supra has been developed to validate Radio Frequency (RF) concepts compatible with this objective. The overall layout and specification is presented together with operation modes and an example of test campaign.

To test on the Frascati Tokamak Upgrade (FTU) the concept of Passive Active Multijunction (PAM) proposed for Lower Hybrid Current Drive (LHCD) on ITER, studies and design have been made in collaboration between ENEA and CEA. The main properties of this launcher have been analyzed. A description of the antenna and the time schedule toward the experiment is given.

The aim of the ICRF system of Tore Supra is to provide high power to the plasma and to sustain long pulse operation. Good results have been obtained at high power in minority heating at 57 MHz on He(H) plasmas: 10 MW of ICRH alone have been coupled and 12 MW of combined heating (minority ICRH+LH) have been sustained for 2 s. The new lateral bumpers installed on one antenna are very satisfactory for power handling. ICRH experiments in 3He minority (42 MHz, B0=3.7 T) have also been carried out. In particular, 6 MW have been coupled in D plasma. The influence of the 3He concentration has been studied. There is experimental evidence of electron heating with mode conversion at high concentration (10–20%) of 3He. Without 3He, 3 MW have been coupled in FWEH on similar D target plasma. A scan in plasma current has been performed (1.5→1.1 MA). The bootstrap current is estimated. Some edge phenomena are observed: heating of the septum and other parts of the Faraday screen occurs when a cyclotron resonance layer is ...

A first conceptual antenna design based on the passive-active multijunction (PAM), was proposed in 93 in the frame of an ITER-TASK [ 1 ]. The present work accounts for the dimensional change of the ITER ports and also proposes a new arrangement of the hyperguides feeding the PAM to allow the flexibility of the N // spectrum which is desirable to cover the variety of envisioned scenarios. 35 to 55 MW of LH power could be injected in 1 port of ITER depending on the actual plasma reflection coefficient.

High power and energy performance have been achieved by the Tore-Supra Lower Hybrid system during the last experimental campaign. Plasma pulses up to 120 s in steady state conditions with a power density up to 24 MW/m2 were obtained. A combination of feedback controls of the LH and plasma parameters have also been used to investigate long pulse and current profile control operating scenarios.

High power and energy performance have been achieved by the Tore-Supra LH system during the last experimental campaign. Far distance coupling as well as plasma pulses up to 120 s in steady state conditions with a power density of 24 MW/m2 were obtained. A new launcher, made with RF components such as mode converters has been designed in order to extend the present TS performance towards quasi continuous operations.

In order to increase the reliability of the launchers for LHCD on Tore Supra, to drive the main part of the plasma current for long pulses (1000 s), new antennae have been studied. Their front tip can be of 2 types: the first one using multijunctions, the second one using active-passive waveguide array to test this new concept for ITER. After a brief description of their design, results of the main studies and tests in order to realize them are given.

The ‘Quasi‐Optical Grill’ (QOG) has been proposed to increase the robustness and the simplicity of a Lower‐Hybrid Waves antenna for reactor application [1]. To compare the performance of a QOG antenna with that of conventional scheme, an experiment was envisaged in TORE‐SUPRA in cooperation with the ENEA. Encouraging modelling results [2] lead to conduct a pre‐design study [3]. The main results of this study are presented in this paper. In particular, an analysis of the practical system has shown a limitation of the injected power due to either the appearance of large electric fields in the resonant structure, or to an increased reflexion linked with departures from ideal conditions. This analysis is based on the results of the diffraction code [2] developed by the ENEA complemented by a simple ‘Fabry‐Perot’ model. A further optimization is thus needed and is being undertaken to assess the future of the concept.

The full Lower Hybrid Current Drive (LHCD) launcher in JET has been completed and installed. Up to 6 MW has been coupled to the plasma and 2 MA full current drive was achieved with 4 MW of LH power. Coupling is improved by using active feedback control on the launcher and plasma position. A new launcher design concept, called Hyperguide, has been developed and tested successfully in JET to replace eventually conventional launchers using multijunctions for next step machines like ITER.

The new family of IR transmitting glasses, the TeX glasses, based on the association of tellurium and halide (Cl, Br, or I) are characterized by a wide optical window extending from 2 to 18 micrometers and a strong stability towards devitrification. Optical fibers drawn from these glasses exhibit low losses in the 7 - 10 micrometers range (less than 1 dB/m for single index fibers, 1 - 2 dB/m for fibers having a core-clad structure). The TeX glass fibers have been used in a remote analysis set-up which is mainly composed of a FTIR spectrometer coupled with a HgCdTe detector. This prototype system permits qualitative and quantitative analysis in a wide wavelength region lying from 3 to 13 micrometers , covering the fundamental absorption of more organic species. The evolution of a lactic and an alcoholic fermentation has been monitored by means of this set-up.

JET was extensively modified in the 1992/93 shutdown. The new pumped divertor and many new systems were brought into operation early in 1994. Operations have progressed to 4MA plasma current and, with substantial additional heating, H-mode confinement results confirm the expected scaling. The high power handling capability of the pumped divertor with sweeping is estimated at 20MW for 20s. H-mode plasmas have large Type I ELMs. With lower hybrid heating alone, 2MA full current drive has been achieved with good efficiency, with ICRF power, effective heating and direct electron heating have been demonstrated.

The JET Lower Hybrid Current Drive (LHCD) prototype system has been in operation in the early part of the 1991 experimental campaign with generated power up to 2.7 MW at 3.7 GHz and coupled power to the plasma up to 2.4 MW. Experiments have focused so far on the determination of the performance limits of the system, both in terms of long pulse operation and of replacement of the ohmically driven plasma current. Further experiments have also been carried out to confirm early findings of central electron heating induced by LH when applied in combination with Ion Cyclotron Resonance Heating (ICRH). Long pulse operation has been obtained on JET with 2 MA, 1.9 T limiter plasmas with 1 minute current flat top where 40% of the required flux is provided by the combined application of the LH and ICRF systems. Full replacement of the inductive current has been observed in 1 MA, 3.4 T ohmic plasmas with 1.9 MW of LH power, with current drive efficiencies up to 〈ne〉 Icd/Plh∼0.35 1020 m −2 A/W with volume averaged ele...