[en] A significant part of the non-inductive in Tore Supra will be driven by a new launcher, to be installed in September 1999. The antenna phase 2 is made of 6 rows with 48 active waveguides and 9 passive ones in each. Passive waveguides are inserted at every 6. active one. This grill has been designed in the frame of the CIEL project. It will inject 4 MW at 3.7 GHz at a safe power density of 25 MW/m² for a pulse length of 1000 s. The radiated spectrum peaks at N_// = 2.03 with a possible variation of ± 0.35 and a FWHM of 0.35. In order to prepare for operation with this grill, the coupling properties and the power directivity of the radiated spectra have been studied as a function of: the electron density and electron density gradient ; the feeding phase shift between the 8 antenna modules; the geometry of the antenna. Furthermore, the interaction of plasma edge electrons with the antenna is analysed and a comparison with the previous Tore Supra antenna is made. This is done for a range of plasma parameters and feeding phase. (authors)

[en] Stationary operations have been achieved at JET in ITBs scenarios, with the discharge time limited only by plant constraints. Full current drive was obtained, all over the high performance phase, with the current density profile frozen by using Lower Hybrid current drive. For the first time a feed-back control on the total pressure and on the electron temperature profile was implemented by using respectively the Neutral Beams and the Ion Cyclotron waves. Although impurity accumulation could be a problem in steady state ITBs, these experiments bring some elements to answer to it. Tokamak operation in enhanced confinement regimes, characterized by edge and/or Internal Transport Barriers (respectively known as H-mode and ITB), is attractive as it represents an important step towards the approach of ignition conditions. Moreover, the necessity of steady state operation in a Tokamak reactor, has led to the concept of the Advanced Tokamak, in which the current density profile is no longer tied to the plasma conductivity and is non inductively driven. Since the bootstrap current is a consequence of the pressure gradient, one of the primary goal of the Advanced Tokamak studies is to maximize the bootstrap fraction, with a proper alignment, both in H mode and in ITB regimes. However, for several reasons, it is difficult to envisage an operational situation in which the bootstrap fraction is close to 100%: for instance, there are few chances of pressure or/and current profile control to optimize the MHD stability. So far, various experiments have been performed with improved confinement regimes lasting up to tens of the confinement time and up to some current relaxation times. In some experiments a large non inductive plasma current (< 75%) was obtained with about 50% from bootstrap and 25% from Neutral Beam Injection (NBI); however, no full current drive operation was achieved and, moreover, with the available heating systems, no active feedback control of the current density and of the plasma pressure profiles was performed. In other experiments quasi-steady state regimes were achieved by operation at high poloidal beta, with a large fraction of bootstrap current and by using the current drive capability of the negative NBI. Finally stationary discharges (∼70 s) in full current drive were achieved in Tore Supra by using LHCD (Lower Hybrid Current Drive). In principle, the simultaneous use of several auxiliary power systems makes possible to sustain high performance regimes using an active control of all the different plasma profiles. The availability of three heating systems (NBI, ICRH (Ion Cyclotron Heating) and LHCD) gives JbT some advantage with respect to other large experiments, allowing, for the first time, two important targets of advanced scenario to be achieved simultaneously: a) plasma configurations with both electron and ion ITBs have been obtained in full current dnve (no inductively driven current) by optimizing the coupling of the LHCD system, the duration of these discharges being constrained only by plant limitations (the toroidal magnetic field flat top) (author)

[en] Full text: Lower Hybrid (LH) is a highly desirable tool for ITER, particularly for the steady state and hybrid scenarios that rely on forming and maintaining a specific q profile. However, coupling of the LH waves in ITER represents a challenge because the launcher, flush with the outer wall, will be at least 12cm away from the last closed flux surface (LCFS), where the electron density (n_e) is predicted to be below the cut-off density. Also, the plasma will be in H-mode with ELMs that could trigger trips of the protection systems, resulting in reduction of the averaged power. To study solutions to improve coupling of LH waves in these conditions, experiments were performed in JET using plasmas with Internal Transport Barrier (ITB) and H-mode, with distance between the LCFS and the limiter, DPL, up to 10 cm. To increase n(e) in front of the launcher, n_e,grill), CD₄ or D₂ is puffed from a specially designed pipe, recently modified to optimise the gas flow. When no gas is puffed, the LH coupling is poor during the high power phase, indicating that n_e,grill is below the cut-off density. With CD₄ puffing, the coupling improves dramatically, and 2.5 MW of LH power is coupled with DPL = 9cm and type I ELMs. Measurements with a reciprocating Langmuir probe show that n_e,grill) is at, or slightly above, the cut-off density, in this case. With D₂ puffing, preferable to CD₄ for ITER because of concerns about T co-deposition, 3MW was coupled successfully with DPL up to 10cm. The ELMs are smaller with D₂, but the improvement in coupling can not be attributed only to this. The use of D₂ or CD₄ was explored further in other plasmas with ITB and H-mode. In all the scenarios, the coupling is better with D₂ than with CD₄. Moreover, the ITB existence and performance are not affected by the D₂. To help understand the processes leading to the increase in n_e,grill), C₂H₆ and C₃H₈ were compared to CD₄ and D₂ in the same scenario. ne scales with the ionisation cross-section in the case of the hydro-carbide gases, but is higher than expected for the D₂, possibly because of higher D₂ recycling from the walls. This paper also presents results from experiments investigating LH counter current drive in JET, in plasmas with reversed magnetic field and plasma current. Preliminary simulations indicate significant current drive efficiency. (author)

[en] Control of impurity transport, and hence radiative losses in tokamaks is becoming an increasingly important issue. This is because the reduction of radiative losses from the hot core is essential for achieving a reactor grade plasma, and at the same time high radiative losses in the divertor region are required, in order to relax the heat load handling requirement for the divertor plates. On TdeV it has been shown that plasma biasing can be an important control instrument to achieve these two apparently contradictory requirements simultaneously. In order to characterize the distribution of the radiative losses in TdeV, the bolometry system has been upgraded. The new system includes a horizontal array of 17 detectors, which views the entire plasma cross section, and two pairs of bolometers, which monitor the plasma in front of the divertor plates, in both the upper and lower closed divertor chambers. A mathematical model appropriate to the TdeV geometry, which takes into account the radial and poloidal distribution of the radiated power, has been proposed and used for unfolding the radiated power distribution on TdeV. It has been observed that negative (or positive) plasma biasing reduces (or increases) the radiation from the plasma core by up to about 30%, whereas the radiative losses in front of the divertor plate, where E x B flow is directed, always increase by a factor of 2 to 3. This is in agreement with the observed increase in the pressure of the divertor chamber. Under standard operating conditions (I_p = 210 KA, n_e = 2.5 x 10¹⁹ m^-3, B_T = 1.5 T), the radiated power from the plasma in front of the divertor plate, where the plasma density is about 2 x 10¹⁸ m^-3 and plasma temperature is about 10 eV, is less than 1% of the ohmic input power. This fraction is expected to increase substantially when LHCD becomes operational on TdeV

No abstract available

[en] The interaction of tokamak plasma edge electrons with the electric near field generated by a lower hybrid slow wave antenna is studied. Antenna field spectra of interest for current drive and/or plasma heating have lobes at high-n_parallel values (n_parallel approx-gt 30) intense enough for resonant acceleration of the relatively cold (∼25 eV) edge electrons. For waveguide electric fields, typically around 3 kV/cm, the higher-order modes overlap in the phase-space [B. V. Chirikov, Phys. Rep. 52, 263 (1979)], so that electron global stochasticity is induced. For Tokamak de Varennes (TdeV) [Dacute ecoste et al., Phys. Plasmas 1, 1497 (1994)] conditions and for 90 degree waveguide phasing, the stochastic limit in the current drive direction is about 2 keV, determined by the last overlapping mode. The progress of electrons through accessible phase space is very efficient: the TdeV 32 waveguide array can accelerate the electrons to the possible limit. An area-preserving map is derived to study the electron dynamics. Surface-of-section plots fully confirm the resonant wave-particle nature of the interaction. copyright 1996 American Institute of Physics

[en] Finite elements offer some interesting possibilities in modelling tokamak edge and divertor plasmas. In particular, they can readily be applied to discretise transport equations on an unstructured triangular mesh which, in turn, is well suited to solve problems in complex geometries. This has recently stimulated a number of projects aimed at developing new plasma edge and divertor models based on this type of approach. In this paper, we present simulation results obtained with the latest version of TOPO, which accounts for impurities via multiple fluid species, as well as for selected drift effects. The code is applied to the new TdeV divertor geometry with particular emphasis on results which can be directly compared with experiment. (orig.)

[en] The data in figure 15 were incorrectly plotted and the graph should be replaced with the corrected version given in this erratum

[en] Recent studies at JET focus on analysis of the lower hybrid (LH) wave power absorption and current drive (CD) calculations by means of a new ray tracing (RT)/Fokker–Planck (FP) package. The RT code works in real 2D geometry accounting for the plasma boundary and the launcher shape. LH waves with different parallel refractive index, $N_{\parallel <!--\; ???\; -->}$, spectra in poloidal direction can be launched thus simulating authentic antenna spectrum with rows fed by different combinations of klystrons. Various FP solvers were tested most advanced of which is a relativistic bounce averaged FP code. LH wave power deposition profiles from the new RT/FP code were compared to the experimental results from electron cyclotron emission (ECE) analysis of pulses at 3.4 T low and high density. This kind of direct comparison between power deposition profiles from experimental ECE data and numerical model were carried out for the first time for waves in the LH range of frequencies. The results were in a reasonable agreement with experimental data at lower density, line averaged values of ${\stackrel{\to <!--\; ???\; -->}{n}}_{\text{e}}\approx <!--\; ???\; -->2.4\times <!--\; ??\; -->{10}^{19}{\text{m}}^{-<!--\; ???\; -->3}$. At higher density, ${\stackrel{\to <!--\; ???\; -->}{n}}_{\text{e}}\approx <!--\; ???\; -->3\times <!--\; ??\; -->{10}^{19}{\text{m}}^{-<!--\; ???\; -->3}$, the code predicted larger on-axis LH power deposition, which is inconsistent with the experimental observations. Both calculations were unable to produce LH wave absorption at the plasma periphery, which contradicts to the analysis of the ECE data and possible sources of these discrepancies have been briefly discussed in the paper. The code was also used to calculate the LH power deposition and CD profiles for the low-density preheat phase of JET’s advanced tokamak (AT) scenario. It was found that as the density evolves from hollow to flat and then to a more peaked profile the LH power and driven current move inward i.e. towards the plasma axis. A total driven current of about 70 kA for 1 MW of launched LH power was predicted in these conditions. (paper)

[en] Full text: A deuterium-tritium (DT) experimental campaign DTE2 on JET scheduled for 2019-2020, will be done in the Be/W vessel and will address essential operational, technical, diagnostic and scientific issues in support of ITER. In preparation for the campaign, developments were performed on JET aiming at studies of α-particles. For studying AEs driven entirely by α-particles, a scenario similar to the TFTR beam “afterglow” was developed for JET. In DT plasmas, after NBI is switched off, α-particles will be the only energetic ions during the time interval between slowingα-down times for NBI-produced ions and α-particles. Detection of α-driven AEs in this time window may help in diagnosing the temporal evolution of the pressure profile and slowingα-down time of α-particles. JET advanced tokamak scenarios with q ≈ 1.5-2.5 were chosen and discharges have been successfully developed. The transport modelling extrapolated to DT predicted that α-particle ≈ 0.1% could be achieved comparable to that in successful TFTR experiments. In “hybrid” scenario plasmas with q₀ ≥ 1, fast ion losses in the MeV energy range were observed during n = 1 fishbones driven by a resonant interaction with D beam ions in the energy range ≤ 120 keV. The losses are identified as an expulsion of DD fusion products, 1 MeV tritons and 3 MeV protons. A mode analysis with the MISHKA code combined with the study of nonlinear waveα-particle interaction with HAGIS show that the loss of toroidal symmetry strongly affects the confinement of high energy tritons and protons by perturbing their orbits and expelling them in a good agreement with experiment. The extrapolation to the case of α-particles in DTE2 hybrid scenarios with similar fishbones has shown an additional α-particle loss of ∼ 1%. (author)