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In this report key aspects of Lower Hybrid Current Drive (LHCD) operation at JET are presented. Amplitude and broadening of the Radio Frequency (RF) spectrum at about 3.7GHz and the frequency shift of the lower sideband were analyzed by means of new diagnostic utilizing RF spectra measurements. Dependencies on plasma density, LH power and the toroidal magnetic field were studied. Plasma rotation profiles with and without LH power were investigated. Power deposition and Current Drive (CD) profiles were assessed by means of a new Ray Tracing / Fokker Planck (RT/FP) code. Significant progress in a number of technological issues including the new supplementary arc protection system based on a visible camera imaging system and operation of the system with waveguides filled with N2 instead of greenhouse SF6 gas was achieved.

An overview of the recent results of Lower Hybrid (LH) experiments at JET with the ITER-like wall (ILW) is presented. Topics relevant to LH wave coupling are addressed as well as issues related to ILW and LH system protections. LH wave coupling was studied in conditions determined by ILW recycling and operational constraints. It was concluded that LH wave coupling was not significantly affected and the pre-ILW performance could be recovered after optimising the launcher position and local gas puffing. SOL density measurements were performed using a Li-beam diagnostic. Dependencies on the D2 injection rate from the dedicated gas valve, the LH power and the LH launcher position were analysed. SOL density modifications due to LH were modelled by the EDGE2D code assuming SOL heating by collisional dissipation of the LH wave and/or possible ExB drifts in the SOL. The simulations matched reasonably well the measured SOL profiles. Observations of arcs and hotspots with visible and IR cameras viewing the LH launc...

Mechanisms of arc formation have been analyzed and the critical electric fields for the multipactor effect calculated, compared to the experimental values and found to be within the normal operational space of the LH system on JET. It has been shown that the characteristic electron energy (20-1000)eV for the highest multipactor resonances (N = 4-9) are within the limits of secondary electron yield above 1 required for multipactoring. Electrons with these energies provide the highest gas desorption efficiency when hitting the waveguide walls. The effect of higher waveguide modes and magnetic field on the multipactor was also considered. The distribution function for electrons accelerated by LH waves in front of the launcher has been calculated. The field emission currents have been estimated and found to be small. It is proposed that emission of Fel5, 16 lines, which can be obtained with improved diagnostics, could be used to detect arcs that are missed by a protection system based on the reflected power. The reliability and time response of these signals are discussed. A similar technique based on the observation of the emission of low ionized atoms can be used for a fast detection of other undesirable events to avoid sputtering ormore » melting of the plasma facing components such as RF antenna. These techniques are especially powerful if they are based on emission uniquely associated with specific locations and components.« less

The aim of the Tore Supra tokamak is to address physics and technology issues of long pulse discharges. For this purpose, Tore Supra is equipped with two actively cooled Lower Hybrid Current Drive (LHCD) antennas (f = 3.7GHz), designed to operate in 1000s long pulses. One of these is the recently installed passive active multijunction (PAM) antenna, whose design is chosen for an LHCD system for ITER. The first experiments with the PAM antenna in Tore Supra have shown extremely encouraging results in terms of reflection coefficient behaviour and power handling. The maximum power and energy reached on the PAM, after ~500 pulses on plasma, was 2.7MW during 78s (exceeding 200MJ injected energy). In addition, 2.7MW has been coupled at a plasma‐antenna distance of 10cm. The coupling behaviour on the PAM, characterised by the fraction of reflected power (RC), shows good agreement with the predictions from the ALOHA coupling code. Full non‐inductive discharges lasting 50s have been sustained with the PAM alone, e...

Heat loads from ICRF and LH wave absorption in the SOL are characterized on JET from the de‐convolution of surface temperatures measured by infrared thermography. The spatial localization, quantitative estimates, parametric dependence and physical origin of the observed heat fluxes are documented. Implications of these observations are discussed for the operation of JET with an ITER‐Like Wall, featuring Beryllium tiles with reduced power handling capability.

In this paper, significant results in the heating and current drive domains obtained at JET in the past few years following systems upgrade and dedicated experimental time, will be reviewed. Firstly, an overview of the new Ion Cyclotron Resonance Frequency (ICRF) heating capabilities will be presented i.e. results from the ITER‐Like ICRF antenna (ILA), the use of External Conjugate‐T and 3dB hybrid couplers to increase the ICRF power during ELMy H‐mode. Furthermore, experiments to study the influence of the phasing of the ICRF antenna on power absorption and coupling will be described. Looking at Low Hybrid (LH) issues for ITER, the effect of the location of gas injection on the LH coupling improvement at large launcher‐separatrix distances will be discussed as the possibility to operate at ITER‐relevant power densities. Experiments to characterise the LH power losses in the Scrape‐Off‐Layer (SOL) and to determine the LH wave absorption and current drive using power modulation will be shown. Finally, plas...

The impact of the Radio Frequency heating and current drive systems on the ITER advanced scenarios is analyzed by means of the CRONOS suite of codes for integrated tokamak modelling. As a first step, the code is applied to analyze a high power advanced scenario discharge of JET in order to validate both the heating and current drive modules and the overall simulation procedure. Then, ITER advanced scenarios, based on Radio Frequency systems, are studied on the basis of previous results. These simulations show that both hybrid and steady‐state scenarios could be possible within the ITER specifications, using RF heating and current drive only.

LH power modulation was recently used at JET to quantify experimentally the LH current drive efficiency and power deposition. A new approach was applied for the analysis of the amplitude and phase of electron temperature Te perturbations δTe(kωo) at different harmonics k of the modulation frequency ωo. A solution of the Fokker‐Planck equation combined with heat transport modelling was used to study the dependence of the ratio of the perturbation of different harmonics k and n, δTe(kωo)/δTe(nωo), and the phase shift of the harmonics δφ(kωo) on the plasma parameters and electron distribution function plateau width, which is directly connected to the current drive efficiency. The results of the modelling were compared with the experimental data to estimate the current drive efficiency. In addition, LH power deposition profiles were deduced from the radial dependence of δφ(ωo). The maximum of the LH power deposition becomes more peripheral and with a reduced current drive efficiency at higher densities. In H‐...

During high power commissioning of the JET LH launcher, the radiation and impurity release has been analyzed from various diagnostics: VUV and visible spectroscopy, bolometry. These two last diagnostics have lines‐of‐sight viewing the launcher and can provide information about the electron and/or impurity source localisation. Using a database of 800 plasmas, it is concluded that the iron contamination (FeXV and FeXXIII) is very low for 94% of the pulses and increases linearly with LH power. During arcs, a strong and fast increase of the radiation along the line‐of‐sight viewing the launcher is observed. This diagnostic could provide a tool for arc detection complementary to the RF measurements aiming at reducing the metal contamination in the plasma.

In recent JET experiments, Lower Hybrid (LH) power losses in the Scrape‐Off Layer (SOL) were characterized using infra‐red (IR) thermography. Hot spots were observed on objects intercepting the field lines passing in front of the LH launcher, i.e. on poloidal limiters and on dumplates located at the top of the tokamak; their locations being in good agreement with magnetic field line tracing using the EFIT equilibrium code. The dumplate temperature was monitored while scanning the launcher position so that the radial distance between field lines intercepting the hot spots and the launcher was increased up to 3.5 cm. The dissipation layer in front of the launcher was estimated to be at least 3.5 cm wide, in agreement with recent measurements on Tore‐Supra, but not with simple models that predict a dissipation layer in the mm range.

Lower Hybrid (LH) wave coupling deteriorates when the system is pulsed with Ion Cyclotron Range of Frequencies (ICRF) antennas magnetically connected to the launcher. This has been attributed to the density modifications by the RF sheaths. Reflection Coefficients (RCs) dependencies are investigated and shown consistent with the sheath physics. Gas puffing near the launcher has been used to improve the coupling. Results from a statistical analysis of a particular set of data are summarised.

In ITER, the requirement for the ICRF antenna is to deliver 20 MW in ELMy H‐mode plasmas with an averaged antenna—plasma separatrix distance of 14 cm [1]. Two major problems will have to be solved: the very fast change in antenna loading during ELMs and the decrease of the loading when the plasma is pushed far away from the antenna. JET has the capability to combine these conditions and for the first time, experiments were performed in ELMy H‐mode at antenna—separatrix distance, referred as ROG, varied from 10 to 14 cm. When ROG was increased, the perturbation caused by ELMs was found to decrease significantly and the loading between ELMs was found to deteriorate to very low values. In order to compensate the latter unwanted effect, different levels of deuterium gas were injected in the edge either from the divertor, the midplane or the top of the tokamak. Using this technique, the loading was increased by up to a factor 6 and up to 8 MW of ICRF power were coupled.

LHCD has been used in JET experiments aimed at producing internal transport barriers (ITBs) in highly triangular plasmas ({delta}{approx_equal}0.4) at high {beta}{sub N} (up to 3) for steady-state application. The LHCD is a potentially valuable tool for (i) modifying the target q-profile, which can help avoid deleterious MHD modes and favour the formation of ITBs, and (ii) contributing to the non-inductive current drive required to prolong such plasma regimes. The q-profile evolution has been simulated during the current ramp-up phase for such a discharge (B{sub 0} = 2.3 T, I{sub P} = 1.5 MA) where 2 MW of LHCD has been coupled. The JETTO code was used taking measured plasma profiles, and the LHCD profile modeled by the LHstar code. The results are in agreement with MSE measurements and indicate the importance of the elevated electron temperature due to LHCD, as well as the driven current. During main heating with 18 MW of NBI and 3 MW of ICRH the bootstrap current density at the edge also becomes large, consistently with the observed reduction of the local turbulence and of the MHD activity. JETTO modelling suggests that the bootstrap current can reduce the magnetic shear (sh) at large radius, potentiallymore » affecting the MHD stability and turbulence behaviour in this region. Keywords: lower hybrid current drive (LHCD), bootstrap current, q (safety factor) and shear (sh) profile evolutions.« less

Good coupling of LH power at plasma‐launcher distance of 15 cm has been obtained at JET. Near‐gas injection is used to increase the density in front of the grill. The role of LH power in the density increase at constant gas level is demonstrated. For the first time at JET the temperature of the hot spots caused by parasitic absorption of LH power has been measured.

The lower hybrid current drive efficiency of 66 Tore Supra pulses has been investigated. The ohmic part of the plasma current (0.6–0.9 MA) is very small (Vloop

In this study, the most likely causes of the enhanced radiation in front of the LHCD launcher are investigated: fast ions from the warm plasma, fast electrons parasitically accelerated in front of the grill and arcs. Evidence for the presence of each of these mechanisms is discussed. The experimental conditions favouring the appearance of these phenomena and their impact on the launcher have also been highlighted.

The improvement of LH coupling with local puffing of D2 gas, which made operation at ITER relevant distances (10 cm) and with ELMs a reality, has been extended to ITER‐ like plasma shapes with higher triangularity. With ICRF, we developed tools such as (1) localized direct electron heating using the 3He mode conversion scenario for electron heat transport studies, (2) the production of 4He ions with energies in the MeV range by 3 ωc acceleration of beam injected ions at 120 keV to investigate Alfven instabilities and test α diagnostics, (3) the stabilisation and destabilisation of sawteeth and (4) ICRF as as a wall conditioning. Several ITER relevant scenarios were tested. The (3He)H minority heating scenario, considered for the non‐activated start‐up phase of ITER, produces at very low concentration energetic 3He which heat the electrons indirectly. For n3He/ne > 2%, the scenario transforms to a mode conversion scenario where the electrons are heated directly. The (D)H minority heating is not accessible as the concentration of C6+ dominates the wave propagation and always leads to mode conversion. The minority heating of T in D is very effective heating for ions and producing neutrons. New results were obtained in several areas of ICRF physics. Experimental evidence confirmed the theoretical prediction that, as the larmor radius increases beyond 0.5 times the perpendicular wavelength of the wave, the second harmonic acceleration of the ions decreases to very small levels. An exotic fusion reaction (pT) must be taken into account when evaluating neutron rates. The contribution of fast particles accelerated by ICRF to the plasma rotation was clearly identified, but it is only part of an underlying, and not yet understood, co‐current plasma rotation. Progress was made in the physics of ELMs while their effect on the ICRF coupling could be minimized with the conjugate‐T matching scheme. The addition of 3 dB couplers is a step in increasing the power capability of the ICRF heating on JET in ELMy plasmas. The installation of an ITER‐like, ELM resilient antenna in 2006 will further improve this and be a test of the ICRF scheme for ITER.

Combined operation of Lower Hybrid (LH) and Ion Cyclotron Resonance Frequency (ICRF) waves can result in a degradation of the LH wave coupling, as observed both in the Tore Supra and JET tokamaks. The reflection coefficient on the part of the LH launcher magnetically connected to the powered ICRF antenna increases, suggesting a local decrease in the electron density in the connecting flux tubes. This has been confirmed by Langmuir probe measurements on the LH launchers in the latest Tore Supra experiments. Moreover, recent experiments in JET indicate that the LH coupling degradation depends on the ICRF power and its launched k//‐spectrum. The 2D density distribution around the Tore Supra ICRF antennas has been modelled with the CELLS‐code, balancing parallel losses with diffusive transport and sheath induced E×B convection, obtained from RF field mapping using the ICANT‐code. The calculations are in qualitative agreement with the experimental observations, i.e. density depletion is obtained, localised mainly in the antenna shadow, and dependent on ICRF power and antenna spectrum.

A significant step towards demonstrating the feasibility of coupling Lower Hybrid (LH) waves in ITER has been achieved in the latest LH current drive experiments in JET. The local electron density in front of the LH launcher was increased by injecting gas (D2 or CD4) from a dedicated gas injection module magnetically connected to the launcher. PLHCD=3MW was coupled with an average reflection coefficient of 5%, at a distance between the last closed flux surface and the launcher of 10cm, in plasmas with an internal transport barrier (ITB) and H‐mode edge, with type I and type III ELMs. Following a modification of the gas injection system, in order to optimise the gas localisation with respect to the LH launcher, injection of D2 proved to be more efficient than CD4. A D2 flux of 5−8×1021el/s provided good coupling conditions at a clearance of 10cm, while when using CD4, a flux of 12×1021el/s was required at 9cm. The plasma performance (neutron rate, H‐factor, ion temperature) was similar with D2 and CD4. An ...

In recent LHCD experiments in JET, a large effort has been placed on improving the coupling of Lower Hybrid waves in Optimised Shear plasmas, i.e. in conditions with internal transport barriers (ITB) or H-modes. Good coupling conditions have been obtained by using gas injection of CD4 near the LHCD launcher. This has resulted in a reflection coefficient as low as 5% during the ITB phase, at a plasma-launcher distance of 6 cm. Results from the coupling experiments have been compared with the SWAN coupling code, which has recently been extended to take into account the poloidal cut of the launcher mouth. The launcher mouth is composed of twelve rows of waveguides, all with different electrical lengths. By introducing the scattering matrices for all rows and calculating the resulting power splitting in the multijunctions, a more realistic model for the coupling has been obtained.

Good coupling of the LH power during the whole internal transport barrier (ITB) phase has been achieved in JET by fitting the plasma shape to the LH antenna shape and by injecting CD4 gas from a pipe near to the LH grill. No adverse effect on the edge pedestal or on the ITB quality, or significant impurity influx is observed. The key is the low ionization potential of all the CD4 dissociation chain, which is completed inside the SOL. Promising perspectives are open for injecting safely LHCD with more power during an ITB.

RF heating and current drive with ion cyclotron waves and Lower Hybrid waves have been crucial for the development of the Optimized Shear scenario on JET to high performance. Peaked electron temperature profiles and improved energy confinement could be obtained with electron heating both from LHCD and ICRH during plasma current ramp up. ICRH and NBI comparisons allow to separate heating and fueling and suggest a dominant role of core heating in the formation of an Internal Transport Barrier (ITB). ICRH and NBI powers are there equivalent. Pressure profile control by varying the composition of centrally peaked ICRF and broader NBI deposition improves MHD stability. Current profile modifications in a wide range have been obtained with LHCD and in combination with NBI during current ramp up. During the high performance phase, however, LH coupling degrades strongly due to the steep edge density gradient resulting in a drop of the density in front of the LH antenna to the cut-off density. High fusion performance achieving simultaneously high beta values and bootstrap currents is predicted in scenario modeling using pressure and current profile control with ICRF and LHCD.

The interaction is studied of TdeV plasma edge electrons with the electric near-field generated by a lower hybrid slow wave antenna. For waveguide electric fields, typically around 3 kV/cm, the higher-order modes overlap, so that the relatively cold (∼25 eV) scrape off layer (SOL) electrons can resonantly accelerate. For TdeV conditions and for 90° waveguide array phasing, the stochastic limit in the co-current direction is about 2 keV, determined by the 5th harmonic—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. We calculate that between 35 and 70 kW is transferred from the wave to the SOL electrons.

Current drive experiments have been performed on TdeV over a wide range of parameters. It is found that the current drive efficiency {eta} does not scale with {ital N}{sub {parallel}} as expected from simple theory. Simulations with the ACCOME code suggest that the reduced efficiency at low {ital N}{sub {parallel}} values is mostly due to a large toroidal upshift in the LH wave spectrum. The loss of suprathermal electrons to the divertor plates was monitored during CD experiments. The power loss increases with decreasing {ital N}{sub {parallel}} values and can be reach up to 15{percent} of the injected rf power in low density plasmas. {copyright} {ital 1996 American Institute of Physics.}