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The lower hybrid current drive (LHCD) system for FTU has a modular configuration that gives it a high degree of flexibility in operations. The system normally operates with six conventional grills (CG), each one made of four superimposed rows of 12 waveguides, distributed on two independent launching structures. This modular configuration has given the possibility of testing on FTU a classical multijunction (MJ) and a passive active multijunction (PAM) that is considered as a privileged candidate for the LH launcher for ITER. The tests have confirmed the good coupling properties of the MJ with plasma densities lower than those normally acceptable for a CG, but at expenses of a reduced power directivity and flexibility in N ∥ . The PAM, with a small reduction on power directivity and flexibility in N ∥ with reference to the MJ, has shown an efficient coupling when working close to cut-off density (0.8 × 10 18 m −3 ) flush to the first wall, a relatively quiet region in the harsh plasma environment of ITER. The paper presents an overview of the performances, in term of coupling characteristics, of the three launchers on FTU and gives a comparative analysis of the results obtained in similar plasma conditions, in particular during the same shot.

The interaction between the lower hybrid current drive (LHCD) launchers and the plasma has been studied during long pulse, high power operation in the Tore Supra tokamak. The main diagnostics used for characterising the plasma–launcher interaction are calorimetry of the energy extracted by the launchers and infrared (IR) imaging of the launchers and their side limiters. The calorimetry has allowed to identify three different heat sources on the LHCD launchers, namely the RF losses in the waveguides, a fraction (∼0.8%) of the total injected energy and, finally, fast ion losses during ion cyclotron resonance heating (ICRH), accounting for ∼1% of the injected ICRH energy. The interaction by fast ions is identified by infrared imaging of the LHCD launchers as a localised hotspot on the ion drift side, below or at the mid-plane.

The 20 MW, 5 GHz, LHCD system for ITER aims mainly at controlling the q-profiles in partially (hybrid regime at I P = 12 MA) and fully non-inductive steady-state operation ( I P = 9 MA). In the last few years several European associations have given their contribution to the solution of many prominent problems on the road to the design of an efficient and reliable system. LH coupling has been achieved on JET in physics scenarios relevant for ITER, while the reliability of the LH for long pulse operation has been ascertained on Tore Supra. A downscaled prototype of the Passive Active Multijunction (PAM) launcher presently proposed for ITER, has successfully demonstrated on FTU its good coupling characteristics, power handling capability and current drive efficiency. An ITER relevant launcher, based on the PAM concept is presently under construction to be installed on TORE SUPRA to investigate the technological solutions (materials, cooling, etc.) envisaged for this launcher.

During high-power, long-pulse transmission from lower-hybrid-range-of-frequency (LHRF) antennas, the waveguide walls outgas as a result of rf-loss-induced heating. If the resulting pressure rise is too high, power transmission will be adversely affected and additional pumping may be required to maintain the pressure at a low enough value. The outgassing rates of waveguides made of various materials (oxygen-free high-conductivity copper, dispersoid copper, copper-coated carbon fiber composite, copper-coated graphite) were measured during rf injection at high power density (50–200MW∕m2) for a duration in the range 100–4700 s. The experiments were performed on a test-bed facility equipped with a 3.7 GHz klystron on multiwaveguide (2 to 18) mock-ups. The effect of the main parameters, namely, the waveguide surface temperature and the initial wall gas loading (“conditioning”), are analyzed in detail. It is concluded that an outgassing rate of 1×10−5(5×10−5)Pam3s−1m−2 at 300 °C (400 °C) can be considered for most materials. The requirement, in terms of additional pumping, for the International Thermonuclear Experimental Reactor LHRF antenna is finally discussed.During high-power, long-pulse transmission from lower-hybrid-range-of-frequency (LHRF) antennas, the waveguide walls outgas as a result of rf-loss-induced heating. If the resulting pressure rise is too high, power transmission will be adversely affected and additional pumping may be required to maintain the pressure at a low enough value. The outgassing rates of waveguides made of various materials (oxygen-free high-conductivity copper, dispersoid copper, copper-coated carbon fiber composite, copper-coated graphite) were measured during rf injection at high power density (50–200MW∕m2) for a duration in the range 100–4700 s. The experiments were performed on a test-bed facility equipped with a 3.7 GHz klystron on multiwaveguide (2 to 18) mock-ups. The effect of the main parameters, namely, the waveguide surface temperature and the initial wall gas loading (“conditioning”), are analyzed in detail. It is concluded that an outgassing rate of 1×10−5(5×10−5)Pam3s−1m−2 at 300 °C (400 °C) can be considered for mo...

On ITER, the LH system aims at supplying 20 MW CW for controlling the q-profiles in partially and fully non-inductive steady-state operation (9 MA). The designed LH system relies on a transmitter made of 24 5 GHz I MW klystrons linked to one antenna based on the passive active multijunction (PAM) concept, via a 60 m long oversized circular transmission line. Through a common European effort including several associations (CEA, ENEA, UKAEA, IST, TEKES and IPP-CZ) many outstanding problems have been solved: coupling has been achieved on JET in a physics scenario similar to ITER, a PAM has been tested with success on the FTU plasma. The reliability of LH for long pulse operation has been ascertained on Tore Supra. In order to routinely realise long pulse operation (1000 s), the Tore Supra LH system is being refurbished within the CIMES project framework where the transmitter is to be equipped with 16 klystrons 700 kW 1000 s at a frequency of 3.7 GHz. A new LH launcher based upon the PAM concept has been studied, designed and is actually under realisation. (c) 2005 Elsevier B.V. All rights reserved.

High-frequency wave systems with high-power density launching capability have been the preferred choice to heat the Frascati Tokamak Upgrade (FTU) because of physics arguments (electron heating at ...

The Passive active multi-junction (PAM) launcher is a robust coupling structure, able to withstand the strong thermal loads and the electromechanical stresses of ITER-like tokamaks. In the frame of a collaboration between ENEA, Frascati and CEA, Cadarache, a PAM antenna has been designed and built to validate on FTU the concept of this launcher. All the technological solutions of an ITER-like launcher will be tested in a full rated experiment to be performed, possibly, on Tore Supra. This paper reports the results of the low power measurements of the microwave characteristics of the PAM for FTU.

This paper outlines the preliminary radiofrequency analysis of the prominent microwave components of the LHCD system for ITER. The general overview and inclusive analysis of the system is given in Ph. Bibet et al. (Overview of the ITER-Feat LH System; this Conference), a companion paper in this Conference. The results of the analysis and optimisation of the most relevant components is reported as computed by the ‘high frequency structure simulator’ (HFSS), a computer code developed by ANSOFT and based on the finite elements method.

The tokamak Tore Supra (TS) has been operated since 1988 in Cadarache Centre. With the Composants Internes et Limiteur project, it will be the only machine to be able to exploit the Advanced Tokamak Regimes in steady state operation with three heating systems ICRH, LH, ECRH. The injected RF power in the Torus is a very important parameter of the total energy balance of TS, for this reason we need the best possible knowledge of these powers. The analysis and the explanation of the different RF techniques at low power which have been used in TS on the three RF systems will be described. Principles of the different measurement systems are given. The calibration methods of these systems are discussed and the present status of the calibration uncertainties of the RF measurements is analysed. Finally, when the generators are connected to a dummy load, RF measurements are compared with the calorimetric power measurements. This study allows preparation of improvements of these systems which will be used in these next steps: Composants pour l'Injection de Matiere et d'Energie Stationnaire project (B. Beaumont, Tore Supra steady state power and particles injection: the CIMES project, in: Proceedings of the 21st SOFT, Madrid, 2000).

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.

Tore Supra inner components are now being replaced by new elements that will be able to extract about 25 MW power flux during long discharges (up to 1000 s). A new project described here, deals with the necessary upgrades of the heating and fuelling systems to match actual Tore Supra extraction capability.

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.

During the 1997 and 1998 Tore Supra shutdown, a first set of new antenna guard limiters was installed on one of the three ion cycloton resonance heating (ICRH) antennas of Tore Supra. This limiter, which was one of the main technological studies of the 1998 campaign, was widely experimented in real plasma conditions, thus allowing the validation in situ, for the first time, of the technology of active metal casting (AMC) for plasma facing components. The huge improvement in the thermal response of the new limiter generation, compared to the old one, is shown on plasma pulses made identical in terms of antenna position and injected RF power profile. By using the infrared cameras installed inside Tore Supra and viewing the antennas front, the power density fluxes received by the carbon fibre composite (CFC) surface of the limiter were evaluated by correlation with the heat load tests made on the electrons beam facility of CEA/Framatome.

Parasitic absorption of the short wavelength modes of the LH spectrum is a probable reason for the hot spots seen in the grill region of several tokamaks. Experiments suggest that the heat loads on the wall structures depend on the coupled power. In this work, the parasitic absorption of LH power was studied with self-consistent particle-in-cell simulations. The launched spectra were obtained from the SWAN coupling code. The power and temperature dependences of the absorption in the near field of the LH grill were investigated with a series of simulations. The parasitic absorption was found to grow from 0.6 to 1.1% when the coupled power increased from 26 to 67 MW/m2. When the edge temperature rose from 12.5 to 100 eV, the absorption increased from 0.4 to 1.7%. The maximum kinetic energies were between 0.6 and 1.8 keV. Estimates for the heat loads and surface temperature of the grill limiter are also obtained. The absorption leads to heat loads between 1.5 and 13 MW/m2 and surface temperatures of 510-2390° C.

Fast electrons generated in front of lower hybrid (LH) grills can cause damage to tokamak divertor and first wall components. It is therefore of interest to identify possible edge electron acceleration mechanisms and suggest measures to limit these. One possible acceleration process described here depends on mode overlap in the antenna electric field spectrum and on the presence of high- modes, resonant with the ambient ( eV) edge electrons. These electrons then diffuse throughout the globally stochastic region of -space whose upper bound is given by the last overlapping mode. It is shown that the described acceleration is seriously aggravated by the presence of higher order waveguide modes generated at sharp waveguide-plasma transitions and that rounding-off the waveguide septa can considerably reduce the fields and electron acceleration associated with these higher order modes. Additional antenna field non-uniformities arising from a grill modular structure and random fields generated at the plasma edge through parametric decay and scattering of the LH slow wave from plasma density fluctuations can further enhance the edge electron energy.

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.

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.

An outgassing rate is measured using a waveguide module during a long pulse r.f. injection at 3.7 GHz in order to operate a lower hybrid current drive (LHCD) antenna in steady state. The waveguide module consists of four sub-waveguides which are made of dispersion-strengthened copper and copper-plated stainless steel with cooling channels to control its working temperature between 100 and 500 °C. The waveguide module shows the high power capability of power density up to 200 MW m −2 after short term conditioning. A low outgassing rate of about 2 × 10 −7 Pa m 3 s −2 m −2 is obtained during r.f. injection up to 150 MW m −2 at a working temperature of T =300 °C after 450 °C baking. Long pulse r.f. injection is effective for reduction of the outgassing by about 1/100. Outgassing during r.f. injection depends on the working temperature but is independent of the r.f. power density after sufficient conditioning. The calculation code taking desorption, adsorption and diffusion processes into account can show time behavior of outgassing in the waveguide module. A quasi-constant outgassing of about 10 −7 Pa m 3 s −1 m −2 is observed at a power density of 150 MW m −2 in 1800 s injection while keeping a saturated temperature at the center of the module below 120 °C by using water cooling. The outgassing properties obtained indicate that steady state operation of an LHCD antenna is feasible without a large pumping system.

The design and the fabrication of a new Lower Hybrid (LH) actively cooled antenna based on the passive active concept is a part of the CIMES project (Components for the Injection of Mater and Energy in Steady-state). The major objectives of Tore-Supra program is to achieve 1000 s pulses with this LH launcher, by coupling routinely >3 MW of LH wave at 3.7 GHz to the plasma with a parallel index n{sub ||} = 1.7 {sup {+-}}{sup 0.2}. The launcher is on its way to achieve its validation tests--low power Radio Frequency (RF) measurements, vacuum and hydraulic leak tests--and will be installed and commissioned on plasma during the fall of 2009.

A prototype passive active multijunction (PAM) launcher for lower hybrid (LH) waves conceptually similar to that foreseen for ITER has been successfully tested on FTU at frequency f = 8 GHz. The power routinely and safely managed for the maximum time allowed by the LH power plant (0.9 s) without any fault in the transmission lines is 250 kW, corresponding to 75 MW m(-2) across the antenna active area and very close to the design value of 270 kW or 80 MW m(-2). The achieved value is at least 1.4 times larger than the ITER request, which would be only 52 MW m(-2), if the 33 MW m(-2) required to the ITER grill in order to couple 20 MW to the plasma, are scaled up linearly with f, from f(ITER) = 5 GHz. This linear scaling of the power handling capability of the LH antennae is indeed conservative with respect to the available data. The test results validate also the other two main expectations relevant to ITER, foreseen by the codes, namely to operate with the grill entirely in the vessel shadow and to still preserve good current drive (CD) efficiency. Even with the grill mouth retracted 2 mm inside the port shadow and with density in front of the launcher very close or even lower than the cut-off value, the PAM reflection coefficient is always

The recent success in coupling lower hybrid (LH) waves in high performance plasmas at JET together with the first demonstration on FTU of the coupling capability of the new passive active multijunction launcher removed major concerns on the possibility of using LH on ITER. LH exhibits the highest experimental current drive (CD) efficiency at low plasma temperature thus making it the natural candidate for off-axis CD on ITER where current profile control will help in maintaining burning performance on a long-time scale. We review recent LH results: long internal transport barrier obtained in JET with current profile sustained and controlled by LH acting under real time feedback together with first LH control of flat q-profile in a hybrid regime with Te ~ Ti. Minutes long fully non-inductive LH driven discharges on Tore Supra (TS). High CD efficiency with electron cyclotron in synergy with LH obtained in FTU and TS opening the possibility of interesting scenarii on ITER for MHD stabilization. Preliminary results of LH modelling for ITER are also reported. A brief overview of ITER LH system is reported together with some indication of new coming LH experiments, in particular KSTAR where CW klystrons at the foreseen ITER frequency of 5 GHz are being developed.

Improvement (up to a factor of approximately 4) of the electron-cyclotron (EC) current drive efficiency in plasmas sustained by lower-hybrid (LH) current drive has been demonstrated in stationary conditions on the Tore Supra tokamak. This was made possible by feedback controlled discharges at zero loop voltage, constant plasma current, and constant density. This effect, predicted by kinetic theory, results from a favorable interplay of the velocity space diffusions induced by the two waves: the EC wave pulling low-energy electrons out of the Maxwellian bulk, and the LH wave driving them to high parallel velocities.

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.

LH is considered to be used in ITER-FEAT steady-state scenario thanks to its highest current drive efficiency in the plasma outer part. The design of a 5 GHz, 20 MW CW LH system has been realised. That relies on a transmitter of 24 1-MW klystrons, and a circular oversized 60-m long transmission line feeding an antenna, based on the passive active multijunction concept. In order to have a high reliability, the power density is limited to 33 MW/m2. After the system description, the results of different studies are given: RF computation, launcher coupling properties, thermal analysis, mechanical stress caused by disruption, neutron activation, acceleration of electrons in the antenna near field.

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.

When the magnetic shear vanishes over a wide spatial region close to the q=2 surface, a strong MHD activity ascribed to a global tearing mode m/n=2/1 is observed, corresponding to a 15% reduction of the current drive efficiency, and a significant flattening of the HXR profile localized in the vicinity of the island. Calculations of the HXR bremsstrahlung and the LH current drive efficiency are reported based on Fokker-Planck calculations to assess the role played by the fastest electrons of the tail, and identify the fraction which is lost by the MHD perturbation.

An overview is presented of the results obtained on JET by the Heating and Current Drive Task Force (TF-H) in the period May 2000 - March 2001. A strongly improved Lower Hybrid (LH) coupling was achieved by optimising the plasma shape and by controlling the local edge density via the injection of CD4. Up to 4 MW have been coupled in type III ELMy H-mode and/or on Internal Transport Barrier (ITB) plasmas with reflection coefficients as low as 4%. Long lasting quasi steady-state ITBs have been obtained by adding the LH current to the bootstrap and beam driven components. Furthermore the use of LH in the pre-heat phase results in electron temperature in excess of 10 keV, deep negative magnetic shear and strongly reduced power threshold for ITB formation. Preliminary results on ICRF coupling are reported including the effect of CD4 injection and the commissioning of the wide band matching system on ELMy plasmas. 1C CD scenarios have been studied in H and 3He minority and used to modify the stability of the sawtooth to influence the formation of seed islands for the appearance of NTM. Up to 3 MW of 1C power was coupled in the high magnetic field fast wave CD scenario. Preliminary MSE measurements indicate differences in the current profiles between -90° and +90° phasing. Careful measurements of the toroidal rotation, in plasmas heated by ICRF only show some dependence on the position of the resonance layer. Finally the use of ICRF minority heating under real-time control, in response to measured plasma parameters to simulate the effect of alpha particles, is presented. ICRF heating results in ITER non-activated scenarios are reported in a companion paper.

In lower hybrid (LH) current drive experiments at Tore Supra and Tokamak de Varennes (TdeV), hot spots and the generation of impurities have been observed. Melting of the grill mouth has also occurred in LH current drive experiments. A possible explanation for these observations is parasitic absorption of the short-wavelength part of the LH spectrum close to the grill mouth.In this work, we investigate the parasitic absorption of the LH power in the edge plasma of Tore Supra and JET with particle-in-cell (PIC) simulations. The LH spectra are calculated with the SWAN coupling code and used in the self-consistent electrostatic PIC code XPDP2 which calculates the absorption.The absorption was calculated for different edge densities and density scale lengths when the coupled LH power densities varied between 25 MW m-2 and 50 MW m-2. The high-n|| part of the LH spectrum was found to be absorbed near the edge within a few millimetres. The absorbed power density varied from 65 kW m-2 to 420 kW m-2 which corresponds to 0.2-0.8% of the coupled power. In an edge plasma having a temperature of 25 eV, the maximum energies of the fast electrons generated by the parasitic absorption were between 0.4 keV and 1.6 keV.

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.

For simplification of Lower Hybrid Current Drive (LHCD) antenna system and reduction of power density in the antenna, three types of poloidal power divider which split the power in three, and 3 × 6 multi-junction module were developed. A good power dividing ratio of 33 ± 4 % was obtained for each of these dividers. In combination with two dividers connected to the 3 × 6 multi-junction module, a good high power handling capability was assessed: 420 kW-100 sec was transmitted without breakdown. Quasi stationary operation for r.f. injection time of 1000 sec at 300 kW was also demonstrated under water cooling. The outgassing rate at that time is in range of 10−7 Pa m3 s−1 m−2 with maximum temperature of 100°C, which requires no active vacuum pumping of the LHCD antenna.

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 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.

Outgassing, induced by very long RF waves injection (up to 6000s) at high power density, is studied with a module, able to be used for a lower hybrid frequency antenna.

In large tokamak such as JET, JT60, Tore Supra, the lower hybrid frequency antennae are composed of hundreds of waveguides and are fed by tens of klystrons. The N // spectrum which is then radiated depends on the geometry and on the wave amplitude and phase. It has to be checked when the total system is linked to the machine. Here four methods using the RF system are described which could be relevant for ITER: - The first method is the standard one. It uses measurements in laboratory to have the antennae electric lengths and measurement on the real system after its calibration. The incident wave is driven by the RF system for each klystron and the incident phase is then measured at the input of the antenna, computed at the antenna mouth and compared to the ones of neighbouring klystron. - The second one uses only the real system. The incident phase in the waveguide i fed by the klystron i is measured at the antenna mouth with the use of a magnetic loop. The other waveguides fed by the same klystron are matched with good loads. The calibration of the RF measurement system is not necessary here. - The third method uses the observation of the amplitude of the reflected signal of the klystron i and i+1 when they are fed together and the incident phase of i is constant whereas the one of i+1 is pulsed. The experiment has to be done on plasma. In this method only the measurement of amplitude is necessary, the components of the antenna must be identical from the reflection point of view. - The fourth one consist in the measurement of the scattering matrix at the input of the antenna on plasma. For N klystrons, N cases of feeding linearly independent must be done in such a way to have a linear system of N × N equations obtained from the measurement of the electric field at the input of the antenna (192 RF signals on Tore Supra). Then the S parameters can be obtained and by using the property of symmetry of the matrix, the measurement system calibration can be checked.

For Tore Supra lower hybrid current drive experiment a 8 MW, 3.7 GHz, 0.36 duty cycle RF transmitter made of 16 klystrons (efficiency 42 %, Uc = 58 kV, Ib = 19 A) with a modulating anode will be built. Two feedback systems will be set up to perform constant RF amplitude and phase for each klystron output. Control, slow protection of the transmitter will be done by control data acquisition, for fast protection an electronic device will be used. A 25 m long pressurized, water cooled transmission line will be built to link the transmitter to 2 antennas. The cooling of the whole system will be made by TS general water cooling system.