Passive Active Multijunction Research Articles

Development of the 3.7 GHz LHCD system on HL-2A

A 2 MW-3.7 GHz lower hybrid current drive (LHCD) system is under development for physics experiments on the HL-2A device. The RF Power is generated by four TH2103A klystron amplifiers and propagates in the TE10 mode through WR284 waveguides. The transmission lines with a length of 20 m to 30 m are pressurized with 2 bars of nitrogen to decrease the possibility of arcing. The launcher, based on the passive-active multi-junction (PAM) concept, has been developed and is currently being realized. It was designed for a power spectrum peaked at N|| = 2.75 with good coupling properties over a wide range of plasma parameters. The four klystrons are fed by a high-voltage power supply (HVPS) based on the pulse step modulation (PSM) concept with a fast switch-off time of less than ten μs. This system is expected to be in operation within 1 years and will explore many international thermonuclear experimental reactor (ITER) related LH experiments in the following years.

Advances in multi-megawatt lower hybrid technology in support of steady-state tokamak operation

It has been demonstrated that lower hybrid current drive (LHCD) systems play a crucial role for steady-state tokamak operation, owing to their high current drive (CD) efficiency and hence their capability to reduce flux consumption. This paper describes the extensive technology programmes developed for the Tore Supra (France) and the KSTAR (Korea) tokamaks in order to bring continuous wave (CW) LHCD systems into operation. The Tore Supra LHCD generator at 3.7 GHz is fully CW compatible, with RF power PRF = 9.2 MW available at the generator to feed two actively water-cooled launchers. On Tore Supra, the most recent and novel passive active multijunction (PAM) launcher has sustained 2.7 MW (corresponding to its design value of 25 MW m−2 at the launcher mouth) for a 78 s flat-top discharge, with low reflected power even at large plasma-launcher gaps. The fully active multijunction (FAM) launcher has reached 3.8 MW of coupled power (24 MW m−2 at the launcher mouth) with the new TH2103C klystrons. By combining both the PAM and FAM launchers, 950 MJ of energy, using 5.2 MW of LHCD and 1 MW of ICRH (ion cyclotron resonance heating), was injected for 160 s in 2011. The 3.7 GHz CW LHCD system will be a key element within the W (for tungsten) environment in steady-state Tokamak (WEST) project, where the aim is to test ITER technologies for high heat flux components in relevant heat flux density and particle fluence conditions. On KSTAR, a 2 MW LHCD system operating at 5 GHz is under development. Recently the 5 GHz prototype klystron has reached 500 kW/600 s on a matched load, and studies are ongoing to design a PAM launcher. In addition to the studies of technology, a combination of ray-tracing and Fokker–Planck calculations have been performed to evaluate the driven current and the power deposition due to LH waves, and to optimize the N∥ spectrum for the future launcher design. Furthermore, an LHCD system at 5 GHz is being considered for a future upgrade of the ITER Heating and Current Drive systems, with a power capability of 20 MW coupled to the plasma using a PAM launcher. An R&D programme is being conducted at CEA/IRFM to develop a BeO vacuum window which is a safety critical component of the transmission line. In addition, a mock-up of a TE10–TE30 mode converter at 5 GHz, designed for a rectangular transmission line, has been manufactured and successfully tested on Tore Supra at low RF power.

Multi-megawatt, gigajoule plasma operation in Tore Supra

Integrating several important technological elements required for long pulse operation in magnetic fusion devices, the Tore Supra tokamak routinely addresses the physics and technology issues related to this endeavor and, as a result, contributes essential information on critical issues for ITER. During the last experimental campaign, components of the radiofrequency system including an ITER relevant launcher (passive active multijunction (PAM)) and continuous wave/3.7 GHz klystrons, have been extensively qualified, and then used to develop steady state scenarios in which the lower hybrid (LH), ion cyclotron (IC) and electron cyclotron (EC) systems have been combined in fully stationary shots (duration ∼150 s, injected power up to ∼8 MW, injected/extracted energy up to ∼1 GJ). Injection of LH power in the 5.0–6.0 MW range has extended the domain of accessible plasma parameters to higher densities and non-inductive currents. These discharges exhibit steady electron internal transport barriers (ITBs). We report here on various issues relevant to the steady state operation of future devices, ranging from operational aspects and limitations related to the achievement of long pulses in a fully actively cooled fusion device (e.g. overheating due to fast particle losses), to more fundamental plasma physics topics. The latter include a beneficial influence of IC resonance heating on the magnetohydrodynamic (MHD) stability in these discharges, which has been studied in detail. Another interesting observation is the appearance of oscillations of the central temperature with typical periods of the order of one to several seconds, caused by a nonlinear interplay between LH deposition, MHD activity and bootstrap current in the presence of an ITB.

Thermal Distributions of Multi-Junction Antenna and PAM Antenna

A multijunction antenna is under development for 4.6 GHz lower hybrid current drive system on experimental advanced superconducting tokamak (EAST). The antenna mouth is exposed to the high temperature plasma directly during tokamak discharge. The thermal distribution of the existing EAST 4.6 GHz multi-junction antenna is analyzed to evaluate the safety of the antenna under high power operation. Result shows that the maximum temperature is 628 °C at the antenna mouth. Then the result is also compared with that of a kind of passive active multijunction (PAM) antenna. The water channels in the multi-junction antenna are located on the upper and bottom of each module, while in the PAM antenna they are drilled behind the passive waveguides. The maximum temperature deposited on the port of PAM antenna is just 285.64 °C, at least two times lower than that of the multi-junction antenna.

Lower Hybrid Current Drive for DEMO: Physics Assessment and Technology Maturity

Recent experiments on lower hybrid (LH) penetration at reactor-relevant densities, together with the recent demonstration of the technological viability of the passive-active multijunction launcher on long pulses, have removed major concerns about the employment of LH waves on next-generation tokamaks, where LH could profitably drive far-off-axis plasma current, allowing current profile control and helping in sustaining burning performance. In this frame and with the aim of being prepared for the design phase of the next experimental reactors, preliminary investigations on the possibility of using LH on DEMO have been started under the supervision of the European Fusion Development Agreement. This paper reports the outcomes of these studies, addressing three main questions: Is LH useful for DEMO? If so, which setting of physics parameters makes it as effective as possible? Last, can available technology fulfill such demands?From the physics viewpoint, deposition sensitivity to launcher poloidal position, scrape-off layer parameters, and peak n‖ have been analyzed, indicating the equatorial injection of 5-GHz waves with n‖peak = 1.8 as the most favorable option. On the engineering side, specific research and development needs have been investigated on the basis of available information and sensible assumptions, showing that most of the components of the transmission line and, of highest priority, radio-frequency vacuum windows demand intense development.

Comparative modelling of lower hybrid current drive with two launcher designs in the Tore Supra tokamak

Fully non-inductive operation with lower hybrid current drive (LHCD) in the Tore Supra tokamak is achieved using either a fully active multijunction (FAM) launcher or a more recent ITER-relevant passive active multijunction (PAM) launcher, or both launchers simultaneously. While both antennas show comparable experimental efficiencies, the analysis of stability properties in long discharges suggest different current profiles. We present comparative modelling of LHCD with the two different launchers to characterize the effect of the respective antenna spectra on the driven current profile. The interpretative modelling of LHCD is carried out using a chain of codes calculating, respectively, the global discharge evolution (tokamak simulator METIS), the spectrum at the antenna mouth (LH coupling code ALOHA), the LH wave propagation (ray-tracing code C3PO), and the distribution function (3D Fokker–Planck code LUKE). Essential aspects of the fast electron dynamics in time, space and energy are obtained from hard x-ray measurements of fast electron bremsstrahlung emission using a dedicated tomographic system. LHCD simulations are validated by systematic comparisons between these experimental measurements and the reconstructed signal calculated by the code R5X2 from the LUKE electron distribution. An excellent agreement is obtained in the presence of strong Landau damping (found under low density and high-power conditions in Tore Supra) for which the ray-tracing model is valid for modelling the LH wave propagation. Two aspects of the antenna spectra are found to have a significant effect on LHCD. First, the driven current is found to be proportional to the directivity, which depends upon the respective weight of the main positive and main negative lobes and is particularly sensitive to the density in front of the antenna. Second, the position of the main negative lobe in the spectrum is different for the two launchers. As this lobe drives a counter-current, the resulting driven current profile is also different for the FAM and PAM launchers.

Experimental characterization and modelling of non-linear coupling of the lower hybrid current drive power on Tore Supra

To achieve steady-state operation on future fusion devices, in particular on ITER, the coupling of the lower hybrid wave must be optimized on a wide range of edge conditions. However, under some specific conditions, deleterious effects on the lower hybrid current drive (LHCD) coupling are sometimes observed on Tore Supra. In this way, dedicated LHCD experiments have been performed using the LHCD system of Tore Supra, composed of two different conceptual designs of launcher: the fully active multi-junction (FAM) and the new passive active multi-junction (PAM) antennas. A non-linear interaction between the electron density and the electric field has been characterized in a thin plasma layer in front of the two LHCD antennas. The resulting dependence of the power reflection coefficient (RC) with the LHCD power is not predicted by the standard linear theory of the LH wave coupling. A theoretical model is suggested to describe the non-linear wave–plasma interaction induced by the ponderomotive effect and implemented in a new full wave LHCD code, PICCOLO-2D (ponderomotive effect in a coupling code of lower hybrid wave-2D). The code self-consistently treats the wave propagation in the antenna vicinity and its interaction with the local edge plasma density. The simulation reproduces very well the occurrence of a non-linear behaviour in the coupling observed in the LHCD experiments. The important differences and trends between the FAM and the PAM antennas, especially a larger increase in RC for the FAM, are also reproduced by the PICCOLO-2D simulation. The working hypothesis of the contribution of the ponderomotive effect in the non-linear observations of LHCD coupling is therefore validated through this comprehensive modelling for the first time on the FAM and PAM antennas on Tore Supra.

Advances in Lower Hybrid Current Drive for Tokamak Long Pulse Operation: Technology and Physics

The paper gives a picture of the present status and understanding of technology and physics of Lower Hybrid Current Drive for long pulse operation in tokamaks, including the development of continuous wave (CW) high power klystrons, and its evolutions towards ITER. 3.7 GH / 700 kW CW klystrons produced in series by Thales Electron Devices are now in operation on Tore Supra. First series of eight klystrons delivered more than 4 MW to sustain non-inductive plasmas during 50 s. Moreover, a prototype of 500 kW CW klystron operating at 5 GHz developed for KSTAR by Toshiba Electron Tubes and Devices, and foreseen for ITER, is able to produce RF output powers of 300 kW / 800 s and 450 kW / 20 s on matched load. The situation on wave coupling and antennas is reported, with the latest Tore Supra results of the new CW Passive-Active Multi-junction (PAM) launcher: the antenna concept foreseen for ITER. First experiments with the PAM antenna in Tore Supra have provided extremely encouraging results in terms of power handling and coupling. Relevant ITER power density of ∼25 MW/m 2 (2.7 MW of power injected into the plasma) has been maintained over ∼80 s. In addition, LH power of 2.7 MW has been coupled at a plasma-antenna distance of 10 cm.

An ITER-relevant passive active multijunction launcher for lower hybrid current drive in JET-grade plasmas

Lower hybrid current drive (LHCD) has been demonstrated to be an important actuator for controlling the current profiles of advanced tokamak plasmas, either in fully non-inductive or in hybrid regimes. To continue such scenario development in JET, and most notably towards extrapolations for ITER, an upgrade of its LH launcher is required to deliver more power (>5 MW) and to deal with a broad array of density situations at the plasma edge more consistently. These may range from steep gradients to ELM activity; extend from as low as the cut-off density nec to large and fast density variations and give rise to strong and rapid reflections of RF power. The passive active multijunction (PAM), advocated as the LH launcher concept for ITER, is claimed to be more resilient to such conditions, run with increased efficiency near nec and even with a small vacuum gap, being also specially appropriate for long pulse operation—all conditions hampering traditional designs. In addition, some of this leverage has been confirmed experimentally at the ITER-relevant FTU with the first PAM ever deployed, and more recently with Tore Supra's PAM. The JET version addressed in this study, with the latter focusing on the physics that drives and supports the adopted technical solutions, allows the injection of up to 5.2 MW at 3.7 GHz and N||peak = 1.9. With directivities better than 70%, reflection coefficients below 2% even for ∼nec and power densities comparable to those foreseen for ITER at 5 GHz, it would validate this concept in ITER-relevant scenarios.

Manufacturing process and tests of a lower hybrid passive active multi-junction launcher for long pulse experiments on Tore-Supra

A new Passive Active Multijunction (PAM) Lower Hybrid heating and Current Drive (LHCD) launcher, has been successfully manufactured and tested on Tore-Supra (TS). The design and the fabrication of this new actively cooled launcher based on the PAM concept, as the present ITER LHCD design, is a major component of the TS CIMES project (Components for the Injection of Matter and Energy in Steady-state), and will play a key role in the TS near term program. To achieve 1000 s pulses with a power flux of 25 MW/m 2 the PAM launcher has been designed for steady state (CW) operation (active cooling) with the objective of coupling 2.7 MW of LHCD power to the plasma at 3.7 GHz with a parallel index N ∥ = 1.7 ± 0.2. The launcher has achieved its qualifications tests, i.e. low power Radio Frequency measurements, vacuum and hydraulic leak tests, and has been installed on Tore-Supra tokamak in September 2009. It is commissioning on plasma started a month later, quickly achieving its design performance of 2.7 MW on a 35 s pulse. After a technical description of the PAM, this paper presents an overview of the project phases (RF optimization, manufacturing and qualification) and concludes with the first experimental results of the PAM.

Manufacturing process and tests of a Lower Hybrid Passive–Active Multijunction launcher for long pulse experiments on Tore-Supra

A new concept of multijunction-type antenna has been developed, the Passive–Active Multijunction, which improves the cooling of the waveguides and the damping of the neutron energy (for ITER) compared to Full Active Multijunction. Due to the complexity of the structures, prototypes of the mode converters and of the Passive–Active-Multijunction launcher were fabricated and tested, in order to validate the different manufacturing processes and the manufacturer's capability to face this challenging project. This paper describes the manufacturing process, the tests of the various prototypes and the construction of the final Passive–Active-Multijunction launcher, which entered into operation in October 2009. It has been commissioned and is fully operational on the Tore-Supra tokamak, since design objectives were reached in March 2010: 2.75 MW – 78 s, power density of 25 MW/m 2 in active waveguides, steady-state apparent surface temperatures <350 °C; 10 cm long distance coupling.

3D electromagnetic analyses during plasma disruption for the Tore Supra lower hybrid passive active multi-junction launcher

A Passive Active Multijunction (PAM) Lower Hybrid heating and Current Drive (LHCD) antenna has been installed on Tore Supra in 2009. The design and the fabrication of this antenna based on the passive-active multijunction concept is a major part of the CIMES project (components for the injection of mater and energy in steady-state) on Tore Supra. The antenna made of copper and stainless steel has to withstand important electromagnetic forces due to eddy currents during plasma disruption. 3D electromagnetic analyses have been performed with the ANSYS code to calculate the eddy currents and the electromagnetic forces.

Thermal and mechanical analysis of ITER-relevant LHCD antenna elements

A 20 MW Lower Hybrid Current Drive system using an antenna based on the Passive-Active Multijunction (PAM) concept is envisaged on ITER. This paper gives an overview of the mechanical analysis, modeling and design carried out on two major elements of the antenna: the grill front face, and the RF feed-through or windows. The front face will have to withstand high heat and fast neutrons fluxes directly from the plasma. It will be actively cooled and present a beryllium coating upon ITER requirement. The RF window being a critical safety importance class component (SIC) because of its tritium confinement function, two of them will be put in series on each line to achieve a double barrier. A design of a water cooled 5 GHz CW RF “pillbox” window capable of sustaining 500 kW of transmitted power is proposed. Both studies allow to move forward, and focus on critical issues, such as manufacturing processes and R&D associated programs including tests of mock-ups.

Coupling characteristics of the ITER-relevant lower hybrid antenna in Tore Supra: experiments and modelling

A new concept of lower hybrid antenna for current drive has been proposed for ITER (Bibet et al 1995 Nucl. Fusion 35 1213–23): the passive active multijunction (PAM) antenna that relies on a periodic combination of active and passive waveguides. An actively cooled PAM antenna at 3.7 GHz has recently been installed on the tokamak Tore Supra. This paper summarizes the comprehensive experimental characterization of the coupling properties of the PAM antenna to the Tore Supra plasmas. In this paper, the electromagnetic properties of the antenna are measured at a reduced power (<1 MW) to allow a systematic comparison with linear wave coupling theory and the associated modelling based on the linear ALOHA code. In a wide range of edge electron densities at the antenna aperture (spanning a factor 20 from 0.5 × nc to 10 × nc where nc is the slow wave density cut-off, nc = 1.7 × 1017 m−3 at 3.7 GHz) and antenna phasing, the ALOHA simulations reproduce the experimental results observed on Tore Supra. In addition, reduced power reflection coefficients (<5%) are measured at a low edge density, close to nc, i.e. in the range 0.5–3 × nc. Measurement and analysis with ALOHA of the antenna–plasma scattering matrices provide explanation of the good coupling properties of the PAM antenna close to nc by highlighting the crucial role of the slow wave intercoupling between active and passive waveguides through the plasma edge. This detailed validation of the coupling modelling is an important step towards the validation of the PAM concept in view of further optimizing the electromagnetic properties of the future ITER antenna.

Benchmark of coupling codes (ALOHA, TOPLHA and GRILL3D) with ITER-relevant Lower Hybrid antenna

In order to assist the design of the future ITER Lower Hybrid launcher, coupling codes ALOHA, from CEA/IRFM, TOPLHA, from Politecnico di Torino, and GRILL3D, developed by Dr. Mikhail Irzak (A.F. Ioffe Physico-Technical Institute, St. Petersburg, Russia) and operated by ENEA Frascati, have been compared with the updated (six modules with four active waveguides per module) Passive-Active Multi-junction (PAM) Lower Hybrid antennas. Both ALOHA and GRILL3D formulate the problem in terms of rectangular waveguides modes, while TOPLHA is based on boundary-value problem with the adoption of a triangular cell-mesh to represent the relevant waveguides surfaces. Several plasma profiles, with varying edge density and density increase, have been adopted to provide a complete description of the simulated launcher in terms of reflection coefficient, computed at the beginning of each LH module, and of power spectra. Good agreement can be observed among codes for all the simulated profiles.

Images of the RF Stability of a Lower Hybrid PAM Launcher

A novel launcher design for heating and current drive of fusion plasmas with lower hybrid waves was proposed. It uses a concept promising resistance to situations expected in the ITER advanced scenarios: the passive-active multijunction (PAM). To study its RF stability against changes in plasma loading, a code was developed, requiring visualization of multidimensional computationally generated data. Images from an application to the PAM under construction for Tore Supra illustrate the importance of visualization techniques and tools.

Images of the Power Splitting Stability of a Lower Hybrid PAM Launcher

A code was created to look into the stability of the spectrum launched by a new lower hybrid antenna used for heating and current drive of fusion plasmas, when the plasma loading conditions vary. The antenna is based on the passive-active multijunction (PAM), a concept promising greater immunity to conditions envisioned in the advanced scenarios for the International Thermonuclear Experimental Reactor (ITER). The code, requiring visualization of multidimensional data, is illustrated with applications to Tore Supra's new antenna.

Images of the TE Wave Inside Tore Supra's LH PAM Launcher

A new lower hybrid launcher based on the passive active multijunction principle and allowing for the withstand of conditions envisioned in the advanced scenarios for the International Thermonuclear Experimental Reactor (large density variations, thermal loads, neutron fluxes, and mechanical stresses) has been designed for the purpose of heating and current drive of fusion plasmas in the Tore Supra tokamak. A 3-D image of the TE wave inside the full waveguide structure of one of its modules is presented.

PAM, multijunction and conventional LHCD grills: Operational experience on FTU

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.

Progress in LHCD: a tool for advanced regimes on ITER

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.