Table of contents

In the editorial of the September issue of this year it was announced that a number of special issues of Nuclear Fusion will be published which will address subjects for which the integration of physics and enabling technology is essential. This November issue is dedicated to Electron Cyclotron Wave (ECW) physics and technology.

This issue is a compilation of 25 papers which form a snapshot of what is currently going on in the research area of ECW physics and technology for fusion devices. The collected articles are mostly derived from conference contributions (EC-12, SMP-2002, IAEA Lyon conference), rewritten and extended for the purpose of this special issue and submitted to our standard double-referee peer review. Several regular submissions, not related to conference contributions, are also included. This compilation is somewhat arbitrary as the editor is dependent on incoming submissions. It was decided not to delay publication of the specialized subject-related papers by commissioning introductory review articles, which would be subject to longer lead-times. Also, a number of regular papers, whose subject would have fitted perfectly in this issue, were published earlier in the year (see below).

The 25 articles are arranged in a sequence from theory, via experiments on heating and current-drive, electron heat-transport studies, application of ECW emission measurements, to ECW sources (gyrotrons), transmission-lines, microwave technology, with one article on an application outside fusion research. One article was submitted as a Letter (Cappa and Castejon) but it has been placed logically within the sequence.

I hope that this special issue will give the interested reader a state-of-the-art picture of the field of ECW physics and technology in nuclear fusion research and be of help for specialists working in the field.

The following articles, which are closely related to the subject of this special issue, were published in earlier issues this year:

A. Manini, J.-M. Moret, F. Ryter and the ASDEX Upgrade Team 2003 Signal processing techniques based on singular value decomposition applied to modulated ECH experiments Nucl. Fusion43 490--511

C. Angioni, T.P. Goodman, M.A. Henderson and O. Sauter 2003 Effects of localized electron heating and current drive on the sawtooth period Nucl. Fusion43 455--468

V. Pericoli Ridolfini et al 2003 Progress towards internal transport barriers at high plasma density sustained by pure electron heating and current drive in the FTU tokamak Nucl. Fusion43 469--478

K. Sakamoto et al 2003 Development of 170 and 110 GHz gyrotrons for fusion devices Nucl. Fusion43 729--737

C.C. Petty et al 2003 Effects of electron trapping and transport on electron cyclotron current drive on DIII--D Nucl. Fusion43 700--707

F. Leuterer, K. Kirov, G. Pereverzev, F. Ryter and D. Wagner 2003 Modulated ECRH power deposition in ASDEX Upgrade Nucl. Fusion43 744--748

R. Prater et al 2003 Discharge improvement through control of neoclassical tearing modes by localized ECCD in DIII-D Nucl. Fusion43 1128--1134

K. Nagasaki, A. Isayama, S. Ide and JT-60 team 2003 Stabilization effect of early ECCD on a neoclassical tearing mode in the JT-60U tokamak Nucl. Fusion43 L7--L10

A power balance for waves in resonant dissipative media is formulated, which generalizes well-known expressions for dielectric wave energy density, wave energy flux, and dissipated power density. The identification of the different terms with wave energy density and flux remains only phenomenological. The result is better viewed as an equation for the evolution of wave intensity. In that form, its consequences are discussed in particular in relation to anomalous dispersion. A discrimination is made between boundary and initial value problems. For boundary value problems, anomalous dispersion is shown not to lead to unphysical results. In contrast, for initial value problems the solution for the evolution of wave intensity is shown to be at fault in the case of anomalous dispersion. Further illustration is provided by consideration of wave dispersion in a medium of charged harmonic oscillators and of ordinary-mode dispersion in plasma. Both are characterized by anomalous dispersion and show marked differences in the solutions of the dispersion relation solved either for complex wave vector at real frequency, k(ω) (applicable to boundary value problems), or for complex frequency at real wave vector ω(k) (applicable to initial value problems).

The conventional ordinary and extraordinary modes in the electron cyclotron range of frequencies are not suitable for heating of and/or driving currents in spherical tori (ST) plasmas. However, electron Bernstein waves (EBWs) offer an attractive possibility for heating and current drive in this range of frequencies. In this paper, we summarize our theoretical and numerical results that describe the excitation of EBWs in ST plasmas when the extraordinary mode or the ordinary mode are coupled into the plasma from an external source. In our discussion on the conversion of the ordinary mode to EBWs (via the slow extraordinary mode) we illustrate very important physics, relevant to this conversion process, that has been ignored in previous studies. This particular physics has to do with the conversion of the slow extraordinary mode to the fast extraordinary mode that can then propagate out of the plasma and thus reduce the mode conversion to EBWs. This reduction in the mode conversion can occur even when the wave numbers are such that the ordinary mode cutoff and the slow extraordinary mode cutoff are coincident in space. Furthermore, we also consider the emission of EBWs from a thermal plasma. This emission mode converts to extraordinary and ordinary modes in the vicinity of the upper hybrid resonance. We describe the general relationship between the conversion coefficients when exciting EBWs using either the extraordinary mode or the ordinary mode, and the emission coefficients when thermally emitted EBWs convert to the extraordinary and ordinary modes.

Basic research on high-power ECRH started 20 years ago at IPP using 28 GHz pulses with 200 kW for 40 ms at the W7-A stellarator. These pilot experiments triggered a strong activity of exploration of the unique capabilities of localized heating and current drive. The achievements in physics were strongly linked with progress in source and transmission line technology. The capability and versatility of ECRH are reviewed using W7-A and W7-AS as example experiments; the latter was shut down on July 31, 2002. The milestone achievements are discussed. Standard heating scenarios such as O-mode and X-mode as well as advanced scenarios like mode-conversion heating via the O–X–B process at different harmonics were investigated and selected results are presented. First experiments with current drive by Bernstein-waves are reported. The physics of wave interaction with stellarator-specific trapped particle populations is discussed. The results from W7-A and W7-AS establish the experimental and technological bases for the 10 MW, CW ECRH system at W7-X, which aims to demonstrate the inherent steady-state capability of stellarators.

Successful heating experiments with second, third and fourth harmonic 140 GHz electron Bernstein waves (EBWs) were performed at an extremely high density of above 3 × 10²⁰ m⁻³ at Wendelstein7-AS (W7-AS). The EBWs were generated by OXB-mode conversion. At medium densities of 1 × 10²⁰ m⁻³, efficient current drive with first harmonic 70 GHz EBWs could be demonstrated in an overdense plasma for the first time.

This paper provides an overview on recent experimental results obtained in ASDEX Upgrade using electron cyclotron heating and current drive. The following topics are covered: determination of the power deposition profile, modulated power deposition, studies of the electron heat transport via power balance and heat wave analysis and a comparison with turbulent transport theory, generation of an internal transport barrier for the electron heat flux, impact of electron cyclotron resonance heating (ECRH) on particle and impurity transport, and studies related to neoclassical tearing modes and to sawteeth.

With the very high electron cyclotron (EC) wave power density achieved in the TCV tokamak, more than 20 MW m⁻³, quasilinear modelling predicts an electron cyclotron current drive (ECCD) efficiency well in excess to the experimental value, by up to a factor of 10. Experimentally, radial transport of suprathermal electrons consistent with a diffusion coefficient larger than 1.5 m² s⁻¹ has been observed. This implies that the radial transport time is of the same order as the electron deflection time, suggesting that the key to resolving the discrepancy is to include radial transport in the kinetic simulations. In this paper we show that with a diffusion coefficient in accordance with the experimental estimation, we can reproduce the observed current drive efficiency in the fully EC current driven plasmas of TCV by solving the Fokker–Planck equation. Experimentally the total wave-driven current is well-known since the current in the Ohmic transformer is set to a constant value. We study the radial profile and the velocity dependence of the radial diffusion coefficient.

A specific model is employed for steady-state electron internal transport barriers, produced by off-axis ECCD, with the electrons divided into two groups according to their energy. A small diffusion coefficient is assigned for the low-energy electrons, while at higher energies the diffusion level is chosen such as to obtain the experimental ECCD efficiency. The total current density, which is the sum of the wave-driven part and the bootstrap current, is found to be hollow, supporting the hypothesis that the reversed shear is the cause of the transport barrier.

The electron cyclotron heating (ECH) system in TCV has recently been upgraded to a total of 4.5 MW of installed power. In order to extend the density range of ECH-heated plasmas up to electron densities of 1.2 × 10²⁰ m⁻³, three 0.5 MW gyrotrons at a frequency of 118 GHz have been added to the existing 3 MW second harmonic system operating at a frequency of 82.7 GHz. The launcher consists of a mirror placed at the top of the vessel that collects the power of the three microwave sources and can be steered radially and poloidally to ensure maximum flexibility. The choice of the top launch scheme results as a compromise between heating of high-density plasmas and maximization of single-pass absorption in Ohmically heated target plasmas. In this launching configuration, the high sensitivity of single-pass absorption to launching geometry has been experimentally demonstrated. A comparison between the experimental results and the linear ray-tracing/absorption code TORAY-GA shows a good agreement for densities up to 6.0 × 10¹⁹ m⁻³.

Electron cyclotron current drive (ECCD) is an important prospective tool for tailoring the current profile in next-step devices. To fill the remaining gaps between ECCD theory and experiment, especially in the efficiency and localization of current drive, a better understanding of the physics of suprathermal electrons appears necessary. In TCV, the fast electron population is diagnosed by a multichordal, spectrometric hard x-ray camera and by a high-field side electron cyclotron emission radiometer. The main modelling tool is the quasilinear Fokker–Planck code CQL3D, which is equipped with a radial particle transport model. Systematic studies of fast electron dynamics have been performed in TCV with modulated or pulsed electron cyclotron power, followed by coherent averaging, in order to identify the roles of collisional relaxation and radial transport in the dynamics of the suprathermal population. A consistent picture is emerging from experiment and modelling, pointing to the crucial role of the radial transport of suprathermal electrons in the physics of ECCD.

The 110 GHz and the new 140 GHz gyrotron systems for electron cyclotron resonance heating (ECRH) and ECCD on TEXTOR are described and results of ECRH experiments with the 110 GHz system are reported. Central ECRH on Ohmic plasmas shows the presence of an internal electron transport barrier near q = 1. This is confirmed by modulated ECRH experiments. A central barrier is also indicated by ECRH in radiatively improved (RI) mode discharges and up to two barriers are seen with ECRH during the current ramp phase. ECRH control of sawteeth is reported for both Ohmic and RI mode target plasmas.

Experiments performed on the FTU tokamak, aimed at validation of physics-based transport models, which yield the electron temperature profile stiffness by using electron cyclotron resonance heating (ECRH), and at the implementation of ECRH-based techniques for plasma real-time control, are presented. ECRH is used to probe transport features, both in the steady state and in response to time-varied heating. The experiments clearly show stiffness in the electron temperature profile response to localized ECRH. The low gradient plasma region near the axis is characterized by low stiffness, in the sense that the temperature gradient can change with increasing heat flux, and low electron thermal diffusivity. Strong stiffness (in the sense that the temperature gradient length 1/L_T = −∇T/T does not change significantly even with significantly different heating profiles and intensity) and high diffusivity are found in the confinement region (0.15 < r/a < 0.5). Particular attention is given to the experimental investigation of the transition layer between low and high diffusivity (and low and high stiffness) regions, which is located at the EC deposition radius r_dep when powerful ECRH is applied. A transition layer, identified by heat waves launched by modulated ECH, is found also in plasmas with dominant Ohmic heating, showing that it is a local plasma feature, and not merely a consequence of a step-wise increase in the conducted heat flux at r_dep. All observations fit well with a critical temperature gradient length modelling of local electron heat transport.

The measurement of the rate of change of stored electron energy when ECRH is switched on (or off) is the basis of a technique for real-time and automatic detection of the absorption layer. Repetitive short pulses are used to increase the signal to noise ratio, with a very low duty cycle, so as not to waste ECRH power during the detection procedure.

The electron heat transport is investigated in ASDEX Upgrade using electron cyclotron heating (ECH) combining steady-state and power modulation schemes. Experiments in which the electron heat flux has been varied in the confinement region while the edge was kept constant were performed. They demonstrate that ∇ T_e and ∇ T_e/T_e can be varied by a factor of 3 and 2, respectively. They allow a detailed determination of the transport characteristics by comparing steady-state and modulation data with modelling. The analyses clearly show the existence of a threshold (∇ T_e/T_e)_crit above which transport increases. Both steady-state and modulation experiments agree with such a transport model. The experiments have been carried out at low density in the L-mode to ensure low electron–ion coupling and good conditions for studying electron heat transport. The experiments were carried out at two different values of plasma current and show that transport increases at low current, as well-known from global scaling laws for confinement time. In the pure off-axis cases the region inside the ECH deposition is just at the (∇ T_e/T_e)_crit threshold, which allows it to be measured directly from the profile of ∇ T_e/T_e deduced from the experimental T_e profile. Using this technique, it appears that the turbulence threshold agrees with that expected from the trapped electron mode driven turbulence. It has the correct absolute value and seems to have the correct radial dependence that is determined by the trapped electron fraction and by the density gradient. It almost does not vary with other plasma parameters. In contrast, the threshold calculated for electron temperature gradient modes is higher than the experimental values of ∇ T_e/T_e and this turbulence is therefore not expected to be excited under these experimental conditions.

The internal transport barrier (ITB) was found by means of heat pulse propagation (HPP) and cold pulse propagation (CPP) analysis in target sawteeth-free plasma. The target sawteeth-free plasma was created either by high-field-side off-axis ECRH (140 GHz) or by low-field-side off-axis ECRH (130 GHz) at normalized minor radius r/a_limiter ≈ ± (0.45). Outward HPP with dynamic electron heat diffusivity (at 0.2 < r/a < 0.37 and R/L_Te = R∇T_e/T_e up to 23, where T_e is the electron temperature and R is the major radius) was created by switching on the on-axis ECRH (130 or 140 GHz) imposed on the background, which was created by off-axis ECRH. Inward CPP was created by turning off the off-axis ECRH. The central T_e responds with significant ∼ 20 ms delay. Slow inward CPP with R/L_Te rising up to 17 on the cold wave front, can be described well by at r/a < 0.3 in a time interval limited by the appearance of the first sawteeth oscillation. Off-axis ECRH forms the region of the improved transport in low-shear zone with q slightly above 1. Later, q reduces due to the current redistribution during CPP (HPP). The improved transport is able to survive under R/L_Te up to 23 (10 is Ohmic value of R/L_Te). In the context of a 'critical gradient' model, the Ohmic value of R/L_Te lies above the 'critical' one because enhanced HPP (induced by sawteeth or on-axis ECRH) is observed at all tokamaks at Ohmic background. Somehow, off-axis ECRH and HPP (CPP) processes increase the 'critical' value of R/L_Te over 2.3 times. The combination of large gradients with low transport means that ITB is present during ∼ 20 ms time interval of CPP (HPP). The analysis of HPP (CPP) at both sides of plasma column simultaneously confirms the validity of the results while also showing a light asymmetry of values.

An analytical solution has been found for the simplified time-dependent transport equation in a finite cylinder. This solution represents a decomposition of the heat source in eigenmodes. Their eigenvalues represent the effectiveness of the response to temperature for each eigenmode, which are different in the stationary regime: the higher the order, the lower the effectiveness. In this case, the response to temperature appears as non-local. On the other hand, the profile resilience mainly results from two effects: the first one is that the lower order eigenmodes are more favoured than the higher order; the second one (volume effect) is that the central source (Ohmic heating) is favoured with respect to the off-axis source (electron cyclotron resonance heating, ECRH) in the contribution to the temperature profile shape. This resilience effect on the temperature profile is a basic and natural property of the diffusion equation in cylindrical geometry. The above analytical solution has been used for the determination of the heat diffusivity χ and the damping time τ_d for the ECRH modulation experiments in the Tore Supra tokamak. The results obtained by this analytical method have been compared to those obtained by two other methods: the FFT and the power balance methods. The FFT is the easiest and the fastest method compared to the others, but it is too sensitive to perturbations such as the sawteeth, which may cause inaccurate results. The analytical solution is the most robust method, because it simulates the whole temperature modulation; hence, the results are less sensitive to other perturbations. Furthermore it contains more information than the FFT method. For 0.2 ⩽ r/a ⩽ 0.7, the values of χ^ANA are very close to those of χ^PB during the ECRH phase. Generally, the values of χ^HP, which is more 'transient', are larger than those of χ^ANA and χ^PB (ECRH).

The knowledge of the electric field created by microwave power injected inside the vacuum vessel is fundamental to understand second harmonic ECRH plasma breakdown in magnetic confinement devices. The simple photon model described in this letter allows to determine the electric field assuming that a homogeneous and stationary energy density state is reached during the first nanoseconds after the wave injection. According to second harmonic breakdown theory, the measurement of the maximum prefill pressure for which breakdown is achieved allows an indirect determination of the required electric field strength. Experiments performed in HSX show that the field value obtained in this way is in very good agreement with the value calculated with the model.

The influence of large- and small-scale magnetohydrodynamic (MHD) modes on plasma transport properties has been studied. Poloidal and toroidal reconstruction of large m = 2 modes and their evolution in time have been analysed, as well as their electron temperature profiles and heat transport properties with respect to the background plasma. The confinement is improved within the magnetic island, compared to the background plasma as evidenced by a secondary temperature maximum at the centre of the island. The perpendicular heat diffusivity inside the island is estimated to be of the order of 0.01 m² s⁻¹, which is close to the neoclassical value. By cross-correlation of different electron cyclotron emission (ECE) channels, temperature fluctuations in the presence of MHD modes are investigated. For m = 2 islands, they are found to be different for the X- and the O-points. Evidence of a small mode with higher m number between the q = 1 and 2 radii is given.

The paper describes the detection of the m/n = 2/1 disruption precursor at TEXTOR. This frequently observed precursor is detected by realtime cross-correlation of two radially separated channels of the heterodyne electron cyclotron emission diagnostic. An electronic module developed to detect the precursor and its functionality is briefly described. A disruption statistic for TEXTOR is presented, from which the probability of the occurrence of the m/n = 2/1 mode is seen. A method to generate reproducible disruption precursors at TEXTOR is also presented. Experiments to avoid disruptions by inducing toroidal velocity shear by tangential neutral beam injection and mitigate disruptions by He-puffing are presented. In the experiments performed, up to 50% of the disruptions could be avoided. Furthermore, calculations of the precursor frequency and its evolution are described and compared with measurements.

This paper reviews theoretical studies on stochastic processes in gyrotrons. Three different types of stochasticity related to high-power gyrotron operation are considered. First, stochasticity in electron residual energies may develop at the resonator exit for some values of operating parameters and complicate the operation of depressed collectors. Second, stochastic radio-frequency oscillations can set in after the electron beam current is raised above certain threshold values. Third, the high-frequency field structure can exhibit spatio-temporal chaos if the cavity diameter is increased too much to minimize Ohmic losses. In high-power gyrotrons the third type of stochasticity is most important, but there are applications in which the two other types should also be taken into account and could even be utilized.

The influence of reflections on the operation of the 2 MW, CW 170 GHz coaxial cavity gyrotron oscillating in the mode is studied. In particular, frequency-independent power reflections, e.g. from plasma or some elements of the transmission line, larger than 1% and occurring in a poor phase may lead to oscillation breakdown and should be avoided. Frequency-dependent reflections from a specially designed diamond window influence mode competition, but contract the oscillation region of the operating mode noticeably only under the unrealistic assumption that all reflected power returns to the cavity. The conclusion is that the gyrotron is sufficiently robust against possible negative effects related to reflections.

The 118 GHz electron cyclotron heating and current drive (ECRH/ECCD) system under development in Cadarache, France, for use on the Tore Supra tokamak (Pain M. et al 1994 Proc. 18th SOFT (Karlsruhe) pp 481–4: Darbos C. et al 2000 Proc. 21st SOFT (Madrid) pp 605–9), is designed to launch 2.4 MW of power for up to 10 min into the plasma. At present two out of six gyrotrons are installed and available for injection of up to 800 kW. This paper concentrates on the generation and transmission of the ECRH/ECCD power for very long pulse operation. The power is injected into the plasma as Gaussian beams by an antenna which, using actively cooled mirrors inside the Tore Supra vacuum vessel, allows extensive control of both the poloidal and toroidal injection angles. The toroidal field on Tore Supra is normally in the range of 3.8–4 T, which for 118 GHz gives almost central deposition at the fundamental electron cyclotron resonance. A pair of actively cooled corrugated mirrors is installed in each matching optics unit at the output of each gyrotron allowing complete control of the polarization of the wave transmitted to the antenna, with the result that pure O-mode—or pure X-mode—power injection can be achieved for all injection angles. In tokamak experiments, a world record energy of 17.8 MJ has been injected into the plasma. New upgraded gyrotrons specified to produce 400 kW for up to 10 min will be introduced over the next 3–4 years.

An electron cyclotron resonance heating (ECRH) system has been designed for JET in the framework of the JET enhanced performance project (JET-EP) under the European fusion development agreement. Due to financial constraints it has been decided not to implement this project. Nevertheless, the design work conducted from April 2000 to January 2002 shows a number of features that can be relevant in preparation of future ECRH systems, e.g. for ITER. The ECRH system was foreseen to comprise six gyrotrons, 1 MW each, in order to deliver 5 MW into the plasma (Verhoeven A.G.A. et al 2001 The ECRH system for JET 26th Int. Conf. on Infrared and Millimeter Waves (Toulouse, 10–14 September 2001) p 83; Verhoeven A.G.A. et al 2003 The 113 GHz ECRH system for JET Proc. 12th Joint Workshop on ECE and ECRH (13–16 May 2002) ed G. Giruzzi (Aix-en-Provence: World Scientific) pp 511–16). The main aim was to enable the control of neo-classical tearing modes. The paper will concentrate on: the power-supply and modulation system, including series IGBT switches, to enable independent control of each gyrotron and an all-solid-state body power supply to stabilize the gyrotron output power and to enable fast modulations up to 10 kHz and a plug-in launcher that is steerable in both toroidal and poloidal angles and able to handle eight separate mm-wave beams. Four steerable launching mirrors were foreseen to handle two mm-wave beams each. Water cooling of all the mirrors was a particularly ITER-relevant feature.

An over-moded evacuated waveguide line was chosen for use in the transmission system for the proposed JET-enhanced performance project (JET-EP) electron cyclotron resonance heating (ECRH) system. A comparison between the quasi-optical, atmospheric waveguide and evacuated waveguide systems was performed for the project with a strong emphasis placed on the technical and financial aspects. The evacuated waveguide line was chosen as the optimal system in light of the above criteria. The system includes six lines of 63.5 mm waveguide for transmitting 6.0 MW(10 s) at 113.3 GHz from the gyrotrons to the launching antenna. The designed lines are on average 72 m in length and consist of nine mitre bends, for an estimated transmission efficiency of ∼90%. Each line is designed to include an evacuated switch leading to a calorimetric load, two dc breaks, two gate valves, one pumpout tee, a power monitor mitre bend and a double-disc CVD window near the torus. The location of waveguide support is positioned to minimize the power converted to higher-order modes from waveguide sagging and misalignment. The two gate valves and CVD window are designed to be used as tritium barriers at the torus and between the J1T and J1D buildings. The last leg of the waveguide leading to the torus has to be designed to accommodate the torus movement during disruptions and thermal cycles. All lines are also designed to be compatible with the ITER ECRH system operating at 170 GHz.

The availability of high power (∼1 MW), long pulse length (effectively cw), high frequency (>100 GHz) gyrotrons has created the opportunity for enhanced scientific results on magnetic confinement devices for fusion research worldwide. This has led to successful experiments on electron cyclotron heating, electron cyclotron current drive, non-inductive tokamak operation, tokamak energy transport, suppression of instabilities and advanced profile control leading to enhanced performance. The key development in the gyrotron community that has led to the realization of high power long pulse gyrotrons is the availability of edge cooled synthetic diamond gyrotron output windows, which have low loss and excellent thermal and mechanical properties. In addition to the emergence of reliable high power gyrotrons, ancillary equipment for efficient microwave transmission over distances of hundreds of metres, polarization control, diagnostics, and flexible launch geometry have all been developed and proved in regular service.

For the upper electron cyclotron wave launcher on the international thermonuclear experimental reactor (ITER), the use of a 'remote steering antenna' based on the imaging properties of square waveguides is planned. To characterize launchers of this type, low-power experiments on a four-side corrugated square waveguide, with a scanning mirror at the input of the waveguide, were performed in the frequency range 140–160 GHz.

It is shown that elliptical polarization, which is necessary for the electron cyclotron current drive (ECCD), can be transmitted without depolarization and that the usable steering range of the antenna is at least 20°.

Low-power measurements demonstrate that mitre bends can be integrated into the waveguide, practically without extra loss. Detailed calorimetric measurements for this set-up confirm this statement for the polarization perpendicular to the scanning plane, whereas excess loss is measured for the parallel polarization. Various modifications of mitre bends are investigated and results are discussed.

The design of a compact matched load for high-power measurements and testing of gyrotrons and transmission lines in ECRH plants for fusion research applications is currently in an advanced phase. The aim is to provide more than 95% absorption and precise calorimetric measurement of the input power in CW. This work is based on the results of tests at high power and short pulse length (140 GHz, 0.5 MW, 0.5 s) on loads installed on the ECRH plant of the FTU Tokamak in Frascati. The loads consist basically of hollow spheres of copper with the inner wall covered by plasma-sprayed lossy ceramics. Tests at higher power and longer pulses on the ASDEX-Upgrade ECRH plant showed, after a number of successful pulses, progressive damage on the absorbing layer, marked by the appearance of electrical arcs. The absorber degradation, showing specific damage patterns, due to exposure to high-power millimetre waves, has been analysed in detail and strategies are proposed, in order to improve the power-handling capabilities and the energy extraction rate. New measurements of millimetric absorption and thermal conductivity have been performed on samples of different ceramics, for choosing the best absorbing layer.

A modified expander mirror surface with a better deposition profile, numerically computed with a multi-reflection model of the sphere, is designed to avoid radiation accumulation close to the entrance port. Improved cooling channels, which in principle can exploit the increased heat transfer rate due to surface boiling, as used in high-performance cooling circuits such as plasma-facing components, will provide 1-MW CW power capability. In this paper, some technical solutions for the construction and the constraints on the allowable deformation during pulses are given.

The ongoing projects of heavy ion accelerators, like LHC at CERN and at GSI in Darmstadt, will require high-intensity ion beams, ≈ 1 emA, of heavy elements like Pb²⁷⁺. Electron cyclotron resonance ion sources (ECRIS) are good candidates for delivering such beams. Now they reach the domain of multi-Tesla magnetic fields and they can be realized almost only by superconducting windings. Correspondingly, the rf generators that can supply these ECRIS are gyrotrons in the 28–37 GHz frequency range, powerful enough to ionize and to heat up the large ECRIS plasmas. The first ECRIS operation at 28 GHz using a gyrotron, which is reported in this paper, was carried out on the superconducting device SERSE. Other ECRIS projects of this new generation are now under-way; they are shortly presented with their expected performances, together with some ECRIS prospects.