Fundamental O-mode Research Articles

Efficient ECCD non-inductive plasma current start-up, ramp-up, and sustainment for an ST fusion reactor

The elimination of the need for an Ohmic heating solenoid may be the most impactful design driver for the realization of economical compact fusion tokamak reactor systems. However, this would require fully non-inductive start-up and current ramp-up from zero plasma current and low electron temperature of sub-keV to the full plasma current of ∼10–15 MA at 20–30 keV electron temperature. To address this challenge, an efficient solenoid-free start-up and ramp-up scenario utilizing a low-field-side-launched extraordinary mode at the fundamental electron cyclotron harmonic frequency (X–I) is proposed, which has more than two orders of magnitude higher electron cyclotron current drive (ECCD) efficiency than the conventional ECCD for the sub-keV start-up regime. A time dependent model was developed to simulate the start-up scenarios. For the Spherical Tokamak Advanced Reactor (STAR) (Menard et al 2023 Next-Step Low-Aspect-Ratio Tokamak Design Studies (IAEA)), it was found that to fully non-inductively ramp-up to 15 MA, it would take about 25 MW of EC power at 170 GHz. Because of the relatively large plasma volume of STAR, radiation losses must be considered. It is important to make sure that high Z impurities are kept sufficiently low during the early current start-up phase where the temperature is sub-keV range. Since the initial current ramp up takes place at a factor of ten lower density compared to the sustained regimes, it is important to transition into a higher bootstrap fraction discharge at lower density to minimize the ECCD power requirement during the densification. For the sustainment phase an array of eight gyrotron launchers with a total of about 60 MW of fundamental O-mode was found to be sufficient to provide the required axis-peaked external current drive. High efficiencies between 19–57 kA MW−1 were found with optimal aiming, and these were resilient to small changes in aiming angles and density and temperature profiles.

ECRH experiments on Tokamaks SST-1 & Aditya-U and ECRH upgradation plan for SST-1

A 42GHz-500kW ECRH system [1-6] is used to carry out various experiments related to plasma breakdown and ECR heating on tokamaks SST-1 and Aditya-U. The system has been upgraded with new anode modulator power supply to launch two ECRH pulses to carry out breakdown and heating simultaneously. In SST-1, ECRH system is used routinely for plasma breakdown at fundamental harmonic, approximately 150kW power is launched for 70ms to 150ms duration and consistent plasma start-up is achieved in SST-1. In the recent experiments, second EC pulse is also launched at the plasma flat-top to heat the plasma, some heating signatures are seen but more experiments will be carried out to confirm the plasma heating with ECRH. In Aditya-U tokamak, simultaneous plasma breakdown and heating experiments are carried out successfully [2]. In the first pulse around 100kW power in fundamental O-mode is launched for 70ms duration for the breakdown at low-loop voltage and around 150kW ECRH power for 50ms duration is launched in second EC pulse to heat the plasma. In case of Aditya-U, plasma heating is observed clearly as soft X-ray signal increases sharply with ECRH. In AdityaU tokamak, deuterium plasma experiments have been carried out and ECRH launched at the flat-top of deuterium plasma current. In deuterium plasma also ECR heating is observed as soft X-ray signal increases with ECH power. For SST-1, ECRH system is being upgraded with another 82.6GHz system, this system would be used to carry out plasma heating and start-up at second harmonic. The 82.6GHz system is already connected with the SST-1 tokamak, the old 82.6GHz-200kW Gyrotron will be upgraded to 400kW system to carry out effective heating experiments on SST-1 at higher ECRH power. The paper discusses the recent results of ECRH experiments carried out on tokamaks SST-1 & Aditya-U and presents the upgradation plan of EC system for SST-1.

Estimation of RF power absorption and stray distribution at plasma breakdown based on the design of ITER ECH&CD equatorial launcher

Based on the latest optical design of ITER ECH&CD equatorial launcher, the power absorption at the plasma breakdown is evaluated with considering reflections by the 3-dimensional first wall structure. The beam diffusion is simulated by a bunch of spreading rays and the reflections on the first wall are analyzed for each ray. The power absorption is evaluated by the optical thickness when the rays pass across the resonant surface. The frequency of EC wave is 170 GHz and the density of concern is from 2.5 × 1015 m−3. As a result, at least 2 MW is absorbed out of 6.7 MW injection from the Top row in the case of fundamental O-mode injection even at the breakdown. In the case of 2nd harmonic X-mode injection, the total absorbing power is 200 kW at the electron density of 1018 m−3 and the electron temperature of 5 eV, which is proportional to the electron density and the electron temperature. It is found that a certain injection angle should be excluded in order to prevent intense stray RF injection up to 4 MW propagating toward specific ports within a few reflections.

Commissioning of Electron Cyclotron Resonance Heating (ECRH) system on tokamak Aditya-U

The Aditya tokamak has been upgraded to Aditya-U by changing it’s vacuum vessel from rectangular to circular to accommodate the diverter coil for shaped plasma. The 42 GHz ECRH (Electron Cyclotron Resonance Heating) system has been integrated with the tokamak. The system is capable to deliver 500 kW power for 500 ms duration. The ECRH system with modified layout has been integrated successfully on Aditya-U; the burn pattern close to tokamak port ensures good Gaussian beam. Earlier the Aditya Tokamak used to operate at lower magnetic field (Max. 1.0 T) and the ECRH assisted low-loop voltage (˜6 V) plasma start-up experiments were carried out at second harmonic. Now the upgraded Aditya is operated at higher magnetic field up to 1.5 T. Initial ECRH experiments have been carried out at fundamental harmonic at magnetic field from 1.2 T to 1.35 T. Approximately 150 kW to 250 kW power in fundamental O-mode is launched from low field side. The ECH pulse of ˜40 ms is launched at the flat-top of plasma current. The effect of ECR heating is seen as some increase in soft X-ray signal. In some small plasma shots, the EC power is launched after the shot and EC breakdown effect has been observed.

Study of off-axis current drive using EC top-launch in KSTAR

The off-axis current drive using electron cyclotron (EC) wave is typically very low due to the low density and temperature and electron trapping effect when it is launched from outside midplane. However, the heating and current drive by EC wave is being regarded as a essential element as an off-axis current drive source together with lower hybrid current drive (LHCD) for advanced tokamak operation research in KSTAR in future. Therefore, the reliable and high efficient ECCD using top launch has been studied for two different launch schemes of down-shift and up-shift resonance with various EC frequencies which will be available in the future KSTAR ECRH system. The ray tracing (GENRAY) simulation studies show that a broad ECCD profile peaked off axis is obtained with a high current drive efficiency for both schemes. It is almost twice as high as that of the outside midplane launch for the fundamental O-mode EC wave.

Turbulence measurements on the high and low magnetic field side of the DIII-D tokamak.

In this paper, we address the challenging question of measuring turbulence levels on the high magnetic field side (HFS) of tokamak plasmas. Although turbulence measurements on the HFS can provide a stringent constraint for the turbulence model validation, to date only low magnetic field side (LFS) measured turbulence has been used in validation studies. To address this issue, an eight channel Correlation Electron Cyclotron Emission (CECE) system at DIII-D was modified to probe both LFS and HFS. In contrast to the second harmonic extraordinary mode electron cyclotron resonance emission that is typically used in CECE, we show that it is possible to probe the HFS using fundamental O-mode electron cyclotron resonance emission. The required hardware modifications for the HFS measurements are presented here, and the potential issues in this measurement are discussed.

Ray Tracing Study on Top ECCD Launch in KSTAR

The current drive efficiency of electron cyclotron (EC) wave is typically low compared with other RF and neutral beam heating system in tokamak. It is known that EC current drive by outboard launch suffers from low current drive efficiency due to electron trapping. However, the heating and current drive by EC wave is being regarded as a strong candidate for DEMO reactor due to the simplicity of the launcher, none of its interaction with plasma, and no coupling issue at the plasma edge. Also, off-axis heating and current drive by EC wave plays an important role of steady state operation optimization. To enhance the current drive efficiency in DEMO-relevant operation condition having high density and high temperature, the top launch of EC wave is recently proposed in FNSF design [2]. In FNSF, a top launch makes use of a large toroidal component to the launch direction adjusting the vertical launch angle so that the rays propagate nearly parallel to the resonance layer increasing of Doppler shift with higher n||. The results shows a high dimensional efficiency for a broad ECCD profile peaked off axis. In KSTAR, the possibility of efficient off-axis ECCD using top launch is investigated using the ray tracing code, GENRAY [3] for the operating EC frequencies (105 GHz or 140 GHz, and 170 GHz). The high current drive efficiency is found by adjusting the toroidal magnetic field and the radial pivot position of the final launcher mirror for fundamental O-mode and second harmonic X-mode. A large Doppler shift is not quite sure in the typical plasma profile in KSTAR, but the simulation results show high current drive efficiency. This paper presents ray tracing results for many cases with the wave trajectories and damping of EC by scanning the launching angle for specific launcher pivot positions and toroidal magnetic field, and two equilibriums of the KSTAR.

Non-inductive initiation of closed flux surfaces by ECH/ECCD on KSTAR using an oblique fundamental O-mode injection from the low-field side

An experiment on non-inductive plasma current start-up by electron cyclotron (EC) heating and current drive (ECH/ECCD) has been carried out on KSTAR by injecting the fundamental O-mode wave from the low-field side obliquely to the toroidal magnetic field. A plasma current up to 14.5 kA is generated by 180 kW of 84 GHz microwave power and the magnetic measurement shows the formation of a large last-closed flux surface with a diameter of 0.4 m. The soft x-ray emission profile and fast CCD images also support the existence of closed flux surfaces. The current of the cross-field-passing electrons (CFPEs) is calculated according to the paper Nucl. Fusion 52 083008 in these experimental conditions, and it is shown that a CFPE current can produce the initial closed flux surfaces. The observed large increase of EC emission supports the generation of energetic electrons, like CFPEs. After the formation of the closed flux surfaces, the pressure-driven current and CFPE current do not flow in the closed flux surfaces. EC-driven current should flow in these surfaces and ramp up the plasma current. It is estimated that an EC-driven current of about one third of the total plasma current flows in the closed flux surface at the last stage.

42GHz ECRH assisted Plasma Breakdown in tokamak SST-1

In SST-1, 42GHz ECRH system has been commissioned to carry out breakdown and heating experiments at 0.75T and 1.5T operating toroidal magnetic fields. The 42GHz ECRH system consists of high power microwave source Gyrotron capable to deliver 500kW microwave power for 500ms duration, approximately 20 meter long transmission line and a mirror based launcher. The ECRH power in fundamental O-mode & second harmonic X-mode is launched from low field side (radial port) of the tokamak. At 0.75T operation, approximately 300 kW ECH power is launched in second harmonic X-mode and successful ECRH assisted breakdown is achieved at low loop_voltage ~ 3V. The ECRH power is launched around 45ms prior to loop voltage. The hydrogen pressure in tokamak is maintained ~ 1×10 -5 mbar and the pre-ionized density is ~ 4×10 12 /cc. At 1.5T operating toroidal magnetic field, the ECH power is launched in fundamental O-mode. The ECH power at fundamental harmonic is varied from 100 kW to 250 kW and successful breakdown is achieved in all ECRH shots. In fundamental harmonic there is no delay in breakdown while at second harmonic ~ 40ms delay is observed, which is normal in case of second harmonic ECRH assisted breakdown.

42-GHz 0.5-MW ECRH System for Tokamaks SST-1 and Aditya

A 42-GHz electron cyclotron resonance heating (ECRH) system will be used to carry out preionization and start-up experiments on tokamaks SST-1 and Aditya. The system would give reliable start-up in the SST-1 tokamak at 1.5-T operating toroidal magnetic field. The fundamental O-mode will be launched from the low-field side of the tokamak. The same system will be used in tokamak Aditya to carry out second-harmonic ECRH-assisted breakdown experiments at 0.75-T operation. The gyrotron capable of delivering 500-kW power will be installed such that it will deliver power to both the tokamaks without dismantling any component. An approximately 50-m-long transmission line will be used to transmit power from the gyrotron to each tokamak. The total transmission loss in the line is less than 20%; in this case, we can launch 400-kW power to carry out reliable ECRH-assisted breakdown experiments at the fundamental and second harmonics. The launcher design is different for both the tokamaks. In Aditya, due to space restriction, a simple waveguide-type launcher is used to launch ECRH power in X-mode at the second harmonic. In the SST-1 tokamak, there are two options to launch ECRH power: 1) from the radial port and 2) from the top port. From the radial port, a conventional ECRH launcher consisting of two mirrors (one focusing and one plane) would be used; however, from the top port, one mirror would be used along with a corrugated waveguide. The VME-based data acquisition and control system will be used for the 42-GHz ECRH system. The slow interlocks would be activated through software while the fast interlocks would be hardwired to remove the high voltage within 10 . This paper discusses the physics and technical aspect of the 42-GHz ECRH system and preliminary design of launchers for SST-1 and Aditya.

Studies on impact of electron cyclotron wave injection on the internal transport barriers in JT-60U weak shear plasmas

Impact of the electron cyclotron range of frequency wave (ECRF) on the internal transport barriers (ITBs) in a weak shear (WS) plasma has been investigated in JT-60U. The fundamental O-mode ECRF of 110 GHz injected obliquely (co-current drive) from the low field side is used. It is observed that the ion temperature (Ti) ITB in a WS plasma can be degraded by ECRF. It is clarified for the first time that the degradation depends increasingly on the EC power (PEC) but decreasingly on the plasma current (Ip). Moreover it is confirmed that ECRF affects the toroidal rotation (Vt) indirectly and results in the flattening of Vt(ρ) and therefore the radial electric field (Er) profiles regardless of the direction of the target Vt(ρ), peaking co or counter direction (relative to the Ip direction). Furthermore, it is recently found that Ti and Vt in the whole ITB region are affected with almost no delay from the EC onset even with off-axis EC deposition. These results indicate that EC injection unveiled a semi-global structure that characterizes Ti ITB in a WS plasma.

Optimization of incident wave polarization for ECRH in LHD

This paper reports on the results of an experimental investigation into electron cyclotron resonance heating (ECRH) using the fundamental O-mode in the large helical device (LHD). The aim of the experiment is to understand the mechanism of the selective excitation of the O-mode EC wave and, thereby, optimize the performance of the ECRH. The polarization angle and the ellipticity of the electric field of the incident high power millimetre waves were varied in the experiment using a pair of grating polarizers. The responses of the plasma stored energy and electron temperature were measured to evaluate the effects. The optimum values of the polarization angle and the ellipticity for the ECRH were identified experimentally. A simple model of power branching between the O- and X-modes is introduced in order to interpret the observed experimental results. Specific to the ECRH in the LHD configuration, the effect of magnetic shear in the plasma peripheral region is considered.

Overview of steady state tokamak plasma experiments in TRIAM-1M

An overview of steady state tokamak studies in TRIAM-1M (R0 = 0.8 m, a × b = 0.12 m × 0.18 m and B = 8 T) is presented. The current ramp-up scenario without using centre solenoid coils is reinvestigated with respect to controllability of the current ramp-up rate at the medium density region of (1–2) × 1019 m−3. The plasma is initiated by ECH (fundamental o-mode at 170 GHz with 200 kW) at B = 6.7 T, and the ramp-up rate below the technical limit of 150 kA s−1 for ITER can be achieved by keeping the LH power less than 100 kW during the current ramp-up phase. The physics understanding of the enhanced current drive (ECD) mode around the threshold power level has progressed from a viewpoint of transition probability. A transition frequency, ftrans, for the ECD transition is determined as a function of PCD. At ∼70 kW no transition occurs for an ftrans value of ∼0.017 Hz, meaning almost zero transition probability. With increasing PCD > Pth, ftrans increases up to 10 Hz, and the transition tends to occur with high probability. The record value of the discharge duration is updated to 3 h 10 min in a low and low power (<10 kW) discharge. The global particle balance in long duration discharges is investigated, and the temporal change in wall pumping rate is determined. Although the density was low, the gas supply had to be stopped at 30 min after the plasma initiation to maintain the density constant. After that the density was sustained by the recycling flux alone until the end of the discharges. In addition to the recycling problem, in the high power and high density experiments, the localized PWI affects the SSO of the tokamak plasma. The effects of enhanced influx of metal impurities (Fe, Cr, Ni, Mo) on sustainment of the high performance ECD plasma are investigated. In order to evaluate the helium bombarding effects on the plasma facing component and hydrogen recycling in the future burning plasma, microscopic damage of metals exposed to long duration helium discharges was studied. The total exposure time was 128 s. From thermal desorption experiments for the specimens the amount of retained helium was evaluated as 3.9 × 1020 He m−2 and the scale length to be ∼1 mm in the SOL.

Physics and Technological Issues for Steady-State Tokamak Operation on TRIAM-1M

Subjects on TRIAM-1M are presented from the viewpoint of plasma physics and reactor technology associated with ‘steady state operation’ (SSO). For a future fusion power plant, for burning plasma, complete steady state operation is required. Control of the density under the complicated plasma-wall interaction, non-inductive current start-up, and sustainment of high performance are key areas in the present and future investigations. The following experimental results are reviewed. First, it is shown that the co-deposition of the metallic impurity and oxygen plays an important role in the temporary change in the wall pumping rate, and a model of the co-deposition probability agreed with the observation. It was also noticed that the thermal release of the hydrogen from the plasma-facing components affects the steady state density operation in the ultra long discharge. It was found that enhanced influx of metal impurities from the hot spot affect the steady state operation of the high performance plasma. Second, helium effects on microscopic damage on metals were studied in helium/hydrogen mixture discharges. A large quantity of dislocation loops and dense fine bubbles were observed by means of TEM even for exposure only 125 seconds in duration. From TDS for the specimens, the amount of retained helium was evaluated to be 3.9 · 10 20 He/m 2 . Third, the physics understanding for the enhanced current drive (ECD) mode around the threshold power level progressed from the viewpoint of transition probability. The forward transition frequency from a non-ECD plasma state to the ECD state was precisely determined under fixed LHCD power. Thus, a statistical probability for ECD transition was determined; that is, the transition behavior around the threshold power could be described in a statistical manner. Transition frequency showed a strong power dependence. Fourth, the current ramp-up scenario without using centre solenoid coils was reinvestigated at higher density, and controllability of the current ramp-up rate was studied. The plasma was initiated by ECH (fundamental O-mode at 170 GHz with 200 kW) at B = 6.7 T, and a ramp-up rate below the technical limit of 150 kA/s for ITER could be achieved by choosing LH power. A model to describe the ramp-up is proposed.

The 110-GHz Electron Cyclotron Range of Frequency System on JT-60U: Design and Operation

The electron cyclotron range of frequency (ECRF) system was designed and operated on the JT-60U to locally heat and control plasmas. The frequency of 110 GHz was adopted to inject the fundamental O-mode from the low field side with an oblique injection angle. The system is composed of four 1 MW-level gyrotrons, four transmission lines, and two antennae. The gyrotron is featured by a collector potential depression (CPD) and a gaussian beam output through a diamond window. The CPD enables JAERI to drive the gyrotron under the condition of the main DC voltage of 60 kV without a thyristor regulation. The gaussian mode from the gyrotron is effectively transformed to HE11 mode in the 31.75 mm diameter corrugated waveguide. About 75% of the output power of the gyrotrons can be injected into plasmas through the waveguides about 60 m in length. There are two antennae to control the deposition position of the EC wave during a plasma discharge. One is connected with three RF lines to steer the EC beams in the poloidal direction. The other is to control the EC beam in the toroidal and poloidal directions by two steerable mirrors.On the operation in 2000, the power of 1.5 to 1.6 MW for 3 s was successfully injected into plasmas using three gyrotrons. Local profile control was demonstrated by using the antennae. This capability was devoted to improve the plasma performance such as high Te production more than 15 keV and suppression of the MHD activities. In 2001, the fourth gyrotron, whose structure was improved for long pulse operation, has been installed for a total injection power of ~3 MW.

Magnetohydrodynamic Stability of Improved Confinement Plasmas in JT-60U

Progress in the understanding of magnetohydrodynamic (MHD) stability is summarized on JT-60U tokamak discharges with improved confinement such as the (hot-ion) H-mode, high-βp mode, high-βp H-mode, and reversed shear discharges. Transport barriers, which are essential for the improved confinement, play key roles in the local and global MHD stability owing to the local large pressure gradient and the related bootstrap current. Disruptive β limits of these discharges are consistent with theoretical ideal kink-ballooning stability limits with low toroidal mode numbers n. Achievable β limit is improved by broadening of the pressure profile with high plasma internal inductance, plasma shaping, and wall stabilization. Edge localized modes (ELMs) and barrier localized modes (BLMs), which are associated with edge and internal transport barriers, respectively, are analyzed carefully. Resistive interchange modes with n ≤ 3 are excited in the negative shear region in reversed shear discharges with the internal transport barrier and lead to major collapse occasionally through nonlinear coupling with a tearing mode in the positive shear region. MHD characteristics of low m/n (m: poloidal mode number) tearing modes, which are attributed to the neoclassical tearing mode, are investigated. Stabilization of tearing modes and control of sawtooth activity are demonstrated using the fundamental O-mode electron cyclotron wave injection. Resistive wall modes associated with current-driven and pressure-driven low n external kink modes are identified.

Noninductive Current Drive and Steady-State Operation in JT-60U

Studies on noninductive current drive (CD) and development of an integrated steady-state high performance operation in JT-60 are reviewed. Experiments on lower hybrid current drive (LHCD) in JT-60 have shown a large noninductive current up to 3.6 MA, high current drive efficiency of 3.5 × 1019 m-2A/W, and a flexible current profile control. Basic studies on LH waves, such as an effect of accessibility condition, fast electron behaviors, and so on, in JT-60 have contributed to understanding LHCD physics. Significant progress in neutral beam current drive (NBCD) has been made in JT-60 by testing the performance of negative ion-based (N) neutral beam injection (NBI) (N-NBI). The CD efficiency of ~1.5 × 1019 m-2A/W and negative ion-based neutral beam (N-NB) driven current of ~1 MA have been demonstrated in N-NBCD. Strongly localized noninductive driven current by electron cyclotron current drive (ECCD) was identified with a fundamental O-mode scheme from a low field side injection. ECCD in JT-60 has shown CD efficiency of 0.5 × 1019 m-2A/W and EC-driven current of 0.2 MA. Modification of local current profile was demonstrated and was used for suppression of neoclassical tearing mode. Based on these developments, two integrated steady-state operation scenarios were developed in JT-60, which are reversed magnetic shear (R/S) plasmas and high βp ELMy H-mode. In these operation regimes, discharges have been sustained near the steady-state current profile under full noninductive current drive (High βp; HHy2 ~ 1.4 and βN ~ 2.5 with N-NB, R/S; HHy2 ~ 2.2 and βN ~ 2 with fBS ~ 80%). High performance plasmas with a high nDoτETio and at high normalized density were also produced under fully noninductive condition in high βp ELMy H-mode and R/S mode.

Electron cyclotron emission refraction effects during edge-localized modes

During `type III' edge-localized modes (ELMs) on Alcator C-Mod, electroncyclotron emission (ECE) diagnostics show brief signal drops of secondharmonic X-mode and signal increases of fundamental harmonic O-mode. Theseare explained in terms of refraction effects and are found to be useful toinfer the associated ELM geometrical dimensions. A new ray tracing code,which can accommodate poloidal variations, has been developed for thisinvestigation. The ELMs are modelled satisfactorily as a density loss froma poloidally elongated region.

Numerical analysis of electron cyclotron current drive with suppressedDoppler broadening

An electron cyclotron current drive by the fundamental O-mode is numericallyanalysed in the off-axis region of a reactor-grade tokamak. By scanning thelaunching direction in both the toroidal and poloidal directions, the optimumdirection of beam injection is obtained to drive a current with a maximumvalue of I/ρw at the desired magnetic surface, where I is thetotal driven current and ρw is the radial width of the drivencurrent. When the current is driven at the position where a ray trajectorybecomes tangent to the magnetic surface (referred to as the `tangentialresonance'), Doppler broadening of the current profile is significantlyreduced and I/ρw reaches a maximum value. In the case oftangential resonance, the driven current width is mainly determined by theeffects of the beam divergence and finite spatial width of the beam at thelaunching position. The width of the driven current is kept in a range of2-5% of the minor radius under the assumption of a beam divergence witha full angle of 2°. The dependence of the driven current on both thelocation of beam injection and the wave frequency is also examined.

Electron-Cyclotron Maser Emission and Propagation in Solar Microwave Spike Bursts

AbstractAn improved model of the generation and propagation of cyclotron-maser radiation in flaring loops is discussed, which incorporates competition between driving of the maser instability and maser-induced relaxation of the unstable plasma. This model enables previous large discrepancies between the build-up, relaxation, and observed timescales to be resolved for solar microwave spike bursts. Also, it implies that emission via fundamental o-mode and second-harmonic x-mode instabilities can compete more effectively against fundamental x-mode emission than has previously been thought. Propagation of the radiation to the observer is discussed both theoretically and with reference to ray tracing calculations and it is shown that the observed levels of MHD waves in the corona make it significantly easier for the radiation to escape than in the unperturbed case. In the absence of nonlinear processes or mode conversion we argue that the escaping radiation is generated by either fundamental o-mode or second-harmonic x-mode instabilities.