ICRF Power Research Articles

The effect of ICRF antenna phasing on metal impurities in TFTR

ICRF power levels of up to 2.8 MW were achieved during the 1988 experimental run on TFTR. Metal impurity concentrations (Ti/el, Cr, Fe, Ni) and Zeff were monitored during ICRF heating by X-ray pulse height analysis and UV spectroscopy. Antenna phasing was the key variable affecting ICRF performance. No increase in metallic impurities was observed for Pr.f.<or approximately=2.8 MW with the antenna straps phased 0- pi , while a measurable increase in titanium (Faraday screen material) was observed for Pr.f.>or approximately=1.0 MW with 0-0 phasing.

Role of the antenna screen angle during ICRF heating in JET

Two comparative heating experiments were carried out to test the role of the antenna screen angle during high power ICRF heating in JET. In the first experiment the magnetic field was practically aligned with the screen elements, whereas in the second experiment the toroidal field was reversed and the angle between the elements and the field was 25°. There were clear differences in the impurity influx, the heating efficiency and the coupling resistance between discharges with normal field and discharges with reversed field. The results indicate the impact of the Faraday screen design on the levels of RF enhanced metal influxes originating at the screen. This should be of particular relevance for the ICRF antennas in a reactor where, in contrast to small devices, most of the interaction between the RF fields and the plasma edge is expected to take place around the antenna structures because of the highly localized RF field patterns. A numerical code based on three-dimensional single-pass and full wave calculations of antenna-plasma coupling, including an arbitrary angle between the screen elements and the magnetic field, can qualitatively reproduce the observed difference in the coupling resistance. The experimental results are discussed with regard to possible mechanisms that can account for the observed differences in the two cases studied.

Invited paper: ICRF/Edge physics research on textor

Extensive investigations of ICRF-induced effects on the edge plasma and on plasma-wall interaction were conducted on TEXTOR under different wall- and limiter as well as plasma- and heating conditions. Several strong effects of ICRF on the edge parameters were observed on TEXTOR, such as density rise, instantaneous electron heating, modification of SOL profiles, influx of light and/or heavy impurities, increased heat flux to the limiters, and production of energetic ions in the SOL. The fast response time of some of the changes and the observation of a maximum in the SOL profile of electron temperature, heat flux and metal sputtering clearly demonstrated that RF power is directly absorbed in the SOL. Estimates of this power amount to several percent of the total RF power launched into the plasma. Plasma-wall interaction during ICRF was substantially reduced by an appropriate choice of the wall conditioning procedures (wall carbonization with liner at 400° C or, above all, boronization). As a result record low values of the radiated power fraction were achieved during ICRF and long pulse, high power, low impurity operation was possible. Further improvement was obtained by ICRF antenna phasing. When ICRF power is coupled to the plasma, several effects on the core and edge plasma influence the operation of the toroidal pump limiter ALT-II. Experimental and theoretical studies were performed to elucidate the mechanisms responsible for the ICRF-induced effects, including the propagation of plasma waves in the edge plasma and nonlinear phenomena such as parametric decay, important changes in the DC current between the antenna structure and the liner due to the sheath effect at the antennas, and the generation of waves at harmonics of the RF generator frequency. Radial profiles of the DC radial and poloidal electric fields as well as a localized RF electric field structure were measured in the SOL using a fast scanning probe.

Effect of RF heating on DC electric fields in the SOL of TEXTOR

Radial profiles of the DC radial and poloidal fields in the scrape-off layer of TEXTOR in ohmic and ICRF heated discharges have been obtained by means of a fast scanning double probe. It is found that ICRF heating significantly changes both field components. Poloidal fields as high as 10 V/cm have been observed at 1 MW of ICRF power. The impact of these DC electric fields on the SOL will be discussed.

Capacitive probe measurements during RF heating on textor

Measurements with a capacitive probe, capable of withstanding the high heat loads during auxiliary heating, have been performed on an ICRF heated tokamak. The amplitude of the signal at the generator frequency measured by the capacitive probe corresponds to values of a few hundreds volts peak to peak at 1 MW of ICRF power, thereby confirming earlier measurements, at lower power on the URAGAN 3 [1,2,3] torsatron and on the RF test facility RFTF [4]. A theoretical model of the capacitive probe shows that these potential oscillations are probably related to ion Bernstein waves.

ICRF-edge plasma interaction studies at MIT

The Alcator C-Mod experiment will study the high power ICRF heating of elongated, diverted plasmas at high density in a compact, high field tokamak. ICRF heating will be carried out in the minority heating regime by fast wave excitation in a deuterium plasma with a 3He or H minority. Because of the compact size of the tokamak, high loading will be required to couple the power at acceptable voltages and there will be high power density at the antenna surface. High edge densities and steep edge gradients are expected, making the ICRF-edge plasma interactions a key issue. Several studies have been undertaken to better understand these effects. Measurements have been made of the short wavelength perturbations to the RF fields caused by various Faraday shield geometries. Magnetic field probes show a perturbation which is largest near the ends and between the shield elements where the RF magnetic field is mostly radial. Electric field probes show a perturbation which is larger in the center of the elements and close to each element. A test antenna is being installed in the Versator II tokamak to study ICRF-edge plasma effects. The RF will be excited in the IBW polarization with κ ‖ chosen for interaction with edge electrons. The properties of different Faraday shield coatings, including titanium carbide and boron nitride, will be explored. It will also be possible to bias the protection limiters of this antenna in order to study the effect of biasing on the plasma density in the vicinity of the Faraday shield and resultant impurity generation.

Power absorption and confinement studies of ICRF-heated plasma in JIPP T-IIU Tokamak

The energy confinement characteristics of ICRF-heated Tokamak plasmas are studied at high input power density approximately 2 MW m-3 volume averaged, on the JIPP T-IIU device (R=0.91 m, a=0.23 m). High electron and ion temperatures (Te approximately 2.5 keV, Ti approximately 2.0 keV, at each maximum) have been achieved by the operation at a plasma current IP of 280 kA, plasma line-averaged electron density ne of 7*1013 cm-3 and input power of 2 MW, with a suppression of total radiation loss (30-40% of the total input power) by a carbon coating on the vacuum vessel. The fraction of ICRF power absorbed by the plasma, alpha , is determined experimentally from the decay of the stored plasma energy just after the ICRF pulse is terminated. The value of alpha increases slightly with increasing electron density and decreases from 90 to 70% as the ICRF power is increased from 1 to 2 MW m-3 volume averaged. The global energy confinement time tau epsilon , defined by WP/(POH+ alpha Pr.f), decreases by a factor of 2 approximately 3 from that in Ohmic plasmas as the heating power increases up to 2 MW. It is found that the energy confinement time has a strong line-averaged electron density dependence as tau E varies as ne0.6, which is obtained by the use of the measured absorbed power, while the Kaye-Goldston scaling predicts tau E varies as ne0.26.

High power ICRF heating experiments on the JIPP T-IIU tokamak

In the JIPP T-IIU tokamak, a high power ICRF heating experiment has been conducted, up to an extremely high power density (∼2 MW·m−3), with a total RF power of PRF = 2 MW. Great attention has initially been paid to the problem of impurities, and it has been found that (a) the adoption of low Z materials for the limiter, (b) in situ carbon coating (i.e. carbonization) and (c) adequate gas puffing synchronized to the RF pulse are very effective in suppressing radiation loss. With these methods, a remarkable reduction in metal impurities (especially in iron impurity) has been achieved; the total radiation loss has been reduced to less than 30-40% of the input power. In these reduced radiation loss plasmas, the characteristics of ICRF heated plasmas have been studied intensively. With an increase in the ICRF heating power, a deterioration of the energy confinement time has been observed, indicating quantitative agreement with the Kaye-Goldston L-mode scaling. It is shown that the so-called profile consistency, which is the leading feature in neutral beam heated plasmas, also holds in ICRF heated plasma. It has been observed that the electron temperature profile only responds to the safety factor q(a) and does not change when the deposition profile is controlled by tailoring the k1 spectrum.

Stability of plasmas sustained by ion cyclotron wave excitation in the central cell of the Tara tandem mirror

The stability of plasmas produced by radio-frequency heating in the ion cyclotron frequency range (ICRF) has been studied in the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research 1986, Proceedings of the 11th International Conference, Kyoto (IAEA, Vienna, 1987), Vol. II, p. 251]. Ion cyclotron wave excitation by a slot antenna provided stability against macroscopic plasma motions in an axisymmetric configuration. The maintenance of macroscopic stability depended on the ICRF power, gas fueling rate, ion cyclotron resonance location, and ω/ωci at the antenna location. The ICRF ponderomotive force model is consistent with many of the observed stability features and predicts that the E+ component of the ion cyclotron wave was responsible for the stabilization. The Alfvén ion cyclotron microinstability was observed when the plasma β⊥ and anisotropy were sufficiently high. Magnetic probe measurements of the unstable mode identified it as an ion cyclotron wave and the instability threshold was within a factor of 2 of the theoretical value.

Experimental evidence for sheath effects at the ICRF antenna and ensuing changes in the plasma boundary during ICRF on textor

Measurements of a generated dc current between a Faraday-shielded ICRF antenna and the wall (liner) were performed on the ICRF-heated TEXTOR plasma. It is observed that the dc current, corresponding to electrons drawn by the antenna, increases by a factor of 8, reaching a value of 80 A for 200 kW ICRF power applied to the antenna. It is shown that this increased dc current is mainly due to the plasma sheath effect at the ICRF antenna. The resulting changes in the plasma edge are very similar to those observed during negative biasing of the ALT-I [1] limiter on TEXTOR. The most important consequences of the sheath effect are the occurrence of a density bump in the scrape-off layer (SOL) due to particle transport changes, and the generation of waves at harmonics of the RF generator frequency.

The effect of ICRF on the Alcator C scrape-off layer plasma

This paper presents a characterization of the Alcator C scrape-off layer (SOL) plasma during ICRF second harmonic hydrogen fast wave heating experiments. The SOL plasma parameters were measured using a multifunctional probe, Janus, which is capable of simultaneously measuring the ion and electron parameters both parallel and antiparallel with respect to the toroidal magnetic field. The probe data indicate, at low value of injected RF power, that there is direct edge heating and density increases at radii greater than that of the antenna Faraday shield. Increasing the injected RF power spreads both the temperature and density increases throughout the edge region, flattening the radial profiles. Varying the position of the resonance layer in the main plasma does not significantly change the effect of ICRF on the SOL parameters. Given this single spatial point characterization of the SOL, a crude estimate of power flow into and through the edge plasma indicate that ≈ 20% of the ICRF power launched from the antenna is absorbed directly in the SOL plasma. Additional observation of the impurity source rates confirms the conclusions of an earlier paper, which attributed increasing central densities of high-Z impurities to the increase in physical sputtering rate at both the ICRF antenna's Faraday shield.

Boronization in textor

The liner and limiters of TEXTOR have been coated in situ with a boron containing carbon film using a RG discharge in a throughflow of 0.8 He + 0.1 B 2H 6 +0.1 CH 4. The average film thickness was 30–50 nm, the ratio of boron and carbon in the layer was about 1:1 according to Auger Electron Spectroscopy. Subsequent tokamak discharges are characterized by a small fraction of radiated power (< 0.3) even during high power ICRF heating (2.6 MW, 1.6 s). A concomitant strong increase of the convective power loading of the limiters is observed. Values of Z eff lower than 1.2 are derived from conductivity measurements. The most prominent change in the impurity concentration compared to good conditions in a carbonized surrounding is measured for oxygen. The value OVI/ n ̄ e of the OVI intensity normalized to the averaged plasma density n e decreases by more than a factor of four. The decrease in the oxygen content manifests itself also as a reduction of the CO and CO 2 partial pressures measured during and after the discharge with a sniffer probe. The carbon levels are reduced by a factor of about two as measured by the normalized intensity CII/ n e of the CII line and via the ratio of the C fluxes and deuterium fluxes measured at the limiter (CI/D α). The wall shows a pronounced sorption of hydrogen from the plasma, easing the density control and the establishment of low recycling conditions. The beneficial conditions did not show a significant deterioration during more than 200 discharges, including numerous shots at ICRH power levels > 2 MW.

Plasma production and heating in a tandem mirror central cell by radio-frequency waves in the ion cyclotron frequency range

Plasma production and heating in the central cell of the Tara tandem mirror [Nucl. Fusion 22, 549 (1982); Plasma Physics and Controlled Nuclear Fusion Research, 1986, Proceedings of the 11th International Conference, Kyoto, Japan (IAEA, Vienna, 1987), Vol. 2, p. 251] have been studied. Using radio-frequency excitation by a slot antenna in the ion cyclotron frequency range (ICRF), plasmas with a peak β⊥ of 3%, density of 4×1012 cm−3, ion temperature of 800 eV, and electron temperature of 75–100 eV were routinely produced. The plasma radius decreased with increasing ICRF power, causing reduced ICRF coupling and saturation of the plasma beta. About 70% of the applied ICRF power can be accounted for in direct heating of both ions and electrons. Wave field measurements have identified the applied ICRF to be the slow, ion cyclotron wave. In operation without end plugging, the plasma parameters were limited by poor axial confinement and the requirements for maintenance of magnetohydrodynamic stability and microstability.

Transport and stability studies on TFTR

During the 1987 run, TFTR reached record values of QDD, neutron source strength, and Ti(0). Good confinement together with intense auxiliary heating has resulted in a plasma pressure greater than 3*105 Pascals on axis, which is at the ballooning stability boundary. At the same time improved diagnostics, especially ion temperature profile measurements, have led to increased understanding of tokamak confinement physics. Ion temperature profiles are much more peaked than previously thought, implying that ion thermal diffusivity, even in high ion temperature supershot plasmas, is greater than electron thermal diffusivity. Based on studies of the effect of beam orientation on plasma performance, one of the four neutral beamlines has been re-oriented from injecting co-parallel to counter parallel, which will increase the available balanced neutral injection power from 14 MW to 27 MW. With this increase in balanced beam power, and the addition of 7 MW of ICRF power it is planned to increase the present equivalant QDT of 0.25 to close to break-even conditions in the coming run.

HCN laser scattering on the JIPP T-IIU tokamak

A 4-channel HCN laser scattering system has been developed and applied to the JIPP T-IIU tokamak. Main aim of this system is to study microturbulence excited in the plasma, especially in the high power ICRF heating experiment. The observed density fluctuations are in a frequency range of the electron diamagnetic drift wave and have broad frequency spectra. The main part of the wavenumber is found to satisfy the condition of\(k_ \bot \rho _i \lesssim 1\). In the heating experiments, deterioration of the energy confinement time is observed, and the density fluctuation level increases with the increase of the heating power.

Coupler impedance calculations for ICRF heating of large tokamaks

A simple two-dimensional model for evaluating the efficiency of ICRF coupling to a slab plasma is developed. The model takes advantage of the very low plasma surface impedance associated with the propagating magnetosonic wave to obtain lowest order analytical expressions for the surface RF magnetic field. The calculations are used to compare the loading characteristics of a simple loop antenna with a cavity resonator for high power ICRF heating.

Plasma heating in JET

Heating the JET plasma well above temperatures reached in the ohmic phase is the aim of the two additional heating systems planned for JET: ion cyclotron resonance heating (ICRF) and neutral beam injection (NBI). Operations with the latter started in February 1986, initially with hydrogen injection, up to a power level of 7 MW. ICRF power has been delivered to the plasma by three antennae and has reached power levels of 6 MW for 2 s. In most experiments, the frequency of the waves is adjusted to position the minority ion resonance layer close to the centre of the discharge, resulting in a centrally peaked power deposition profile. Results have also shown that the increase of the volume averaged electron temperature was much less dependent on the radial position of the ion resonance layer than were the central temperatures of both ions and electrons which can both approach 5 keV. In cases of 'on axis' heating, large increases in the sawtooth activity is observed with sawtooth periods exceeding 0.3 s. Initial results with NBI have shown a decrease of the global energy confinement time with additional power similar to the one observed with ICRF heating.

High power ICRF heating on the divertor tokamak ASDEX

First ICRF experiments on ASDEX have been performed at 67 MHz, corresponding to 2 Omega CH-heating of a hydrogen plasma at B0=2.2 T. Despite divertor operation ICRH is accompanied by a significant increase of impurity production which can drastically be reduced by means of wall carbonisation. RF power up to 2.3 MW is routinely coupled to the plasma for pulse lengths of up to 1 sec. The RF heating is found to depend strongly on plasma preheating. In combination with neutral beam injection the ICRF heating efficiency is even higher than the one of NI. Confinement degrades with ICRH to values in between NI-L-type and OH confinement.

Slow-wave antenna coupling to ion Bernstein waves for plasma heating in ICRF

The coupling of ICRF power from a slow-wave antenna to a plasma with finite temperature is examined. A heuristic model, allowing explicit representations of ion Bernstein waves, fast waves and slow waves, is used to clarify how the antenna power is partitioned into the various wave energy fluxes. This model is complemented quantitatively by a more elaborate and realistic computer model. It is shown that such antennas can be highly efficient in transferring most of the antenna power directly to ion Bernstein waves, with only a very small fraction going into fast waves. The potentiality of this coupling scheme for plasma heating in ICRF is briefly discussed.

ICRF heating of currentless plasma in Heliotron E

In the Heliotron E device, a non-axisymmetric helical system, ICRF heating experiments were carried out for the first time, using fast-mode and slow-mode waves. In the fast-wave heating experiment, ICRF power of up to 550 kW was emitted during 15 ms by four antenna loops. Effective heating of a current-less ECRH-produced target plasma was observed over a wide density range. The plasma loading resistance of an antenna loop reached about 5 Ω. This is a value comparable with that of tokamak experiments. The increments of ion and electron temperatures by fast-wave heating were about 200–230 eV at an electron density of about 3 × 1019m−3. Minority heating and pure second-harmonic heating have almost the same efficiency ((1–2) × 1019eV·m−3·kW−1) during the short RF pulse used (t ≤ 15 ms). The energy transfer rate from the waves to ions and electrons could be explained by mode conversion. The signals of toroidal eigen-modes were experimentally observed and radial mode numbers could be determined using a simple model. In the slow-wave heating experiment, the upper density limit of effective heating appeared to be in qualitative agreement with wave theory.