Antenna Loading Resistance Research Articles

ICRF coupling in ASDEX upgrade magnetically perturbed 3D plasmas

The RF properties of the four ion cyclotron range of frequencies (ICRF) antennas in the ASDEX Upgrade tokamak are characterized in H-mode magnetically perturbed 3D discharges. An n = 2 magnetic perturbation (MP) field is applied and rigidly rotated, which allows diagnosing the separatrix displacement and consequent coupling change. We find the antenna loading resistance to be coherently modified by the resulting non-axisymmetric plasma equilibria, thus becoming a function of the applied MP field poloidal mode spectra. We perform a detailed statistical analysis, which correlates the change in loading resistance to the fast wave R-cutoff layer movements. From it, a 1D scaling is derived that differs from previous studies evaluated in pure axisymmetric plasma conditions. This experimentally derived scaling is used to predict the average loading resistance change of the ITER ICRF antenna under applied MPs. ICRF coupling simulations using measured 1D density profiles are performed with the RAPLICASOL code, in order to investigate the predictive capabilities of numerical state of the art tools. We find that both 1D conventional scaling laws and 1D numerical simulations fail to capture the 3D physics, and can substantially overestimate the measured loading resistance change up to a factor of ∼3.

Pre-Stub Tuner for Reduction of Radio-Frequency Voltage Along Transmission Line in EAST-ICRF System

The ion cyclotron range of frequency (ICRF) heating system of the Experimental Advanced Superconducting Tokamak (EAST) is characterized by high radio-frequency (RF) power up to 12 MW and wide frequency range over 25 to 70 MHz. A high RF power transmission system composed of a liquid impedance matching device, ceramic feedthrough, decoupler, and ICRF heating antenna with four straps has been in operation for some years. In a high-power ICRF experiment, one issue that needs to be solved is the high RF voltage on the coaxial transmission line between the ICRF antenna and the impedance matching device, which is caused by low antenna loading resistance compared to the characteristic impedance of the transmission line. A stub tuner is employed to reduce the RF voltage in the EAST ICRF power transmission system. Two methods to reduce RF voltage using short-circuited and open-circuited stub tuners are introduced in detail. The optimized position and length of the stub tuner are analyzed and calculated to achieve a smaller voltage reduction ratio (VRR) on the transmission line. The test with the stub tuner to reduce the RF voltage of the transmission line is carried out, and a RF VRR of ∼0.57 is achieved. The RF voltage on the transmission line is significantly reduced, and the capability of the transmission power is obviously improved. Ohmic losses caused by the surface resistance of the conductor of the coaxial transmission line are also decreased, and the probability of breakdown within the transmission line is reduced under high RF power operation.

Investigation of the coupling properties of the ion cyclotron fast wave under applied magnetic perturbations and MHD phenomena in ASDEX Upgrade

The modulation of the ion cyclotron fast wave coupling to the plasma due to non-axisymmetric changes of the distance antenna-R-cutoff is studied. These changes can arise when magnetic perturbation (MP) fields are used, or when MHD activity is present. The application of MP fields can excite a low field side midplane plasma kink response that amplifies the vacuum perturbation field, leading to appreciable 3D plasma displacements. This effect is studied via NEMEC simulations. Rigid rotation of the MP field is found to produce a coherent antenna loading resistance modulation, suggesting an interplay between the non-axisymmetric magnetic field structure and the wave coupling properties. MHD modes are shown to introduce similar loading resistance oscillations, coherent with the mode rotation frequency. The case of a (2,1) mode is presented.

Improvement of ICRF antenna loading by local gas injection on ASDEX Upgrade

Local gas injection has recently been tested as a tool to improve coupling of ion cyclotron range of frequency (ICRF) waves to tokamak plasmas, which relies on the tunnelling of the fast wave between the antenna structure and the cut-off layer within the plasma. It is found that antenna loading resistance is substantially increased when using a local gas inlet at a deuterium gas injection rate of 5 × 1021 el s−1 or higher, and that the proximity of the gas inlet to the antenna is the key factor to optimize the improvement in coupling due to gas injection.

Radio-Frequency Heating in Straight Field Line Mirror Neutron Source

There is a need in a practical scenario for ion heating up to high temperatures in a mirror based neutron source. Such a scenario could be developed with the ion cyclotron heating. Fundamental harmonic ion cyclotron heating of deuterium and second harmonic heating for tritium are studied numerically from the point of view of antenna coupling and heating efficiency. The behavior of the antenna loading resistance and radio-frequency power shine-through the cyclotron zone with the heating frequency, plasma density and temperature and the antenna position is analyzed.

Study of the Antenna Loading Resistance of the LHD ICRF Antenna

Ion cyclotron range of frequencies (ICRF) heating is used to heat plasma in magnetically confined fusion plasma experiments. ICRF heating has been used in the Large Helical Device (LHD) and contributes to high-power steady-state experiments. Antenna loading resistance is important in ICRF heating; a high loading resistance is required for high-power injection. Many elements influence the antenna loading resistance. Here, the dependence of the loading resistance on various parameters is investigated. The loading resistance is very low at lower wave frequencies. High-power injection using such frequencies was difficult in plasma heating experiments in the LHD. The loading resistance increases with the plasma density. The distance between the antenna and the plasma boundary is closely related to the plasma edge density. It is important to keep the antenna away from the plasma and also keep the loading resistance at a certain level in steady-state operation for the various types of plasma. The effect of additional heating and magnetic field strength are also investigated. These results will contribute to the design of new ICRF antennas, the ICRF heating experiment in the LHD, and ICRF heating in future plasma devices.

Modeling electromagnetic field excitation and rf power absorption in a large helicon plasma

Wave propagation and rf power absorption are considered in a large helicon plasma source excited by a flat antenna along the magnetic field. Various single-, four- and six-turn antennas of different dimensions are examined comparatively. The simple physical model and computation code accounting for the most important features of the system are introduced. Profiles of the rf magnetic field and absorbed power and the antenna loading resistance are computed over a broad range of parameters. Computation results are interpreted considering propagation characteristics of the waves, such as the group velocity and characteristic angle, and are compared with measured data.

High-voltage test of feedthroughs for a high-power ICRF antenna

A feedthrough is one of the most important parts of an ion cyclotron range of frequencies antenna for plasma heating in nuclear fusion devices. It must transmit RF power while keeping the antenna in high vacuum, and support antenna elements against electromagnetic forces and thermal stress. The standoff voltage, a key property for high-power transmission, is experimentally studied for six different types of feedthroughs. The Princeton Plasma Physics Laboratory (PPPL), Massachusetts Institute of Technology (MIT), Japan Atomic Energy Research Institute and Oak Ridge National Laboratory types show a standoff voltage of 40-52 kV with the pulse length 1 s, which indicates the possibility of power injection 1-1.6 MW in the present 50-/spl Omega/ systems. If a 30-/spl Omega/ feedthrough designed for the International Thermonuclear Experimental Reactor antenna, has a similar standoff voltage, 3-4 MW can be injected. Here, the antenna loading resistance is assumed as 3 /spl Omega/. In particular, the PPPL and MIT types show higher standoff voltages, which is inferred to be due to the design of the base flange covering the ceramic end. Multipactoring is observed in the voltage range less than 0.9-1.6 kV as predicted, but it is not a crucial problem. Therefore, a key point in designing a high-voltage standoff feedthrough is the optimization of the base flange and the ceramic ends.

Upper limit to the monotonic increasing dependence of the plasma density on the magnetic field in helicon discharges

The line integrated electron density and the ion saturation current are measured for a wide range of the magnetic field including the lower hybrid resonance region in helicon plasmas and the results are compared with the calculated ones based on the particle and power balance. The linear dependence of the magnetic field on the plasma density is not preserved when the magnetic field is higher than its value corresponding to the lower hybrid resonance ( B LH ), deviating from the monotonic increasing dependence, as the calculated antenna loading resistance is sharply decreased across B LH . It has been also observed that oscillations in the ion saturation current sharply increases as B crosses B LH .

Low-power fast wave antenna loading as a radio-frequency sheath diagnostic

Radio-frequency (RF) sheaths induced by the RF E∥ component are a ubiquitous feature of Ion Cyclotron Range of Frequency (ICRF) heating and current drive. An important consequence of RF sheaths is the power dissipation, Psh, caused by ions accelerated in the rectified sheath potential and flowing into material boundaries, such as the antenna structure and nearby limiters. It is shown that the RF sheath-power dissipation yields an antenna loading resistance RL larger than the usual fast wave (FW) loading at very low RF power PRF (typically below 100 kW), because Psh scales as ‖Erf‖, whereas the FW coupled power scales as ‖Erf‖2. The curve RL(PRF) has a maximum at PRF=0 and rapidly decreases with power until asymptoting at the usual FW loading. The ratio RL(0)/RL(∞) is a measure of the average RF sheath voltage on the antenna and nearby limiter surfaces. It is suggested that this technique could be used to measure the RF sheath properties of different antennas or operational regimes (e.g., different phasings) before attempting high-power operation. A generalized loading model is applied to interpret recent measurements on the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. I, p. 69] for a four-strap antenna with and without a Faraday screen. The possible use of this effect as a diagnostic for the scrape-off layer density is also discussed.

Performance of toroidally wide-separation loop antennae for JT-60U ICRF experiments

Abstract The performance of two array antennae of phased 2 × 2 loops for JT-60U is presented in the initial operation for higher harmonic ion cyclotron resonant frequency (ICRF) experiments. The array antenna is designed so as to have a sharp coupled power spectrum of n  peak ≈ 3 with Δn  ≈ 2.5 when out of phase with toroidally adjacent loop currents ((π, 0) phasing) at 116 MHz by separating the loops widely by 0.44 m. The maximum coupled power up to the present is 6.4 MW with (π, 0) phasing at Bt = 4 T, which corresponds to the power density of about 6.1 MW m−2. Anomalous antenna breakdown is observed during combined neutral beam injection (NBI) and ICRF heating for the particular magnetic field strength Bt = 3.0 T at which an ion cyclotron resonance layer of 7ωcD is located near the antennae. A fairly large antenna loading resistance is obtained, i.e. 4 Ω for (π, 0) phasing at 0.13 m of the distance δSF between the separatrix and the Faraday shield, and 5 Ω for in-phase ((0, 0) phasing) at very large distances of more than 0.33 m. results show the decrease in the loading resistance at the L to H transition is about 30%, from 5 to 3.7 Ω with (0, 0) phasing at δSF = 0.32 m. The ratio of the incremental radiation loss to ICRF power decreases with increasing ICRF power and is saturated at small values less than 10% at power levels more than 4 MW. The increase in the radiation loss seems to be mainly caused by the light impurities from the first wall.

Plasma production by helicon waves

High density plasma production using m=+1 and m=-1 helicon waves is studied. Characteristics of cylindrical helicon waves including effects of a vacuum gap between the plasma and the conducting wall and of a non-uniform density profile are presented. The m=+1 and m=-1 helicon modes are separately excited by a helical antenna, and the dependences of plasma density and antenna loading resistance on RF power are different for these modes.

Analysis of low frequency fast wave current drive for ITER

A low frequency fast wave current drive scenario for ITER (f ~ 17 MHz < fcT, where fcT is the cyclotron frequency of tritium ions) has been analysed using a 1-D full wave code (TASK/W1). The current drive efficiency varies significantly with changes in the electron density owing to effects of the cavity resonance, even for the standard value of the peak of the parallel wave number N|| chosen for ITER (peak of a main lobe, N||(p)=2.3). The peak value of the current drive efficiency, according to the cavity resonance, agrees with that for the single wave mode for a large number of antennas in the toroidal direction. The antenna loading resistance as well as the current drive efficiency reach a maximum value nearly at the cavity resonance corresponding to that of the eigenmode. Therefore, it may be possible to optimize the current drive efficiency by optimizing the antenna coupling resistance to its maximum value. Frequency feedback control with a combination of instantaneous bandwidths and several preset frequencies is considered to achieve and to maintain good current drive efficiency against a possible change of the electron density. It is shown that this scheme allows good current drive efficiency (γ ~ 0.5 × 1020 A.W-1.m-2 with N||(p)=1.5), similar to that for neutral beam current drive

Coupling of fast waves in the ion cyclotron range of frequencies to H-mode plasmas in DIII-D

Measurements of low power (≃ 1 mW) antenna loading are used to study the coupling of a compact loop antenna structure to plasmas in the divertor configuration in DIII-D heated by neutral beam injection (NBI) or electron cyclotron heating (ECH). When a transition to the H-mode regime occurs during NBI, the antenna loading resistance drops by approximately a factor of two. This coupling decrease is due to a steepening of the edge density profile near the separatrix, accompanied by a reduction in edge density in the scrape-off layer. During edge localized modes, the opposite effects occur, and the antenna coupling increases transiently. The loading measurements are compared with theoretical calculations which take into account the measured density profiles as well as the conducting side-walls of the recessed antenna housing. Absolute agreement between the theoretical and the experimental results is obtained, including the correct dependence on the density, antenna position, RF frequency and antenna geometry. The theoretical interpretation of the results is discussed, together with the technological implications for future high power experiments.

Ion cyclotron heating in TMX-U

Ion cyclotron heating (ICH) is applied to TMX-U to improve the thermal barrier performance by reducing the passing ion collisionality. During its development, measurements of the antenna loading resistance, Rp, and the absorption efficiency, η, were compared with calculations with the antenna design code ANTENA over a wide range of densities and frequencies. Good agreement in Rp was obtained in the short wavelength slow wave regime but not for long wavelength fast waves because the experimental magnetic field gradients are not modelled in ANTENA. Similarly, η is much larger experimentally (40%) than in ANTENA (10%) due to the magnetic beach in TMX-U. In its application, ICH successfully decreased the passing ion collisionality tenfold but did not extend thermal barrier plugging to higher density, indicating that collisional barrier filling is not currently limiting TMX-U performance.

Propagation and absorption of ICRF waves in helical plasmas

In a study of the structure of propagation and absorption of waves in the ion cyclotron range of frequencies (ICRF) in stellarator/heliotron devices, the full Maxwell equation, formulated as a stationary boundary-value problem, is solved numerically in a straight helical configuration. A cold plasma approximation is employed to obtain the conductivity tensor. The shapes of plasma boundary, vessel wall and antenna can be chosen arbitrarily by using a finite-element method. Two-ion hybrid resonance heating by the fast wave is studied. The wave field solution shows tunnelling across the evanescent layer, formation of the cavity resonance and absorption near the hybrid resonance surface. The dependence of the loading resistance on the axial wave number shows a shift due to the helical pitch. The dependence of the loading resistance and the deposition profile on the plasma parameters is also studied. In Heliotron-E-grade plasmas, the fast wave can propagate into the central region of the plasma and the antenna loading resistance is as large as it is in tokamaks. A model of the modular torsatron ATF is studied and good coupling of the fast wave to the plasma is also predicted in this device.

Antenna Loading and Electron Heating Experiments of ICRF Wave in TNT-A Tokamak

Antenna loading resistance and electron heating effects of ICRF wave were investigated in TNT-A tokamak. Lodaing resistance increased with the mean plasma density and decreased with the input power. The effect of the distance between the plasma and antenna surface on loading resistance was studied and had good agreements with the calculated results. The increase in the soft Xray emissivity was larger in the presence of ion-ion hybrid and/or ion cyclotron resonance layer in the plasma than that in the absence of them. With the absorbed power up to two times of the ohmic power, the central electron temperature increased by 20%, the soft Xray emissivity increased by 80% and the mean plasma density decreased by 10%, while the total radiation loss increased slightly (by 15%).