Lower Hybrid Wave Coupling Research Articles

Effect of gas injection during LH wave coupling at ITER-relevant plasma–wall distances in JET

Good coupling of lower hybrid (LH) waves has been demonstrated in different H-mode scenarios in JET, at high triangularity (δ ∼ 0.4) and at large distance between the last closed flux surface and the LH launcher (up to 15 cm). Local gas injection of D2 in the region magnetically connected to the LH launcher is used for increasing the local density in the scrape-off layer (SOL). Reciprocating Langmuir probe measurements magnetically connected to the LH launcher indicate that the electron density profile flattens in the far SOL during gas injection and LH power application. Some degradation in normalized H-mode confinement, as given by the H98(y,2)-factor, could be observed at high gas injection rates in these scenarios, but this was rather due to total gas injection and not specifically to the local gas puffing used for LH coupling. Furthermore, experiments carried out in L-mode plasmas in order to evaluate the effect on the LH current drive efficiency, when using local gas injection to improve the coupling, indicate only a small degradation (ΔILH/ILH ∼ 15%). This effect is largely compensated by the improvement in coupling and thus increase in coupled power when using gas puffing.

Long distance coupling of lower hybrid waves in JET plasmas with edge and core transport barriers

Efficient coupling of lower hybrid (LH) waves in conditions close to those foreseen in ITER has been obtained in advanced scenario plasmas in the JET tokamak. Up to 3 MW of lower hybrid current drive (LHCD) power has been coupled at a distance between the separatrix and the launcher of 11 cm, in the presence of edge localized mode activity. The key to the improved LH wave coupling is local control of the Scrape-Off-Layer (SOL) density through gas injection in the region magnetically connected to the launcher. This increases the electron density in front of the launcher so as to improve the coupling of the LH waves, i.e. reduce the reflected power in the launcher. The average power reflection coefficient was 5.7% with gas injection, at 11 cm distance between the separatrix and the launcher. A change in the gas injection design has made the gas puffing more efficient, making the use of D2 injection possible as an alternative to CD4. At similar injected electrons/s rates, D2 gives higher electron density in the SOL than CD4, resulting in better LH coupling with D2. The possibility of using D2 instead of the earlier used CD4 is an encouraging result in view of ITER operation, as CD4 may not be compatible in ITER due to the problem of tritium retention in deposited carbon layers.

Electromagnetic particle-in-cell simulations of a lower hybrid grill

The coupling of lower hybrid waves to the plasma is a crucial issue for efficient current drive in tokamaks. In this work, the coupling problem is attacked with a new tool in this context: an electromagnetic particle-in-cell (PIC) code, XOOPIC (Verboncoeur J P et al 1995 Comp. Phys. Comm. 87 199). A model for a grill with 32 waveguides is constructed using perfectly conducting walls. The wave propagation in the waveguides and the coupling to plasma is followed. The wave–plasma interaction is studied and the evolution of the launched spectrum is resolved in the near-field of the grill. The reflection coefficients in the individual waveguides are determined.

Lower hybrid wave coupling and propagation in the presence of nonthermal edge electrons

In order to improve our understanding of lower hybrid current drive experiments in high power regimes, where fast electrons (up to few keV) are generated in front of the lower hybrid (LH) grill, an analysis of LH wave coupling and propagation is carried out using a formulation of the dielectric tensor that incorporates the fast electron population (Thot≲5 keV). It is found that both the real and imaginary parts of the dielectric tensor are modified for waves with |N∥|≳10, which can interact with the fast electrons through Landau damping. Consequently high N∥ waves can be absorbed within less than 1 cm, or even cut-off at the grill–plasma interface. Antenna–plasma coupling code results show that the power available to accelerate electrons in front of the grill (launched power with |N∥|>10) decreases in the presence of fast electrons. Also, despite strong modifications of the dielectric tensor, and of the power transfer to high N∥ waves, the presence of fast electrons (Thot≲5 keV) leads to only moderate changes of antenna reflectivity and directivity.

Nonlinear coupling of lower hybrid waves to the kinetic low‐frequency plasma response in the auroral ionosphere

A hybrid kinetic‐fluid model is developed which is relevant to lower hybrid spikelets observed in the topside auroral ionosphere [Vago et al., 1992; Eriksson et al., 1994]. In contrast to previous fluid models [Shapiro et al., 1995; Tam and Chang, 1995; Seyler, 1994; Shapiro et al., 1993] our linear low frequency plasma response is magnetized and kinetic. Fluid theory is used to incorporate the nonlinear wave coupling. Performing a linear stability analysis, we calculate the growth rate for the modulational instability, driven by a lower hybrid wave pump. We find that both the magnetic and kinetic effects inhibit the modulational instability.

Ion Bernstein wave antenna loading measurements on the DIII-D tokamak

Antenna loading measurements carried out during high power ion Bernstein wave (IBW) heating experiments on the DIII-D tokamak indicate that efficient, direct coupling to the IBW at ω ≲ 2ωci as predicted by linear coupling theory did not occur. Whereas strong peaking in the loading resistance near cyclotron harmonics is predicted for high edge densities (ω < ωLH|edge), the observed loading resistance is nearly independent of the toroidal magnetic field. The loading anomaly can be explained by invoking the ponderomotive force which decreases the edge density immediately in front of the antenna, thus allowing coupling to the cold plasma lower hybrid wave (LHW). A linear LHW coupling code including the modified density profile due to the ponderomotive force reproduces the measured dependence of antenna loading on toroidal magnetic field, edge density, antenna frequency and antenna phasing. Further evidence for the ponderomotive force is obtained from reactive loading measurements which indicate that the plasma is pushed away from the antenna as the radiofrequency power level is increased. The results indicate that the lack of central ion heating observed during DIII-D IBW experiments may be due to a lack of efficient mode transformation from the coupled LHW to a centrally propagating IBW, possibly as a result of nonlinear mechanism(s)

Lower hybrid wave coupling in TORE SUPRA through multijunction launchers

The TORE SUPRA lower hybrid current drive experiments (8 MW/3.7 GHz) use large phased waveguide arrays, four rows of 32 active waveguides and two passive waveguides for each of the two grills, to couple the waves to the plasma. These launchers are based on the 'multijunction' principle which allows them to be quite compact and is therefore attractive for the design of efficient multi-megawatt antennas in NET/ITER. Extensive coupling measurements have been performed in order to study the radiofrequency (RF) characteristics of the plasma loaded antennas. Measurements of the plasma scattering coefficients of the antennas show good agreement with those obtained from the linear coupling theory (SWAN code). Global reflection coefficients of a few per cent have been measured in a large range of edge plasma densities (0.3 × 1018 m-3 ⩽ neg ⩽ 1.4 × 1018 m-3) or antenna positions (0.02-0.05 m from the plasma edge) and up to a maximum injected RF power density of 45 MW/m2. When the plasma is pushed against the inner wall of the chamber, the reflection coefficient is found to remain low up to distances of the order of 0.10 m

Lower hybrid wave coupling in the Wega Tokamak

The experimental data of the HF coupling measurements of the 4 waveguide grill of the Wega Tokamak have been compared with a linear coupling theory using a step density model. In order to minimize specific boundary effects which are not taken into account in the theory, the authors made use of data obtained when only the central waveguides are fed. The plasma density and its gradient at the mouth of the grill are estimated from probe measurements made with plasma conditions similar to those of the experimental coupling data. The qualitative agreement is always very good and a quantitative agreement is obtained in a relatively high density regime. The validity of the step density model is supported by the density dependence of the phases of the reflected signals.

Non-linear coupling to lower-hybrid waves in a tokamak plasma

The non-linear coupling of lower hybrid waves excited by a waveguide array is examined numerically and analytically in a model of a tokamak plasma. The non-linearity considered is the ponderomotive force and the fields are evolved self-consistently in time to the steady state. Simplified coupling models are derived for the loading of large and small arrays and for different phasings of the waveguides. It is found that the reflection coefficient can be modified considerably by the non-linear effects and that the non-linear modification of the power spectrum is especially large when the array excites two resonance cones. The fields filament and the spectrum is broadened and upshifted. The travelling-wave excitation, which excites a single resonance cone and is considered for current drive, is less affected non-linearly; the fields do not filament and the spectrum remains narrow.

THE CHARACTER OF THE COUPLING OF LOWER HYBRID WAVE WITH GRILL IN TOKAMAK

The influence of the abrupt density change near the limiter radius in Tokamak on the coupling of lower hybrid wave with grill is considered. It is discovered that the serious wave reflection, which exists in the region of the abrupt density change, would reduce the coupling efficiency of the grill. A standing wave is formed between the region of the abrupt density change and vacuum chamber wall by the reflected wave. The power and the energy flux spectra would be changed and more sensitive to the plasma boundary condition. In addition, the accessibility criterion of LH wave would be loosened.