Coupling and Absorption of Lower Hybrid Waves in Current Profile Control Experiments in the JET Tokamak

Current drive with radio frequency waves in the lower hybrid frequency range is an important method of current profile control via non-inductive current drive in tokamaks. This thesis is concerned with profile control experiments, carried out with the lower hybrid current drive system in the Joint European Torus (JET) tokamak, with the aim to explore scenarios for operation in high confinement regimes with peaked pressure profiles. Good coupling of the lower hybrid waves from the antenna to the plasma is needed for efficiency. The power reflection coefficient depends on electron density in front of the waveguides, which leads to a dependence on the plasma-antenna distance. For a large antenna, as used in JET, the plasma configuration plays a role. This is linked to the connection length of the magnetic field lines in front of the antenna. The effect of gas injection near the antenna is investigated. At a given plasma-antenna distance, the reflection coefficient can be decreased, but if large gas flow is used the density of fast electrons is decreased. The profile of the lower hybrid driven current in the JET plasmas is usually peaked off-axis, which is advantageous for current profile control in order to suppress magnetohydrodynamic instabilities and improve confinement. The broad profiles can be explained by an increase in parallel wave number of the launched wave, experienced during propagation in plasmas with large elongation. At high electron density and low magnetic field, the wave penetration is limited. The current drive efficiency of the lower hybrid waves is analysed in conditions with high power and off-axis deposition. At high electron temperature and low electron density, local overdriving of the plasma current can take place in the region where the lower hybrid driven current exceeds inductive current. The effect of a negative toroidal electric field must then be taken into account. Finally, applications of current profile control with lower hybrid waves in JET are demonstrated

Coupling and Absorption of Lower Hybrid Waves in Current Profile Control Experiments in the JET Tokamak

Current drive with radio frequency waves in the lower hybrid frequency range is an important method of current profile control via non-inductive current drive in tokamaks. This thesis is concerned with profile control experiments, carried out with the lower hybrid current drive system in the Joint European Torus (JET) tokamak, with the aim to explore scenarios for operation in high confinement regimes with peaked pressure profiles. Good coupling of the lower hybrid waves from the antenna to the plasma is needed for efficiency. The power reflection coefficient depends on electron density in front of the waveguides, which leads to a dependence on the plasma-antenna distance. For a large antenna, as used in JET, the plasma configuration plays a role. This is linked to the connection length of the magnetic field lines in front of the antenna. The effect of gas injection near the antenna is investigated. At a given plasma-antenna distance, the reflection coefficient can be decreased, but if large gas flow is used the density of fast electrons is decreased. The profile of the lower hybrid driven current in the JET plasmas is usually peaked off-axis, which is advantageous for current profile control in order to suppress magnetohydrodynamic instabilities and improve confinement. The broad profiles can be explained by an increase in parallel wave number of the launched wave, experienced during propagation in plasmas with large elongation. At high electron density and low magnetic field, the wave penetration is limited. The current drive efficiency of the lower hybrid waves is analysed in conditions with high power and off-axis deposition. At high electron temperature and low electron density, local overdriving of the plasma current can take place in the region where the lower hybrid driven current exceeds inductive current. The effect of a negative toroidal electric field must then be taken into account. Finally, applications of current profile control with lower hybrid waves in JET are demonstrated

The Role of the Plasma Current in Turbulence Decrease with Lower Hybrid Waves

We study the effect of lower hybrid waves on edge turbulence using a set of two Langmuir probes inserted in the scrape-off layer of the Tore Supra tokamak. We use the cross-correlation coefficient to assess the interplay between turbulence and the LH waves. We found that turbulence is affected at high plasma current even at relatively low LH powers. When the plasma current is below 1 MA, this effect disappears. Finally, we show that the level of fluctuations and the cross-correlation amplitude are correlated whereas the increase of the first leads to the increase of the second. The role of the plasma current could reflect that the exchange of the RF waves with edge turbulence is occurring in the closed field lines rather in the open field lines range

Comparative modelling of lower hybrid current drive with two launcher designs in the Tore Supra tokamak

Fully non-inductive operation with lower hybrid current drive (LHCD) in the Tore Supra tokamak is achieved using either a fully active multijunction (FAM) launcher or a more recent ITER-relevant passive active multijunction (PAM) launcher, or both launchers simultaneously. While both antennas show comparable experimental efficiencies, the analysis of stability properties in long discharges suggest different current profiles. We present comparative modelling of LHCD with the two different launchers to characterize the effect of the respective antenna spectra on the driven current profile. The interpretative modelling of LHCD is carried out using a chain of codes calculating, respectively, the global discharge evolution (tokamak simulator METIS), the spectrum at the antenna mouth (LH coupling code ALOHA), the LH wave propagation (ray-tracing code C3PO), and the distribution function (3D Fokker-Planck code LUKE). Essential aspects of the fast electron dynamics in time, space and energy are obtained from hard x-ray measurements of fast electron bremsstrahlung emission using a dedicated tomographic system. LHCD simulations are validated by systematic comparisons between these experimental measurements and the reconstructed signal calculated by the code R5X2 from the LUKE electron distribution. An excellent agreement is obtained in the presence of strong Landau damping (found under low density and high-power conditions in Tore Supra) for which the ray-tracing model is valid for modelling the LH wave propagation. Two aspects of the antenna spectra are found to have a significant effect on LHCD. First, the driven current is found to be proportional to the directivity, which depends upon the respective weight of the main positive and main negative lobes and is particularly sensitive to the density in front of the antenna. Second, the position of the main negative lobe in the spectrum is different for the two launchers. As this lobe drives a counter-current, the resulting driven current profile is also different for the FAM and PAM launchers

Influence of Li conditioning on Lower Hybrid Current Drive efficiency in H-mode and L- mode plasmas on EAST

The lower hybrid current drive efficiency on the EAST tokamak is estimated on a large database of low loop voltage discharges (VL of these discharges, can account for the high efficiency according to the expected scaling with Zeff and . Modelling with a ray-tracing code coupled to a Fokker-Planck solver supports this result, assuming that the fast electron transport is reduced in the zero loop voltage discharge with high efficiency

Influence of Li conditioning on Lower Hybrid Current Drive efficiency in H-mode and L- mode plasmas on EAST

The lower hybrid current drive efficiency on the EAST tokamak is estimated on a large database of low loop voltage discharges (VL of these discharges, can account for the high efficiency according to the expected scaling with Zeff and . Modelling with a ray-tracing code coupled to a Fokker-Planck solver supports this result, assuming that the fast electron transport is reduced in the zero loop voltage discharge with high efficiency

Lower hybrid current drive experiments in support of high confinement long pulse operation in EAST

The lower hybrid current drive (LHCD) system plays a crucial role in the mission of the Experimental Advanced Superconducting Tokamak (EAST) and is a prerequisite for reaching long pulse, high confinement plasmas on EAST [1, 2]. LHCD experiments and modelling [3] have been carried out on EAST in 2015-2016, with the aim to optimising EAST long pulse scenarios, and at the same time gain experience for the exploitation of WEST [4]. Experiments have been carried out to study the LH current drive efficiency in different plasma configurations (Upper Single Null and Lower Single Null). The effect of the gas feed location on the LH wave coupling was investigated by comparing gas fuelling from high field side, low field side and upper divertor. In view of long pulse H-mode scenarios, a series of H-mode experiments were conducted where all the heating power was provided by RF heating methods only, i.e. LHCD, ECRH and ICRH. H-modes were sustained in both Upper Single Null (W divertor) and Lower Single Null (carbon divertor) configurations, with loop voltage maintained as low as 50 mV

Lower hybrid current drive experiments in support of high confinement long pulse operation in EAST

The lower hybrid current drive (LHCD) system plays a crucial role in the mission of the Experimental Advanced Superconducting Tokamak (EAST) and is a prerequisite for reaching long pulse, high confinement plasmas on EAST [1, 2]. LHCD experiments and modelling [3] have been carried out on EAST in 2015-2016, with the aim to optimising EAST long pulse scenarios, and at the same time gain experience for the exploitation of WEST [4]. Experiments have been carried out to study the LH current drive efficiency in different plasma configurations (Upper Single Null and Lower Single Null). The effect of the gas feed location on the LH wave coupling was investigated by comparing gas fuelling from high field side, low field side and upper divertor. In view of long pulse H-mode scenarios, a series of H-mode experiments were conducted where all the heating power was provided by RF heating methods only, i.e. LHCD, ECRH and ICRH. H-modes were sustained in both Upper Single Null (W divertor) and Lower Single Null (carbon divertor) configurations, with loop voltage maintained as low as 50 mV