Lower Hybrid Wave Coupling Research Articles

First realization of LHW–plasma coupling feedback control for long-pulse operation in EAST

To sustain good lower hybrid wave (LHW)–plasma coupling for long-pulse plasma operation, for the first time, coupling feedback control is designed and realized in EAST using a proportion integration differentiation method by choosing the reflection coefficient (RC) of LHW power as the reference for gas-puffing feedback, and including one pulse test and multi-pulse experiments. Experiments show that such feedback control can work correctly and maintains good LHW–plasma coupling effectively for a long time, suggesting the possibility of feedback control application on LHW–plasma coupling in long-pulse plasma. Furthermore, during the feedback control process of multi-pulse supersonic molecular beam injection (SMBI), the stored energy changes from 29 kJ to 58 kJ, and the energy confinement factor (H 89) increases from 0.98 to 1.45, implying a positive effect of coupling feedback on plasma performance. Experiments between SMBI puffing and the gas puffing system, fed by a piezoelectric valve near the antenna, are further investigated, showing that the response time of the RC with SMBI is faster than that by the piezoelectric valve. In addition, SMBI puffing on the electron-drift side of the LHW antenna is a little quicker than that on the ion-drift side. Studies suggest that such feedback control is effective for long-pulse LHW–plasma coupling, and the gas puffing by SMBI on the electron-drift side of the LHW antenna could offer an effective way to sustain good LHW coupling in steady-state operation in the future. Further optimization will be continued at a later date.

Improvement of lower hybrid current drive systems for high-power and long-pulse operation on EAST

Aiming at high-power and long-pulse operation up to 1000 s, some improvements have been made for both 2.45 GHz and 4.6 GHz lower hybrid (LH) systems during the recent 5 years. At first, the guard limiters of the LH antennas with graphite tiles were upgraded to tungsten, the most promising material for plasma facing components in nuclear fusion devices. These new guard limiters can operate at a peak power density of 12.9 MW/m2. Strong hot spots were usually observed on the old graphite limiters when 4.6 GHz system operated with power >2.0 MW [B. N. Wan et al., Nucl. Fusion57(2017) 102019], leading to a reduction of the maximum power capability. With the new limiters, 4.6 GHz LH system, the main current drive (CD) and electron heating tool for EAST, can be operated with power >2.5 MW routinely. Long-pulse operation up to 100 s with 4.6 GHz LH power of 2.4 MW was achieved in 2021 and the maximal temperature on the guard limiters measured by an infrared (IR) camera was about 540 °C, much below the permissible value of tungsten material (∼1200 °C). A discharge with a duration of 1056 s was achieved and the 4.6 GHz LH energy injected into the plasma was up to 1.05 GJ. Secondly, the fully-active-multijunction (FAM) launcher of 2.45 GHz system was upgraded to a passive-active-multijunction (PAM), for which the density of optimum coupling was relatively low (below the cut-off value). Good coupling with reflection coefficient ∼3% has been achieved with plasma-antenna distance up to 11 cm for the new PAM. Finally, in order to eliminate the effect of ion cyclotron range of frequencies (ICRF) wave on 4.6 GHz LH wave coupling, the location of the ICRF launcher was changed to a port that is located 157.5° toroidally from the 4.6 GHz LH system and is not magnetically connected.

The effects of E r × B t drift on LH wave coupling with divertor configuration at EAST

The coupling characteristics of the 4.6 GHz lower hybrid (LH) grill and the factors affecting LH wave coupling, mainly drift, with divertor configuration in anti-clockwise/clockwise magnetic field direction are studied. Based on the Langmuir probe measurement, the relationship between the reflection coefficient (RC) of LH power and the edge density at the grill mouth () measured by Langmuir probes is investigated. The optimum LH wave coupling with RC less than 2% is achieved when is in the range of –, which is quite close to the theoretically predicted value. In addition, the experimental results show that drift in the divertor scrape off layer plays a dominant role in affecting and LH wave coupling. Studies show that the discrepancy in and LH wave coupling in different magnetic field directions is mainly attributed to the different directions of drift, whereas the corresponding discrepancy with different plasma currents is mainly ascribed to the change in drift intensity. Therefore, for the upper single null divertor configuration, a lower/higher plasma currents is more favorable for increasing and improving LH wave coupling in anti-clockwise/clockwise . The above studies are of great significance for optimizing and improving LH wave coupling in the extension of the plasma operation regime.

Initial results in SST-1 after up-gradation

SST-1 Tokamak has recently completed the 1st phase of up-gradation with successful installation and integration of all its First Wall components. The First Wall of SST-1 comprises of ∼ 3800 high heat flux compatible graphite tiles being assembled and installed on 132 CuCrZr heat sink back plates engraved with ∼ 4 km of leak tight baking and cooling channels in five major sub groups equipped with ∼ 400 sensors and weighing ∼ 6000 kg in total in thirteen isolated galvanic and six isolated hydraulic circuits. The phase-1 up-gradation spectrum also includes addition of Supersonic Molecular Beam Injection (SMBI) both on the in-board and out-board side, installation of fast reciprocating probes, adding some edge plasma probe diagnostics in the SOL region, installation and integration of segmented and up-down symmetric radial coils aiding/controlling plasma rotations, introduction of plasma position feedback and density controls etc. Post phase-I up-gradation spanning from Nov 2014 till June 2016, initial plasma experiments in up-graded SST-1 have begun since Aug 2016 after a brief engineering validation period in SST-1. The first experiments in SST-1 have revealed interesting aspects on the ‘eddy currents in the First Wall support structures’ influencing the ‘magnetic Null evolution dynamics’ and the subsequent plasma start-up characteristics after the ECH pre-ionization, the influence of the first walls on the ‘field errors’ and the resulting locked modes observed, the magnetic index influencing the evolution of the equilibrium of the plasma column, low density supra-thermal electron induced discharges and normal ohmic discharges etc. Presently; repeatable ohmic discharges regimes in SST-1 having plasma currents in excess of 65 KA (qa ∼ 3.8, BT = 1.5 T) with a current ramp rates ∼ 1.2 MA/s over a duration of ∼ 300 ms with line averaged densities ∼ 0.8 × 1019 and temperatures ∼ 200 eV with copious MHD signatures have been experimentally established. Further elongation of the plasma duration up to one second or more with position and density feedback as well as coupling of Lower Hybrid waves are currently being persuaded in SST-1 apart from increasing the core plasma parameters with further optimizations and with wall conditioning.

Density modification in the scrape-off-layer during LHW coupling at EAST

The results presented in this paper are an extension of our recent studies (Kong et al 2013 Plasma Phys. Control. Fusion 55 065007, 065008) on lower hybrid wave (LHW) coupling. The effects of LHW power, edge recycling, edge-localized-mode (ELM) activity and gas puffing on the scrape-off-layer (SOL) density modifications will be further studied in this paper. The SOL density profile usually depends on the competition between an increment by ionization of neutrals and a depletion by the ponderomotive force (PMF) in L-mode plasma. Experimental results indicate that the SOL density with a lower edge recycling intensity could be reduced by a higher LHW power because ionization of a few neutrals cannot provide sufficient electrons to increase the SOL density and thus degrade the LHW coupling. In addition, in H-mode plasma, it is observed that the significant reduction of RC is associated with higher LHW power compared with the oscillation of RC during ELM activity, implying the priority of neutral gas ionization by LHW power. Studies also indicate that gas puffing in the GIM_e side could be more efficient in improving LHW coupling due to local ionization, inferred by the luminosities on the surface of the poloidal limiter (PL) and confirmed by SOL density measurement. The evolution of edge density with various puffing rates exhibits increments of far SOL density focused on the narrower layer several centimeters in front of the PL, possibly suggesting that the ionization of neutral gas mainly occurs in this region.

Operation and coupling of LH waves with the ITER-like wall at JET

In this paper important aspects of the lower hybrid (LH) operation with the ITER-like wall (ILW) [] at JET are reported. Impurity release during LH operation was investigated and it was found that there is no significant Be increase with LH power. The concentration of W was analysed in more detail and it was concluded that LH negligibly contributes to its increase. No cases of W accumulation in LH-only heating experiments were observed so far. LH wave coupling was studied and optimised to achieve the level of system performance similar to before ILW installation. Measurements by Li-beam were used to study systematic dependencies of the scrape-off layer (SOL) density on the gas injection rate from a dedicated gas introduction module and the LH power and launcher position. Experimental results are supported by SOL transport modelling. Observations of arcs in front of the LH launcher and hotspots on magnetically connected sections of the vessel are reported. Overall, a relatively trouble-free operation of the LH system up to 2.5 MW of coupled radio frequency power in L-mode plasma was achieved with no indication that the power cannot be increased further.

Experimental Investigation of Lower Hybrid Wave Coupling in HT-7

Experiments on lower hybrid wave (LHW) coupling were investigated in the HT-7 tokamak. Good coupling of LHW plasma has been demonstrated at different conditions in the HT-7 tokamak. Relevant results have proved that LHW-plasma coupling is affected by the phase difference between adjacent waveguides. Furthermore, the edge density around the grill and relevant coupling can be adjusted by changing the plasma line average density or the gap value between the LH grill and the last closed flux surfaces (LCFS). It is found that the coupling of LHWs becomes poor when the edge density around the LH grill is large enough in the HT-7 tokamak, and that coupling remains good with a proper edge density. With increasing LHW power, it is also found that the reflection coefficients (RCs) increase due to non-linear effects under conditions of low edge recycling, but can decrease under high edge recycling. The edge density depends mainly on the competition between the ponderomotive force (PMF) and the edge recycling intensity in the HT-7 tokamak.

An overview of FTU results

Since the 2010 IAEA-FEC Conference, FTU has exploited improvements in cleaning procedures and in the density control system to complete a systematic exploration of access to high-density conditions in a wide range of plasma currents and magnetic fields. The line-averaged densities at the disruptive limit increased more than linearly with the toroidal field, while no dependence on plasma current was found; in fact, the maximum density of 4.3 × 1020 m−3 was reached at B = 8 T even at the minimum current of 0.5 MA, corresponding to twice the Greenwald limit. The lack of plasma current dependence was due to the increase in density peaking with the safety factor. Experiments with the 140 GHz electron cyclotron resonance heating (ECRH) system were focused on the sawtooth (ST) period control and on the commissioning of a new launcher with real-time steering capability that will act as the front-end actuator of a real-time system for ST period control and tearing mode stabilization. Various ECRH and electron cyclotron current-drive modulation schemes were used; with the fastest one, the ST period synchronized with an 8 ms modulation period. The observed period variations were simulated using the JETTO code with a critical shear model for the crash trigger. The new launcher was of the plug-in type, allowing quick insertion and connection to the transmission line. Both beam characteristics and steering speed were in line with design expectation. Experimental results on the connection between improved coupling of lower hybrid waves in high-density plasmas and reduced wave spectral broadening were interpreted by fully kinetic, non-linear model calculations. A dual-frequency, time-of-flight diagnostic for the measurement of density profiles was developed and successfully tested. Fishbone-like instabilities driven by energetic electrons were simulated by the hybrid MHD-gyrokinetic XHMGC code.

Modeling of the nonlinear mode coupling of lower hybrid waves in tokamak plasmas

A nonlinear wave equation is derived to describe the nonlinear mode coupling of lower hybrid (LH) waves launched in tokamak plasmas, driven by low-frequency (LF) (⩽10 MHz) ion-sound evanescent modes (quasi-modes). The spectrum of the LF fluctuations is calculated considering the beating of the LH wave at the radiofrequency (RF) operating line frequency (pump wave) with the noisy background of the RF power generator. This spectrum is calculated in the frame of the kinetic theory, following a perturbative approach. Numerical solutions of the nonlinear LH wave equation show the evolution of the nonlinear mode coupling in condition of a finite depletion of the pump power. Under operating conditions generally met in the experiments, the sidebands, excited by the noise of RF power generators and amplified by the nonlinearity, broaden the launched antenna spectrum, and determine the LH propagation and deposition behavior.

Parametric coupling of lower hybrid wave with gyrating ion beam driven ion cyclotron instability in a plasma

A lower hybrid wave, launched into a tokamak for supplementary heating in the presence of neutral beam turned gyrating ion beam, is seen to excite some prominent channels of parametric decay. The beam driven deuterium cyclotron mode is further destabilized by the lower hybrid pump through the nonlinear 4-wave coupling, involving higher and lower frequency lower hybrid sidebands, when ω0/k0zv0z=(1−ωLH2/ω02) , where ωLH is the lower hybrid frequency, ω0 and k0z are the frequency and parallel wave number of the pump wave, and v0z is the velocity of ion beam parallel to the magnetic field. The growth rate increases with parallel wave number of the ion-cyclotron mode. The pump is also susceptible to parametric upconversion into an upper sideband shifted by the frequency of the negative energy deuterium cyclotron mode. For typical parameters, the growth rate of this channel is around one fiftieth of deuterium cyclotron frequency and falls off with the transverse wave number of the mode.

Effects of plasma configuration, ELM and gas puffing on LHW coupling during H-mode in EAST

Couplings of lower hybrid wave (LHW) with different divertor configurations are studied in EAST. With an anti-clockwise toroidal magnetic field and similar plasma parameters, experimental results show that the best coupling occurs in the lower single null (LSN) configuration, whereas the worst occurs in the double-null plasma. Furthermore, for the case of clockwise toroidal magnetic field, the coupling of LHW becomes better in the upper single null configuration and worse in the LSN plasma. Such phenomena show that the LHW coupling with different divertor configurations is possibly related to the flux induced by Er × Bt and edge recycling intensity represented by Da, where Er is the radial electric field in the scrape-off layer. In addition, various edge-localized modes (ELMs), including its intensity and frequency, have impacts on LHW coupling. With increasing ELM frequency in low edge recycling, the intensity of Da would decrease and the associated coupling of LHW should deteriorate. For the case of comparable edge density, the coupling of LHW is almost not influenced by the ELM crash. Results indicate that the changes among Da intensity, ELM frequency and the reflection coefficients of LHW power are self-consistent. Studies show that by gas puffing the nearby LH grill can improve the coupling of LHW during H-mode in EAST. Meanwhile, it is observed that the frequency of ELM should decrease and the plasma confinement should be improved with proper gas puffing, whereas excessive gas puffing should increase the frequency of ELM during H-mode in the case of good LHW coupling. Results also indicate a degradation in confinement performance at increasing puffing rate.

Study and optimization of lower hybrid wave coupling in the experimental advanced superconducting (EAST) tokamak

The results presented in this paper are an extension of our recent (Kong et al 2012 Plasma Phys. Control. Fusion 54 105003) studies on lower hybrid wave (LHW) coupling. By optimizing the shape of the LH grill, the misalignment between the poloidal limiter (PL) and the LH grill is nearly eliminated and the coupling of LHW is improved, especially on the top row, although some discrepancies are still present in the case with low edge density. Density modifications both by LHW and ion cyclotron range of frequency (ICRF) power are studied in EAST. Experimental results show that the edge density modification in front of the LH grill during LHW power depends mainly on the competition between ponderomotive force (PMF) and the ionization of neutral gas, provided by gas puffing and edge recycling. However, the local edge density during ICRF power can be reduced rapidly. Furthermore, such a modification is more obvious with higher ICRF power and the relevant mechanism of density modification by ICRF power can be related to RF sheaths. In addition, another analogous effect of RF sheaths on the coupling of LHW is also investigated, i.e. the density convection induced by Er × B drift. The changes in LHW coupling associated with different ICRF antennas are discussed and it is shown that in some cases the coupling on the lower rows of the LH grill is improved possibly due to magnetic connection between ICRF antennas and the LH grill. The local coupling of LHW can be improved by gas puffing from gas introduction modules (GIM) on both sides of the launcher, but it is difficult to judge which one is more beneficial due to errors in measurements. Experimental results with gas (D2) injection during ICRF power clearly show that the coupling of LHW on the upper rows will be first improved by gas injection on the electron side and the coupling on the lower rows will be effectively improved by gas injection on the ion side. The results are consistent with the mapping of field lines.

Effects of LH power on SOL density profiles and LH coupling on Alcator C-Mod

A swept-frequency X-mode reflectometer has been used to measure the scrape-off-layer (SOL) density profiles with and without lower hybrid (LH) power at three poloidal locations adjacent to the LH launcher for various plasma parameters in order to understand the coupling of LH waves on Alcator C-Mod. LH power has been observed to create significant poloidal SOL density profile asymmetries that are correlated with visible video camera images of emissivity patterns in front of the LH launcher. The observed density profile asymmetries depend on LH power, , magnetic geometry and magnetic field direction. A 2D diffusive–convective model has been used to show that these density profile modifications are consistent with a LH vortex, where LH power drives E × B drifts that then modify the SOL density profile. In particular, the simulations show that the density profile can possibly create a net poloidally averaged density depletion in front of the waveguide rows. A LH slab coupling model is then used to show that the simulated reflection coefficients strongly depend on the poloidal density profile asymmetries. The simulated LH power reflection coefficients agree with the experimental reflection coefficients only after the observed density depletion is included in the model.

Experimental investigation of the potentials modified by radio frequency sheaths during ion cyclotron range of frequency on EAST

Recent ion cyclotron range of frequency (ICRF) experiments combined with lower hybrid wave (LHW) on EAST show that LHW coupling can be strongly modified when the LHW launcher is connected magnetically to a powered ICRF antenna. Using Langmuir probes, investigation of radio frequency (RF)-enhanced potential and local plasma parameters under an applied ICRF pulse is carried out on EAST. When the ICRF antenna connected to the probe is powered, localized high positive peaks appear on the floating potential. The dependence of floating potential modifications on various parameters is investigated. These initial results are shown to be consistent with RF sheath physics. A new four-strap antenna is modelled with the high frequency structural simulator (HFSS) code for evaluation of the parallel electrical fields (E∥) and sheath potentials. It is found that the E∥ fields with the (0, 0, π, π) phase are much higher than those with other phases. Only the (0, π, π, 0) phasing has a significant reduction potential compared with the original two-strap antenna at the dipole phase.

Experimental Investigation of Nonlinear Coupling of Lower Hybrid Waves on Tore Supra

The coupling of lower hybrid waves from the two multijunction-type antennas installed on Tore Supra is investigated. At low power good agreement between the measurement of the power reflection coefficient and the computation with the ALOHA code is found for most cases. Details on electron density measurements, documented from embedded Langmuir probes, are discussed. At high power, departure from the linear theory is clearly seen in many cases. Ponderomotive forces depleting the electron density in a thin plasma layer in front of the antennas are likely to be responsible for the increase of power reflection coefficients measured at the input of the antennas. The decrease of the antenna directivity resulting from weaker wave coupling accounts for the experimental reduction of current drive efficiency.

Investigation of lower hybrid wave coupling and the related effects of ion cyclotron range of frequencies in EAST

Effective coupling for lower hybrid waves (LHWs) is achieved by adjusting the launcher position and optimizing the plasma configuration in L-mode in EAST. It is found that, compared with other divertor shapes, the plasma with double null shows better coupling performance at the same position of lower hybrid (LH) grill, especially in the case of a large safety factor near the separatrix (q95) and a large edge recycling (Dα) intensity. The ion cyclotron range of frequency (ICRF) power has a significant impact on LH wave coupling when the ICRF antenna is magnetically connected to the LH grill. The asymmetry effects in the poloidal direction on reflection coefficients are obtained with a low edge density during ICRF power application. The origin of such a relevant asymmetry with ICRF is different from LHWs. Results not only suggest that ICRF power could modify the density in the local scrape-off layer (SOL), but also indicate that density convection in the SOL could be easily obtained with a low edge density. One promising alternative for eliminating the negative impact on LHW coupling induced by ICRF is gas (D2) injection both from the electronic side and ionic side in EAST.

Influence of gas puff location on the coupling of lower hybrid waves in JET ELMy H-mode plasmas

Reliable coupling of the lower hybrid current drive (LHCD) to H-mode plasmas in JET is made feasible through a dedicated gas injection system, located at the outer wall and magnetically connected to the antenna (Pericoli Ridolfini et al 2004 Plasma Phys. Control. Fusion 46 349, Ekedahl et al 2005 Nucl. Fusion 45 351, Ekedahl et al 2009 Plasma Phys. Control. Fusion 51 044001). An experiment was carried out in JET in order to investigate whether a gas injection from the top of the torus, as is foreseen for the main gas injection in ITER, could also provide good coupling of the LH waves if magnetically connected to the antenna. The results show that a top gas injection was not efficient for providing a reliable LHCD power injection, in spite of being magnetically connected and in spite of using almost twice the amount of gas flow compared with the dedicated outer mid-plane gas puffing system. A dedicated gas injection system, set in the outer wall and magnetically connected to the LHCD antenna, is therefore recommended in order to provide the reliable coupling conditions for an LHCD antenna in ITER.

Simulation on effect of poloidal power spectrum on lower hybrid wave propagation

The coupling of lower hybrid wave to the plasma is a crucial issue for efficient current drive in tokamaks. This paper establishes a new coupling model which assumes the antenna to be a curved face and the plasma to be a cylinder. Power spectrum considering the coupling between wave-guides in both poloidal and toroidal direction is simply estimated and discussed. The effect of the poloidal wave vector on wave propagation, power deposition and driven current is also investigated with the help of lower hybrid current drive code. Results show that the poloidal wave vector affects the ray tracing, and also has effect on power deposition and driven current. The effect of the poloidal wave vector on power deposition and driven current profile depends on plasma parameters. Preliminary studies suggest that it seems possible to control the current profile by adjusting the poloidal phase difference between the waveguide in poloidal direction.

Lower Hybrid Current Drive in Tore Supra

Since the mission of Tore Supra is to produce quasi-steady-state discharges, the lower hybrid current drive (LHCD) system constitutes the most important method of additional heating and noninductive current drive. A description of the LHCD system is given, including the different launcher designs developed for the Tore Supra long-pulse program. Following the completion of the Composants Internes et Limiteur project, together with the installation of a high-performance LHCD launcher, world record discharges, injected and extracted energy exceeding 1 GJ, were obtained in 2003. With the flexibility of lower hybrid (LH) waves to tailor the current profile, an enhanced performance regime, the so-called LHEP, has been maintained in quasi-steady-state discharges. Detailed measurements of the fast electron distribution have allowed us to constrain LHCD ray-tracing models and to quantify parametric dependencies describing the fast electron tail. Localized heat loads on the LHCD launchers due to interaction with fast particles have been measured and quantified, using infrared imaging and calorimetric measurements on water-cooled plasma facing components. Furthermore, experimental results in the area of LH wave coupling are presented.

Effects of ICRF induced density modifications on LH wave coupling at JET

Lower hybrid (LH) wave coupling is modified and usually degraded when the system is powered simultaneously with the ion cyclotron range of frequency (ICRF) antennas magnetically connected to the launcher. This has been attributed to scrape-off layer density modifications by the RF sheaths. LH reflection coefficients dependences on various parameters are investigated and shown to be consistent with RF sheath physics. Gas puffing near the launcher has been used to improve the coupling of LH waves.