Lower Hybrid Current Drive Plasmas Research Articles

Density scaling of error field penetration in radio-frequency-dominant heating plasmas in the EAST tokamak

The density scaling of n = 1 error field penetration is investigated under radio-frequency (RF)-dominant heated L-mode discharges in the EAST tokamak. It is found that the density scaling of the threshold field strength for error field penetration in lower-hybrid current drive (LHCD) plasmas is about , where b r represents the error field amplitude. The results show a weaker density dependence compared to that observed in the previous ohmic heated discharges (Wang et al 2018 Nucl. Fusion 58 056024). For a better understanding of the density scaling, it is compared with field penetration theories, and it is found that it lies in the Waelbroeck regime. The observed scaling is consistent with that evaluated with the magnetohydrodynamic (MHD) theory on error field penetration, for which all the physical parameters are determined experimentally. Due to the density dependence of LHCD heating efficiency, the stronger negative correlation between density and temperature results in a weaker density scaling in these LHCD plasmas. Using realistic parameters under LHCD and ohmic heating as input, respectively, the numerical results based on a reduced MHD model reproduce well the scaling from the error field penetration theory and the observations. Besides, the density scaling in various tokamak operational regimes is also numerically investigated. This provides an excellent validation of MHD theory on error field penetration in the RF-dominant heated L-mode discharges.

Effect of edge plasma density on hot spot in LHCD plasma in EAST

Abstract Hot spot is a serious challenge limiting long pulse operation with LHCD (lower hybrid current drive) in tokamak. In order to mitigate the hot spot in guard limiter and improve LHCD capability, effect of edge plasma density on inducing hot spot and current drive has been studied in EAST. The temperature in the guard limiter of the LH antenna, inducing hot spot directly, increases with edge density and LH power. Studies show that the hot spot is mainly ascribed to the heat flux in front of LH antenna. Further simulation indicates that such spot corresponds to the peak position of edge density due to local LH electric field. In addition, due to the stronger parametric instability (PI) behavior in the case of higher edge density, the current drive capability decreases with edge density. Strike-point splitting behaviors appear with density increase, in agreement with current profiles in the edge region and the reduction of total driven current, suggesting that more power is deposited in edge region, which does more contribution to hot spot. Above studies offer one possible idea to optimize the edge density so as to satisfy the coupling, mitigate the heat flux in the guard limiter, and improve current drive capability for fusion device.

Correlation between the onset of parametric instability of lower hybrid waves and modification in edge plasma current profile on EAST

The plasma current profile modification in the edge region is observed in LHCD (lower hybrid current drive) plasmas for the first time in EAST, correlated with an increased level of PI (parametric instability) activities with the decrease in LH (lower hybrid) source frequency or the increase in plasma density. With the increase in the PI strength, an increase in the edge current is observed despite of a reduction in current drive capability. In these plasmas, strike-point splitting behaviors are also observed, implying the modification of the magnetic geometry in the scrape-off-layer. Results show the effects of LH frequency and plasma density on power dissipation in the edge region, suggesting a role of anomalous wave power damping on the modification of current profile at the plasma edge. These novel results are significant in that they give insight for the first time into how nonlinear wave-plasma interactions such as PI may directly impact the edge current profile, the control of which is critical in order to achieve optimized modes of operation in a steady state fusion reactor.

Transport of runaway electrons in sawtooth active plasmas in the HT-7 tokamak

The magnetic fluctuation level is an important parameter in lower hybrid current-drive (LHCD) plasmas because LHCD has been used to improve plasma performance extensively in the HT-7 tokamak. We investigate the transport of runaway electrons in LHCD plasmas with strong sawteeth in the HT-7 tokamak. A delay time was found to exist between the peak of the soft X-ray sawteeth and the peak of the hard X-ray flux. Based on the delay time, we obtained the transport coefficient of the runaway electrons and the averaged magnetic fluctuation level in sawtooth activity of the plasmas. We found that the diffusion coefficient of the runaway electrons was on the order of Dr ∼ 1 m2s−1 and that the magnetic fluctuation level was on the order of b ∼ 10−6.

New findings about sawtooth oscillations in LHCD plasmas on the HT-7 tokamak using tomography of soft x-ray signals

Two new physical phenomena have been observed during the sawtooth (both with mid-cursor and without precursor) crash in lower hybrid current drive (LHCD) plasmas on the HT-7 tokamak by using tomography of high-resolution soft-x-ray emission together with the singular value decomposition technique. Firstly, there is no displacement of the hot core during the mid-cursor-sawtooth (sawtooth oscillations with mid-cursor) crash; secondly, magnetic reconnection does not occur during the precursorless-sawtooth (sawtooth oscillations without precursor) crash. These observations demonstrate that the crashes of the two typical sawtooth oscillations (mid-cursor-sawtooth and precursorless-sawtooth) are inconsistent with both full reconnection theory and Wesson's theory.

Recent progress in RF heating and long-pulse experiments on EAST

New capabilities including radio-frequency (RF) powers and diagnostics have been developed since the last IAEA meeting. Divertor performance has been systematically assessed for both single null and double null configurations with normal and reversed BT directions to address asymmetries correlated with classic drift and the issues of heat load on divertor plates. 1 MW lower hybrid wave power injection has produced fully non-inductive diverted plasma discharges. Such discharges have been readily extended to well over 1 min duration. With the assistance of lower hybrid current drive (LHCD), EAST can be operated at 1 MA with both RTEFIT/Iso-flux or pre-programmed control algorithms. A later operation mode could produce an improved confinement plasma by increasing the ohmic power through regulating the loop voltage. Both direct electron and ion heating by ion cyclotron resonant frequency power have been observed for the first time on EAST. Intrinsic toroidal rotation and momentum study in ohmic and LHCD plasmas reveals the importance of the balance between neoclassical viscosity and neutral friction at the edge and finds clear linkage between edge and core toroidal rotation. The expected capabilities on EAST in the next two years will allow access to high-performance regimes, such as H-modes, and enable specific physics studies.

Particle confinement and coherence of turbulence with lower hybrid current drive on HT-7 tokamak

A improved particle confinement plasma produced by using lower hybrid current drive (LHCD) has been obtained on the HT-7 superconducting tokamak. It is found that the particle confinement time increases with the LHW power, but saturates at a critical power (PLHW∼300 kW). Therefore, the fluctuation level and coherence of turbulences are investigated by a CO2 collective scattering system. It is observed that the power spectrum for OH discharge is almost the same as for the LHCD plasma, but the value of cross-spectrum between kθ=12 cm−1 and kθ=20 cm−1 is reduced with increasing LHW power. The possible mechanisms responsible for improved particle confinement and the evolution of density profile are studied in detail in LHCD plasma.

Generation of runaway electrons during deterioration of lower hybrid power coupling in lower hybrid current drive plasmas in the HT-7 tokamak

Efficient coupling of lower hybrid (LH) power from the wave launcher to the plasma is a very important issue in lower hybrid current drive (LHCD) experiments. The large unbalanced reflections in the grill trigger the LH protection system, which will trip the power, resulting in the reduction of the coupled LH power. The generation of runaway electrons has been investigated in LHCD plasmas with deterioration of LH coupling in the HT-7 tokamak. The deterioration of LH coupling results in an increase of the loop voltage and a more energetic fast electron population. These two effects favor the generation of a runaway population. It is found that most of the fast electrons generated by LH waves through parallel electron Landau damping were converted into a runaway population through the acceleration from the toroidal electric field when significant deterioration of LH coupling occurs.

The effect of collisional dissipation on the radial profiles of current driven by the lower-hybrid waves

A simulation of the collisional dissipation of fast electrons has been conducted in lower-hybrid-current drive (LHCD) tokamak plasmas. The simulation results show that the dissipation of fast electrons will counteract the effect of radial diffusion to some extent, thereby making the LHCD current profile closer to the power deposition profile. It seems that the dissipation is helpful to maintain a steady state “hole current profile” in LHCD plasmas; thereby, the desired reversed magnetic shear should be maintained.

Current Profile Estimation in Full LHCD Plasmas using Hard X-Ray Measurement along the Top and Bottom Identical Line of Sight on TRIAM-1M

A new technique to measure the current profile in plasmas with asymmetric distribution function such as lower hybrid current drive (LHCD) by using hard X-ray (HXR) energy spectrum measurement along the top and bottom identical line of sights (ILOS) is proposed and is applied to the full and partially LHCD plasmas on the TRIAM-1M tokamak at the first time in the world. The pitch angles were measured at R - R0 = ±2.5 cm, where R, R0 mean the major radii of the ILOS and the magnetic axis, respectively. The measured pitch angle of the magnetic field inverted at the magnetic axis estimated magnetic measurement in partially LHCD plasmas. This indicates that the difference of the measured pitch angles is caused by the plasma current in the plasma and this new method is available in detecting the current profile in tokamaks. In full LHCD plasma, no difference between the HXR signals along the top and bottom ILOS appear. This indicates that the current density around the magnetic axis was reduced compared with that in partially LHCD plasmas. This observation is no contradiction with power deposition of LHCD.

Mode behavior during sawtooth activity in LHCD plasmas on the HT-7 tokamak

Different types of m/n=1/1 mode during the central relaxation oscillations are observed in lower hybrid current drive (LHCD) plasma on the HT-7 tokamak. In this Letter, the difference of the mid-mode and the precursor on the sawtooth oscillation are reported using two-dimensional tomography of soft X-ray signal. It is shown that the growth rate of magnetic island of the mid-oscillation is smaller than that of the precursor oscillation, and the hot core in the mid-oscillation case does not move outward to the vicinity of the q=1 surface. However, in the precursor oscillation case, the colder plasma develops a magnetic island on the former magnetic axis after the hot core expulsion, from the contour map of the soft X-ray emissivity.

Dynamics of runaway electrons in lower hybrid current drive plasmas in the HT-7 tokamak

In lower hybrid current drive (LHCD) plasmas with non-zero loop voltage, the fast electron tail can act as a seed population of runaways. The runaways are found to be enhanced in the presence of lower hybrid (LH) waves when the fast electron tail extends above the critical velocity for electrons to run away. In runaway discharges, the LH waves may suppress runaway electrons due to the drop in the electric field linked to the non-inductive current drive. In this paper, the behaviour of runaway electrons in an extensive range of LHCD plasmas is presented.

Electron Heating of LHCD Plasma in HT-7 Tokamak

Electron heating via lower hybrid current drive (LHCD) has been investigated in HT-7 superconducting tokamak. Experiments show that the central electron temperature Te0, the volume averaged electron temperature <Te> and the peaking factor of the electron temperature QTe = Te0/<Te> increase with the lower hybrid wave (LHW) power. Simultaneously the electron heating efficiency and the electron temperature as the function of the central line-averaged electron density (ne) and the plasma current (Ip) have also been investigated. The experimental results are in a good agreement with those of the classical collision theory and the LHW power deposition theory.

Improved confinement mode induced by a MARFE during lower hybrid current drive in the HT-7 tokamak

Lower hybrid current drive (LHCD) experiments have been carried out to achieve high performance for long pulse operation in the Hefei tokamak-7 (HT-7) superconducting tokamak. Multifaceted asymmetric radiation from the edge (MARFE) phenomena and an improved confinement mode induced by a MARFE are observed in LHCD plasmas when PLH>160 kW and the edge safety factor q(a) is slightly less than 6.5. It is found that an improved confinement mode induced by a MARFE, characterized by Hα line emissions drops and the line-averaged density increase is triggered in the MARFE discharges. The MARFE event occurs at t=1120 ms following the L–H transition, and the improved confinement phase exists for about 65 ms from t=1140 ms after the L–H transition. The LHCD plasma results basically are typical with MARFEs and it is an improved confinement mode; they are unique in that they appear at very low values in the HT -7 tokamak.

Investigation of the Energy Confinement in Ohmic and LHCD Plasmas in HT-7

Investigation of the energy confinement in ohmic and lower hybrid current drive (LHCD) plasmas in HT-7 has been performed. In ohmic discharges at low densities the global energy confinement time τE increases almost linearly with the density, saturates at a critical density (2.5 × 1013/cm3 for HT-7) and is nearly constant at higher densities. The energy confinement time is in good agreement with the Neo-Alcator scaling law at different densities and currents. In the LHCD plasmas the global energy confinement time similar to that of the L-mode discharges has been observed to be in good agreement with the low confinement mode (L mode) scaling law of ITER89-P in higher electron density and plasma current.

Measurement of the non-thermal bremsstrahlung emission between 20 and 7000 keV in the HT-7 Tokamak

Hard X-ray (HXR) emission in the energy range of 20–7000 keV during lower hybrid current drive (LHCD) experiments in the HT-7 superconducting tokamak has been measured. A vertical multi-channel CdTe detector array is used to measure the fast electron bremsstrahlung (FEB) emission in the energy range 20–300 keV. Three tangentially arranged NaI detectors are used to measure the HXR emission in the energy range 500–7000 keV. In LHCD plasmas with non-zero loop voltage, the fast electron tail extends out to several hundreds keV. The energetic tail acts as seeds of runaways in high loop voltage environments. In this paper, the diagnostics system is described, and the dynamics of runaways in LHCD discharges is presented by the combination of FEB and energetic HXR measurements.

Study of MARFE during lower hybrid current drive in HT-7 tokamak

Lower hybrid current drive (LHCD) experiments have been carried out to achieve high performance for long pulse operation in the HT-7 superconducting tokamak. Multifaceted asymmetric radiation from the edge (MARFE) phenomena, is observed in LHCD plasmas when P LH > 160 kW and edge safety factor q ( a ) is slightly less than 6.5 and has been studied in this Letter. Studies show that the critical density of MARFE onset is observed in the region of Z eff 1 / 2 f GW = 0.6 – 0.9 , where f GW = n ¯ e n GW (here n ¯ e is the maximum line average electron density and n GW is the Greenwald density). These MARFEs generally appear to have the same characteristics as high f GW MARFEs and are positionally stable throughout the LHCD pulse.

Observation of the m = 1 mode during the ramp phase of the sawtooth oscillations in LHCD plasmas on the HT-7 tokamak

A new oscillation with m = 1 and n = 1, where m and n are the poloidal and toroidal mode numbers, respectively, has been observed with lower hybrid current drive (LHCD) on the HT-7 tokamak. This mode appears during the sawtooth ramp, saturates, damps and finally disappears before the subsequent sawtooth collapse and before the appearance of the precursor oscillations. The necessary condition for its observation is that the sawtooth period exceeds 6–7 ms and that the LHCD power exceeds the threshold value of about 90 kW. We propose that the mode may be destabilized by the current density gradient, the evolution of which is determined by the suprathermal electrons generated by the LHCD.

Recent progress on steady-state high-performance plasma research in the Hefei Tokamak-7

High performance discharges under steady-state condition in the Hefei Tokamak-7 (HT-7) have been investigated. Lower hybrid current drive (LHCD) was used both for sustaining plasma current and current density profile control. The experiments demonstrated that features of ion Bernstein wave heating in controlling the electron pressure profile can be integrated into LHCD plasmas and the assist localization of LHCD. This local synergy effect was used to tailor the current density profile in a negative shear configuration and to control the internal transport barrier (ITB) under steady-state conditions. The high performance with a stationary ITB at the footprint of the minimum q and βNH89&amp;gt;2 was sustained for &amp;gt;220 τE or &amp;gt;20 τCR. The fraction of noninductive plasma current was larger than 80% in such discharges with considerable bootstrap current. The duration at H89&amp;gt;1.2 and βN∼1 has been extended to nearly 8 s, longer than 400 τE. More than 90% of the plasma current was sustained by LHCD and bootstrap current. High-plasma performance and sustaining time was limited by the magnetohydrodynamic instabilities and the recycling, which caused an uncontrollable rise of the electron density.

Measurements of Boundary Plasma in Synergy Discharges of IBWand LHCD on the HT-7 Tokamak

By applying ion Bernstein wave (IBW) heating into the lower hybridcurrent drive (LHCD) plasma, improved confinements have been obtainedin the HT-7 tokamak. The central electron temperature was doubled andthe storage energy was increased significantly. The core electrondensity and temperature were broadened and their profiles near the edgewere steepened. A transport barrier has been formed in the vicinity ofthe limiter radial location. An enhanced shear in poloidal phasevelocity was found in the same region with reduction of the fluctuationlevels and the coherences between fluctuations. The results suggestthat the improved confinement in the IBW and LHCD plasma is at leastpartially due to the modification of shear in poloidal velocity andthen the suppression of fluctuations and fluctuation induced fluxes viade-correlation effect.