Interferometric study of density fluctuations in a tokamak plasma

Density fluctuations in the LT-4 tokamak plasma are investigated using a Phase Scintillation Interferometer operating at 10.6/Ltm which is sensitive to density fluctuations of δnₑ/nₑ> 10⁻¹⁴. The plasma is imaged across a linear detector array which can be rotated to record projections in any direction, from toroidal to poloidal. The theory of forward scattering from plasmas is developed from the Rytov approximation and aspects of the Fourier diffraction projection theorem relevant to plasma scattering. The result is a clear conceptual picture of diffraction from arbitrary extended refractive media, from which important analytical tools are developed. The Phase Scintillation Interferometer is used to image density perturbations produced by large scale magnetohydro dynamic (MHD) modes in the plasma associated with Mimov oscillations. Structural characteristics are determined, and a comparison between experimental and computed projections of the Dubois model is made which shows that the density fluctuations are consistent with a model of rotating magnetic islands. Island widths and local magnetic field fluctuations are determined and are found to compare well with measured poloidal magnetic field fluctuations. The interferometer is used in conjunction with other diagnostics to investigate minor and major disruptions in LT-4. The time frequency distribution is introduced as an important analytical tool in the characterization of the various regimes of MHD activity. Frequency and amplitude variations of an m = 3 mode during current rise appear correlated with variations in toroidal loop voltage. The mode is also found to persist throughout the whole discharge and to play a part in mode locking which precedes major disruptions. Mode frequencies are found to vary in a regular way with the safety factor q(a). Precursor oscillations before minor and major disruptions are identified. A strong m — 1 type of internal relaxation is found to follow rapid growth and locking of an m = 2 mode during minor disruptions. The interferometer is also applied to the measurement of fine scale density fluctuations in the LT-4 tokamak during periods of low level MHD activity. Line integral measurements indicate an edge fluctuation level of about 10% and broad band spectra typical of strong turbulence. Anisotropy in the spectrum of fluctuations perpendicular to the magnetic field is observed. This observation runs counter to reported measurements of isotropic fluctuations made on other tokamaks using small angle scattering techniques. Very long correlation lengths along the field lines are observed, which are consistent with nearly all models of turbulence in tokamak plasmas. The images are numerically filtered so as to isolate and display counter-propagating structures in the turbulent flow

Mitigation of plasma-wall interactions with low-Z powders in DIII-D high confinement plasmas

Experiments with low-Z powder injection in DIII-D high confinement discharges demonstrated increased divertor dissipation and detachment while maintaining good core energy confinement. Lithium (Li), boron (B), and boron nitride (BN) powders were injected in high-confinement mode plasmas ($I_p=$1 MA, $B_t=$2 T, $P_{NB}=$6 MW, $\langle n_e\rangle=3.6-5.0\cdot10^{19}$ m$^{-3}$) into the upper small-angle slot (SAS) divertor for 2-s intervals at constant rates of 3-204 mg/s. The multi-species BN powders at a rate of 54 mg/s showed the most substantial increase in divertor neutral compression by more than an order of magnitude and lasting detachment with minor degradation of the stored magnetic energy $W_{mhd}$ by 5%. Rates of 204 mg/s of boron nitride powder further reduce ELM-fluxes on the divertor but also cause a drop in confinement performance by 24% due to the onset of an $n=2$ tearing mode. The application of powders also showed a substantial improvement of wall conditions manifesting in reduced wall fueling source and intrinsic carbon and oxygen content in response to the cumulative injection of non-recycling materials. The results suggest that low-Z powder injection, including mixed element compounds, is a promising new core-edge compatible technique that simultaneously enables divertor detachment and improves wall conditions during high confinement operation

In-situ coating of silicon-rich films on tokamak plasma-facing components with real-time Si material injection

Experiments have been conducted in the DIII-D tokamak to explore the in-situ growth of silicon-rich layers as a potential technique for real-time replenishment of surface coatings on plasma-facing components (PFCs) during steady-state long-pulse reactor operation. Silicon (Si) pellets of 1 mm diameter were injected into low- and high-confinement (L-mode and H-mode) plasma discharges with densities ranging from $3.9-7.5\times10^{19}$ m$^{-3}$ and input powers ranging from 5.5-9 MW. The small Si pellets were delivered with the impurity granule injector (IGI) at frequencies ranging from 4-16 Hz corresponding to mass flow rates of 5-19 mg/s ($1-4.2\times10^{20}$ Si/s) at cumulative amounts of up to 34 mg of Si per five-second discharge. Graphite samples were exposed to the scrape-off layer and private flux region plasmas through the divertor material evaluation system (DiMES) to evaluate the Si deposition on the divertor targets. The Si II emission at the sample correlates with silicon injection and suggests net surface Si-deposition in measurable amounts. Post-mortem analysis showed Si-rich coatings of varying morphology mainly containing silicon oxides, with SiO$_2$ being the dominant component. No evidence of SiC was found, which is attributed to low divertor surface temperatures. The Si-rich coating growth rates were found to be at least $0.4-0.7$ nm/s, and the erosion rate was $0.1-0.3$ nm/s. The technique is estimated to coat a surface area of at least 0.94 m$^2$ on the outer divertor. These results demonstrate the potential of using real-time material injection to grow silicon-rich layers on divertor PFCs during reactor operation

Implementation of AI/Deep Learning Disruption Predictor into a Plasma Control System

This paper reports on advances to the state-of-the-art deep-learning disruption prediction models based on the Fusion Recurrent Neural Network (FRNN) originally introduced a 2019 Nature publication. In particular, the predictor now features not only the disruption score, as an indicator of the probability of an imminent disruption, but also a sensitivity score in real-time to indicate the underlying reasons for the imminent disruption. This adds valuable physics-interpretability for the deep-learning model and can provide helpful guidance for control actuators now that it is fully implemented into a modern Plasma Control System (PCS). The advance is a significant step forward in moving from modern deep-learning disruption prediction to real-time control and brings novel AI-enabled capabilities relevant for application to the future burning plasma ITER system. Our analyses use large amounts of data from JET and DIII-D vetted in the earlier NATURE publication. In addition to when a shot is predicted to disrupt, this paper addresses reasons why by carrying out sensitivity studies. FRNN is accordingly extended to use many more channels of information, including measured DIII-D signals such as (i) the n1rms signal that is correlated with the n =1 modes with finite frequency, including neoclassical tearing mode and sawtooth dynamics, (ii) the bolometer data indicative of plasma impurity content, and (iii) q-min, the minimum value of the safety factor relevant to the key physics of kink modes. The additional channels and interpretability features expand the ability of the deep learning FRNN software to provide information about disruption subcategories as well as more precise and direct guidance for the actuators in a plasma control system

Linear ideal MHD predictions for n = 2 non-axisymmetric magnetic perturbations on DIII-D

An extensive examination of the plasma response to dominantly n = 2 non-axisymmetric magnetic perturbations (MPs) on the DIII-D tokamak shows the potential to control 3D field interactions by varying the poloidal spectrum of the radial magnetic field. The plasma response is calculated as a function of the applied magnetic field structure and plasma parameters, using the linear magnetohydrodynamic code MARS-F (Liu et al 2000 Phys. Plasmas 7 3681). The ideal, single fluid plasma response is decomposed into two main components: a local pitch-resonant response occurring at rational magnetic flux surfaces, and a global kink response. The efficiency with which the field couples to the total plasma response is determined by the safety factor and the structure of the applied field. In many cases, control of the applied field has a more significant effect than control of plasma parameters, which is of particular interest since it can be modified at will throughout a shot to achieve a desired effect. The presence of toroidal harmonics, other than the dominant n = 2 component, is examined revealing a significant n = 4 component in the perturbations applied by the DIII-D MP coils; however, modeling shows the plasma responses to n = 4 perturbations are substantially smaller than the dominant n = 2 responses in most situations

Mitigation of plasma-wall interactions with low-Z powders in DIII-D high confinement plasmas

International audienceExperiments with low-Z powder injection in DIII-D high confinement discharges demonstrated increased divertor dissipation and detachment while maintaining good core energy confinement. Lithium (Li), boron (B), and boron nitride (BN) powders were injected in high-confinement mode plasmas ($I_p=$1 MA, $B_t=$2 T, $P_{NB}=$6 MW, $\langle n_e\rangle=3.6-5.0\cdot10^{19}$ m$^{-3}$) into the upper small-angle slot (SAS) divertor for 2-s intervals at constant rates of 3-204 mg/s. The multi-species BN powders at a rate of 54 mg/s showed the most substantial increase in divertor neutral compression by more than an order of magnitude and lasting detachment with minor degradation of the stored magnetic energy $W_{mhd}$ by 5%. Rates of 204 mg/s of boron nitride powder further reduce ELM-fluxes on the divertor but also cause a drop in confinement performance by 24% due to the onset of an $n=2$ tearing mode. The application of powders also showed a substantial improvement of wall conditions manifesting in reduced wall fueling source and intrinsic carbon and oxygen content in response to the cumulative injection of non-recycling materials. The results suggest that low-Z powder injection, including mixed element compounds, is a promising new core-edge compatible technique that simultaneously enables divertor detachment and improves wall conditions during high confinement operation

Sustained suppression of type-I edge-localized modes with dominantly n = 2 magnetic fields in DIII-D

Type-I edge-localized modes (ELMs) have been suppressed in DIII-D (Luxon et al 2003 Nucl. Fusion 43 1813) H-mode discharges with a H98Y2 confinement factor near 1.0 using magnetic perturbations (MPs) with dominant toroidal mode number n = 2. This expand

Recent Progress on Microwave Imaging Technology and New Physics Results

Strike-point sweeping and real-time-controlled (RTC) impurity seeding are both expected to be needed on JET following its upgrade to an all-metal wall with enhanced neutral-beam heating, thereby anticipating exhaust-control requirements plus the materials planned for ITER. Preliminary trials in the previous carbon device have combined these techniques in high-triangularity type I H-mode plasmas, using a VUV spectroscopic signal for feedback control of nitrogen injection. Compared with earlier unswept feedforward counterparts, similar strong mitigation of divertor heat load between ELMs was achieved in swept RTC cases for less than half the integrated nitrogen input and correspondingly less adverse effect upon other properties. Both sweeping and RT control contributed to this improvement. Time-average normalized energy confinement < H(98y)>(t) similar to 1, Greenwald density fraction < f(Gwd)>(t) similar to 0.9 and particularly purity denoted by effective ionic charge < Z(eff)>(t) approximate to 1.7, all remained closer to good reference levels. Transient effluxes in ELMs were also less affected, however, and would require separate active control

Isotopic Scaling of Confinement in Deuterium-Tritium Plasmas

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