2D BES, UF-CHERS, and CXI at DIII-D
Achieving fusion energy will require a comprehensive understanding of turbulence and instabilities in high temperature magnetically confined plasma. This research program develops and operates a suite of spectroscopic instruments to observe the detailed dynamics of plasma turbulence and instabilities at the DIII-D National Fusion Facility, one of the most sophisticated and well-diagnosed fusion experiments in the world.
This collaborative program has developed several advanced diagnostic systems to measure turbulence and other instabilities. A high performance density fluctuation diagnostic system, Beam Emission Spectroscopy (BES), provides 2D fluctuation imaging measurements at high spatial (~1 cm) and high time resolution (up to 1 microsecond) by utilizing high-efficiency optical components (optical fibers, specialized light filters, lenses), customized electronics and light detectors. An ion temperature and rotation velocity fluctuation diagnostic system, Ultra-Fast Charge Exchange Recombination Spectroscopy (UF-CHERS), has also been deployed at DIII-D and has been upgraded with a much improved detector system. We are also developing a charge-exchange-emission-imaging (CXI) system to achieve higher spatial resolution (~3 mm) fluctuation measurements in the important boundary zone by observing emission from charge exchange reactions between carbon ions and neutral beam atoms. CXI will measure small scale pedestal instabilities and sharp edge gradients. These spectroscopic systems probe the nature, characteristics and scaling properties of turbulence in DIII-D plasmas.
2D BES at NSTX-U
UW has developed and deployed a high-performance Beam Emission Spectroscopy (BES) diagnostic system on NSTX-U to observe the plasma fluctuations associated with turbulence and instabilities. These instabilities include ion temperature gradient-driven turbulence, trapped-electron modes, microtearing modes and kinetic ballooning modes that exist at the ion gyroradius scale (~1 cm in NSTX-U plasmas); meso-scale instabilities at low-order rational surfaces; and Alfven instabilities that drive the loss of energetic particles. The recently expanded BES diagnostic features 64 channels in a 2D viewing geometry to image turbulence and measure radial and poloidal characteristics, flows, and wavenumber spectra.
The 2D BES system developed and implemented by UW will facilitate measuring and understanding the turbulence and instabilities in high-performance NSTX-U plasmas. The upgraded features of NSTX-U include higher magnetic field (1 T), higher current (2 MA), long pulse operation (5 seconds), and additional tangential beam injection for toroidal rotation and off-axis current drive. NSTX-U will probe low collisionality regimes projected to exhibit improved confinement. The high-normalized pressure and tangential beam will modify the safety factor profile that enables steady-state advanced scenarios and strongly impacts turbulence and turbulent transport. Macroscopic stability, transport, turbulence and current drive are tightly coupled in these high-performance scenarios. The distinct physics of the edge and pedestal region is a topic of special emphasis due to unresolved questions of the LH transition, steep gradients in pressure and flows, and tight coupling to core performance. The proposed research program contributes to these scientific areas.
2D BES at W7-X
Turbulent transport in W7-X will be a complex interaction of ITG/TEM instabilities, about 50 degrees of freedom for 3D shaping, connection length variation, magnetic curvature and wells, ion-root and electron-root radial electric field solutions for ambipolarity, and weak damping of zonal flows. To investigate the physics of turbulence and transport in W7-X, the University of Wisconsin-Madison is conducting a feasibility study for 2D multi-field turbulence measurements with a fluctuation Beam Emission Spectroscopy (FBES) diagnostic system. The feasibility study will assess the diagnostic and technical considerations for 2D measurements of density and flow field fluctuations, but also state-of-the-art techniques for ion temperature, electric potential, impurity, and magnetic field fluctuations. The feasibility study will also address technical solutions for high throughput optical measurements in a superconducting, long-pulse device with high heat loads on plasma-facing components.
BES and real-time ML for plasma control
This multi-institution project will develop a hierarchy of Artificial Intelligence(AI)/Deep Learning(DL)/Machine Learning (ML) techniques for real-time fusion plasma prediction and control. Facets of the project are to 1) enable real-time analysis of high resolution diagnostics, 2a) label and predict proximity to instability limits, 2b) produce real-time control-relevant predictions of plasma evolution that are difficult to obtain from physics simulations alone, and 3) manipulate experimental actuators in real-time. The UW BES group will develop and deploy “edge ML” resources for the real-time analysis and featurization of 2D BES data at DIII-D. The central plasma control system will integrate real-time signals from BES edge-ML calculations for real-time prediction and control.
Experimental study of turbulent impurity transport in 3D magnetic fields
Detailed understanding and control of impurity transport is essential for the success of magnetic fusion as an energy source. Impurities dilute the D-T fuel and degrade the energy confinement of high temperature plasmas via intense line radiation such that the accumulation of these charged particles must be avoided. The impurity transport can be described by neoclassical and turbulent contributions. While turbulent transport is mainly diffusive, neoclassical transport has a strong convective term. In particular for stellarators, this is challenging as neoclassical transport in these 3D devices is typically directed inwards. A certain level of diffusive transport from turbulence might therefore be required for stable plasma operation. However, it is for instance not clear whether the required level of turbulence would overly degrade the energy confinement via enhanced heat transport. This proposal will therefore investigate turbulent impurity transport at the Helically Symmetric eXperiment (HSX) in Madison, USA and at the Wendelstein 7-X (W7-X) stellarator in Greifswald, Germany. The two experiments feature different 3D magnetic field topologies and allow transport studies from different perspectives. Existing laser ablation systems at both experiments will be equipped with optimized and calibrated glass targets that will allow active impurity injections with a well-controlled amount of particles. Moreover, a high speed charge exchange recombination spectroscopy system will be installed to monitor characteristic line radiation emitted by the injected impurities. The spectroscopy measurements will provide absolutely calibrated impurity densities with excellent spatial resolution and will resolve the inward movement of impurities toward the plasma core following the injections. Combined with further improvements of the analysis and modelling tools, details on the diffusive and convective impurity transport can thus be obtained. Resulting transport profiles will then be compared with neoclassical expectations. Moreover, trends of the remaining turbulence-induced (anomalous) transport will be compared with results from linear and non-linear gyrokinetic simulations. In addition, the observed levels of impurity transport will be compared with experimental heat diffusivities. This will permit the identification of conditions with good energy confinement and a well controlled impurity behavior.
Exploring ion heat transport during neutral beam heated plasmas at W7-X
The stellarator concept is an attractive approach for fusion energy production, as the confining 3D magnetic field structure can be generated without strong internal plasma currents. However, recent results from the neoclassical-transport-optimized W7-X stellarator show that the required excellent ion heat confinement is only observed during transient phases in presence of peaked density profiles. Theory suggests that the improved confinement is related to a strong reduction in ion-temperature gradient (ITG) turbulence in presence of local density gradients, but more studies addressing this important problem from different perspectives are required. Here, we propose to perform a detailed comparison of experimental data vs. theory for W7-X plasmas with strong neutral beam injection (NBI) heating.
NBI heat deposition profiles and the level of charge exchange losses will be determined and validated using Balmer alpha spectroscopy, as these two quantities are essential for accurate calculations of the ion heat diffusivity during NBI heated experiments. Moreover, information on the ion heat-pulse diffusivity will be obtained from NBI modulation experiments. Such perturbation experiments are a widely applied method in fusion research and have become feasible at W7-X thanks to a new charge exchange spectroscopy system operated by UW Madison that allows for measurement of local impurity ion temperatures at different radial positions.
The obtained heat diffusivities will be compared to neoclassical transport predictions to estimate the remaining anomalous transport. The resulting anomalous heat diffusivities will then be analyzed using linear and non-linear gyrokinetic simulations. Synthetic diagnostics will be developed to additionally compare the gyrokinetic simulation results to fluctuation measurements that will provide a better understanding of the specific impact of the 3D geometry and profiles on turbulence. In addition, the validated nonlinear simulations will themselves be used to develop reduced turbulent transport models. The vision is to work towards the identification of mechanisms to alter the magnetic field configuration resulting in reduced ITG turbulent transport.
Measurement of turbulent electric fields in high temperature plasmas
Detailed understanding of turbulence and magneto hydrodynamic (MHD) modes is essential for the success of magnetic confined fusion as these mechanisms are responsible for a significant reduction of the energy confinement time in high temperature plasmas. Here we propose to further develop a diagnostic technique for local measurements of electric and magnetic field fluctuations. The measurement technique is based on the analysis of the Doppler shifted, Stark split radiation present along neutral beams. The Stark splitting appears due to strong v x B electric fields, experienced by the injected neutrals, as well as ambient electric fields. First experimental studies at the DIII-D National Fusion Facility demonstrated excellent sensitivity using a new prototype high throughput spectrometer. This spectrometer will be further improved using custom-made lens systems that minimize photon losses and a new detector solution will be tested and implemented. Moreover, a second high throughput, high-speed spectrometer will be installed that — together with the existing spectrometer — will allow simultaneous measurements at two neighboring locations inside the plasma. Thus, cross-correlation studies, as well as studies of the radial dependence of the impact of modes and turbulence on the plasma confinement, will become possible. For the quantitative interpretation of the measurement data, detailed modelling of synthetic spectra will be performed using the FIDASIM code which considers the detailed 3D shape of neutral beam injected systems, as well as various effects that affect the spectral shape of the Stark spit beam emission. Finally, dedicated experiments will be conducted at the DIII-D tokamak to evaluate the performance of the two spectrometer systems and to demonstrate their improved capabilities in resolving MHD- and turbulence-induced fluctuations.
Entangled two-photon absorption to pump excited populations
Entangled two-photon absorption (ETPA) has the potential to pump a fluorescing excited state population with low incident intensity and high state selectivity for innovative quantum-enhanced plasma spectroscopy measurements. The time-frequency entanglement of entangled photon pairs allows for the simultaneous arrival of entangled pairs at the target location and a narrow bandwidth for the sum frequency. The simultaneous arrival gives a linear scaling of the ETPA cross section with the incident photon flux compared to a quadratic scaling for two-photon absorption (TPA) with classical light. ETPA reduces incident flux requirements for chemical sensing and biological microscopy by up to seven orders of magnitude compared to classical TPA, and we are hopeful to realize a similar quantum enhancement for ETPA in plasma. The lower incident flux for ETPA can be compatible with a low-intensity continuous laser, in contrast to classical TPA which requires a high-intensity, pulsed laser. Finally, the narrow bandwidth of the sum frequency can precisely target an excited state with minimal contamination into non-target states, and the two-photon process is inherently compatible with cross-beam spatial localization. Potential diagnostic schemes include pumping low-n states for low-Z impurities, high-n Rydberg states for high-Z impurities, and charge-exchange populations.