PSFC Student Seminars

Modeling LH wave reduction through SOL blobs using synthetic turbulence data

Bodhi Biswas

MIT

Lower hybrid (LH) waves are an efficient means to drive off-axis current in a tokamak. Presently, both ray-tracing and full-wave simulations are unable to match experimental current drive (CD) profiles in Alcator C-Mod. The likely cause is scrape-off-layer (SOL) turbulence interactions that affect wave propagation. Synthetic SOL turbulence that account for intermittent blob-like structures is coupled to the ray-tracing/Fokker-Planck model GENRAY/CQL3D. In a slab geometry, refraction through blob-like turbulence is shown to result in increased wave scattering compared to previous models that assumed non-intermittent turbulence. This model is next used to study the effects of SOL refraction on power deposition and CD in an Alcator C-Mod geometry. Initial results show that the presence of SOL blobs lead to higher on-axis damping and smoother current profiles, which better match experiment.

5:00pm  |  NW17-218

Studying lower hybrid wave propagation and absorption with full-wave simulations

Sam Frank

MIT

In the simulation of lower-hybrid current drive in tokamaks ray-tracing is currently the workhorse simulation tool used to design experiments. However, ray-tracing has yet to be extensively validated against full-wave simulations. Due to recent advancements in computation it is now possible to simulate lower-hybrid wave propagation in medium-sized tokamaks by a direct solve of the wave equation after it has been Fourier analyzed for a single frequency. Simulations such as these are of significant interest since they are capable of simulating weak-damping scenarios in modern tokamaks where current ray-tracing techniques’ assumptions could possibly break down. However, calculations of the non-Maxwellian damping of the lower-hybrid wave requires an iteration between the full-wave solver and a 3D Fokker-Planck solver in order to self-consistently model the wave fields. Techniques for iteration between the TORLH full wave and the CQL3D Fokker Planck codes by coupling the two codes with a quasi-linear RF diffusion coefficient will be shown and the results of these iterations and their implications for lower-hybrid current drive theory will be discussed.

5:00pm  |  NW17-218

The low frequency edge oscillation in I-mode

William McCarthy

MIT

The I-mode confinement regime is characterized by H-mode like thermal confinement, L-mode like particle confinement and being ELM free, making it a good candidate for reactor scenarios. The Weakly Coherent Mode, a broad fluctuation (∼200 kHz central frequency on C-mod) localized to the pedestal region is thought to cause the enhanced particle transport. A second mode, with much lower frequency (∼15 kHz), has been observed in I-mode discharges. The mode spans the last closed flux surface and can be seen on divertor Langmuir probes as spikes in ion saturation current, and in a variety of other diagnostics. This mode likely contributes to I-mode transport. A database containing a large number of I-mode discharges has been assemble to investigate key questions: parameter space dependence on mode existence, central frequency and frequency width. Temporal dynamics of the mode have been explored using a scanning Langmuir Mach probe with a Mirror Langmuir probe bias system.

5:00pm  |  NW17-218

Learning pedestal dynamics via training reduced models against experimental tokamak plasmas

Abhilash Mathews

MIT

The outer edge region of high confinement tokamak plasmas, known as the pedestal, is associated with the formation of transport barriers. This strongly influences energy and particle confinement, and in turn the energy gain of tokamaks which is crucial for upcoming devices (e.g. SPARC, ITER), yet a fully predictive model of pedestal structure is currently lacking. Pedestal pressure is constrained by magnetohydrodynamic limits due to edge localized modes (ELM), but a general model of pedestal density and temperature in ELM-suppressed regimes is absent. Therefore, this work explores potential methods for evaluating reduced plasma transport models across the pedestal against experiment. Towards this goal, an adaptive Gaussian process regression routine for automating time-dependent​ analysis of the pedestal has been developed and will be outlined. This tool can assist with interpretive modelling, improving inputs for simulations sensitive to gradients, validation efforts, and generating training data for supervised machine learning. ​

5:00pm  |  NW17-218

Parameter Dependencies of the Configuration-Dependent 1-2 kHz Fluctuation in W7-X

Sean Ballinger

MIT

A 1–2 kHz fluctuation is present in a large fraction of discharges in the W7-X stellarator's standard magnetic field configuration with edge iota=0.97 and 5/5 island structure. The fluctuation is present in edge electric and magnetic fields, as well as plasma density, temperature, and radiated light. The surface temperature of divertor tiles can fluctuate by 2.5 ºC at 1–2 kHz, indicating that the fluctuation also has an impact on power handling in W7-X. Its cause and precise location in the plasma are still unknown, and a variety of factors determine its peak frequency and power. In this work, a survey using magnetic pick-up coil data finds that the toroidal bootstrap current is positively correlated with the fluctuation's peak frequency and power. The fluctuation power is highest at high electron cyclotron heating power and decreases with plasma density. A stellarator power plant with higher heating power might therefore need to contend with a stronger fluctuation in divertor heat flux. Finally, shedding some light on the location of the fluctuation in the plasma, electron cyclotron temperature measurements show that the electron temperature fluctuates throughout the plasma minor radius, but with gaps where no fluctuation is observed.   

5:00pm  |  NW17-218

Design evaluation methods and regulatory frameworks for off-site commercial fusion hazard

Patrick White

MIT

The successful commercialization of fusion energy will require the development or adoption of design evaluation methods to assess the safety of fusion facilities and regulatory frameworks to oversee design and operation. The design evaluation methods and regulatory frameworks selected for commercial fusion will have a significant impact on the both the physical design and economic viability of future facilities. Understanding these impacts, and selecting appropriate methods and frameworks will be one of the keys to the successful development and deployment of fusion energy. In this talk, we will identify major offsite hazards of commercial-scale fusion and estimate the potential offsite consequences of worst-case releases of tritium. The use of different design and analysis methods to reduce these consequences to acceptable levels will be presented in the context of their compatibility with common design evaluation methods. Finally, impacts of these design evaluation methods on potential regulatory frameworks and overall licensing burden for commercial fusion will be discussed.

5:00pm  |  NW17-218

Hysteresis as a probe of turbulent bifurcation in intrinsic rotation reversals on Alcator C-Mod

Norman Cao

MIT

Analysis and modeling of a new set of rotation reversal hysteresis experiments unambiguously show that changes in turbulence are responsible for the intrinsic rotation reversal and the Linear to Saturated Ohmic Confinement (LOC/SOC) transition on Alcator C-Mod. Plasmas on either side of the reversal exhibit different toroidal rotation profiles and therefore different turbulence characteristics despite profiles of density and temperature that are indistinguishable within measurement uncertainty. The deactivation of subdominant (in linear growth rate and heat transport) ITG and TEM-like instabilities in a mixed-mode state is identified as the only possible change in turbulence within a quasilinear transport approximation across the reversal which is consistent with the measured profiles and the inferred heat and particle fluxes. This indicates an explanation for the LOC/SOC transition that provides a mechanism for hysteresis through the dynamics of subdominant modes and changes in their relative populations, and does not involve a change in most (linearly) unstable ion-scale drift-wave instability.

5:00pm  |  NW17-218

Measurements of ion-electron equilibration using ion stopping power measurements

Patrick Adrian

MIT

During the thermonuclear-burn phase of an inertial confinement fusion (ICF) implosion, alpha particles primarily deposit energy to the electron which drive the electrons out of thermal equilibrium with the ions. Since the fusion rate is sensitive to the ion temperature, accurate models for ion-electron equilibration are required to capture the thermal evolution of both species. Currently, there are numerous theoretical studies which model the equilibration process in conditions relevant to the hot spot of an ICF implosion. However, there is a lack of experimental data to constrain these models. Here we present precision measurements of ion-electron equilibration rates in the core of exploding pusher implosions at OMEGA. This is indirectly done by measuring the stopping power at low velocities, which is dictated by the same transport coefficient as ion-electron equilibration. The work was supported by DOE, NLUF, CoE and LLE.  

5:00pm  |  NW17-218

Scalings for laser driven proton acceleration in the multi-ps regime

Raspberry Simpson

MIT

In an effort to investigate proton acceleration in the unique laser parameter regime that Advanced Radiographic Capability (ARC) inhabits, a series of experiments were performed at the National Ignition Facility with the TITAN laser at the Jupiter Laser Facility (JLF). TITAN is a high repetition rate laser, which allowed for a detailed study of how accelerated proton energies scale with key laser parameters like pulse length, laser energy and laser focal spot size. This work details the results of this study and presents a new preliminary scaling for accelerated proton energies via TNSA for multi-ps lasers.

5:00pm  |  NW17-218

Using Secondary DT Neutrons to Infer Fuel Convergence and Areal Density Asymmetries in NIF Implosions

Brandon Lahmann

MIT

In deuterium-filled inertial confinement fusion (ICF) implosions, DD-tritons can undergo secondary fusion reactions with the thermal deuterium plasma to create secondary DT neutrons. On the National Ignition Facility (NIF), both the primary reactions (via DD-neutrons) and the secondary DT neutrons are routinely measured from several lines of sights using neutron time of flight (nTOF) spectrometers. The ratio of these secondary and primary reactions are used to infer the areal density (ρR) and the convergence of the fuel region. Additionally, the shape of the secondary DT neutron spectra can be used to infer the final asymmetry of the imploded capsule. Convergences inferred using x-ray imaging techniques are consistently larger than those inferred by this secondary DT neutron technique. These apparent discrepancies are not currently understood, but potential explanations are discussed.

5:00pm  |  NW17-218

Design and installation of a 1D Lyman-alpha camera for Edge Neutral Studies on DIII-D

Aaron Rosenthal

MIT

Recently, a newly developed 1-D Lyman-alpha diagnostic was installed on DIII-D. The diagnostic consists of two pinhole cameras providing edge Lyman-alpha emission profiles on the low field and high field side of the tokamak. The Lyman-alpha camera is intended to provide an improved characterization of neutrals for DIII-D by measuring the Lyman-alpha brightness. The views of the camera were informed by a synthetic diagnostic using SOLPS simulations of edge Lyman-alpha brightness. The camera is intended to investigate divertor leakage, main chamber fueling and radial particle transport. This talk will focus on the design challenges, fabrication and installation of the pinhole camera on DIII-D.

5:00pm  |  NW17-218

Validation of impurity transport models in tokamak plasmas

Francesco Sciortino

MIT

High performance tokamak operation places challenging constraints on the presence of impurities in both the core and edge plasma regions. Consequently, neoclassical and turbulent impurity transport must be understood, controlled and optimized for integrated scenarios.  In this talk, I will introduce some aspects of particle transport modeling and experiments. In addition, I will describe recent attempts to experimentally test predictions from both neoclassical and turbulent transport theory.

5:00pm  |  NW17-218

Dielectric Multipactor with High Power mm-Waves

Sam Schaub

MIT

5:00pm  |  NW17-218

Implementation of Gas Puff Imaging (GPI) system on TCV and investigation of the GPI shadowing effect

Woonghee Han

MIT

Gas-Puff Imaging (GPI) measures the spatially-resolved fluctuations in the plasma edge and SOL by imaging emission from a local gas puff. In August 2018, a GPI system has been installed on TCV in Switzerland. First data were obtained in December 2018 and there is great signal-to-noise ratio for views around the spatial peak of the emission. GPI also captured the propagation of a blob in the SOL. It is typically assumed that the fluctuation in the emission in GPI is correlated with the plasma fluctuations without regarding the neutral density fluctuations. However, according to the GBS simulation result the neutral density fluctuation has significant impacts on the light emission. The neutral fluctuation results in the GPI “shadowing” effect which is expected to be prominent at the distance from the nozzle larger than the neutral mean-free-path. Experiments have been designed to explore the shadowing effect by comparing pairs of cases with different neutral mean-free-path in an identical background plasma.

5:00pm  |  NW17-218

Transport of alpha particles by 3D fields and MHD modes: perspectives from theory and modeling with implications for experiment

Elizabeth Tolman

MIT

Next-generation tokamak experiments operating with DT fuel will have a significant population of energetic alphas from fusion. Good confinement of these alpha particles is important to experiment performance. Two effects which can degrade alpha confinement are ripple and Alfvén Eigenmodes. In this tutorial-style talk, we provide a brief introduction to three theoretical and computational methods for studying transport of alphas by these effects: collisionless guiding center calculations, drift-kinetic theory, and Monte Carlo orbit-following codes.  In addition, we offer a preview of recent work to advance understanding in these areas.

5:00pm  |  NW17-218

Imaging Helium emission on TCV

Bryan Lee Lineman

MIT

The ratio of atomic line intensities from species of the same charge state are functions of Te and ne.  Experiments were performed last December with the aim of utilizing this relationship to measure Te and ne in over the divertor in 2D space on TCV.  Seven Helium lines were imaged simultaneously in the divertor with the new MANTIS diagnostic. This experiment and preliminary data analysis will be discussed at this talk.   
 

5:00pm  |  NW17-218

Observations of kinetic and multi-ion effects in DT and D3He implosions relevant to ICF shock phase

Neel Kabadi

MIT

During the shock-convergence phase of ICF implosions there are steep spatial gradients and the ion mean free path becomes long compared to the system size, indicating that multi-ion and kinetic effects may be important. Yet, almost all ICF simulations use an average-ion hydrodynamic approach. Previous work has indicated substantial burn averaged species separation and possibly other kinetic effects in D³He plasmas with conditions relevant to the NIF shock-phase. In this presentation I will show recent work conducted on the Omega laser facility recreating these conditions in DT plasmas. Both DT and D3He burn averaged observables are well modeled using an equilibrating two ion temperature model with little to no species separation. 

5:00pm  |  NW17-218

Results from fusion-based backlighter development at the OMEGA laser

Graeme Sutcliffe

MIT

Laser-driven implosions of D³He-filled capsules which generate mono-energetic 14.7-MeV and 3.0-MeV protons are used on the OMEGA and NIF lasers for both radiography and stopping-power studies. A new tri-particle mono-energetic backlighter based on a DT³He gas-filled capsule implosion that provides 14.7-MeV and 3.0-MeV protons plus 9.5-MeV deuterons from the T+³He reaction has now been demonstrated on OMEGA. Initial tests using 860 µm OD thin glass capsules filled with DT³He fuel were promising. Preliminary radiographs of laser-driven foils and measurements of stopping power in cold beryllium were made with the backlighted particles and the results are shown.

5:00pm  |  NW17-218

Physics precursors of disruptions and the challenge of predicting them on multiple tokamaks

Kevin Montes

MIT

5:00pm  |  NW17-218

Multipactor and dark current studies of a 17 GHz standing wave accelerator structure

Haoran Xu

MIT

We report our study on the internal dark current generation by multipactor inside a 17 GHz single cell standing wave disk-loaded waveguide accelerator structure. 

5:00pm  |  NW17-218

Design and characterization of a megawatt-class laser-driven semiconductor switch with application to high-gradient accelerator testing

Julian Picard

MIT

Laser-driven semiconductor switches (LDSS) are of great use in producing short RF pulses in the microwave to far-infrared range. Such short pulses have potential applications ranging from the investigation of material properties to testing of high-gradient accelerator concepts and DNP-NMR.

5:00pm  |  NW17-218

The 1-2 kHz fluctuation in W7-X and its impact on divertor heat loads

Sean Ballinger

PSFC

Fast-camera imaging in the visible range has been used to study the motion of high-density filaments in tokamak edge plasmas, shedding light on their role in boundary layer transport.

5:00pm  |  NW17-218

Diagnosing fuel areal density asymmetries in deuterium-tritium inertial

Raspberry Simpson

PSFC

In this work, we discuss the knock-on deuteron data obtained in the 1-D cryogenic DT Campaign at OMEGA. Preliminary data analysis reveals that there is a systematic asymmetry in the direction of one of the charged particle spectrometers of approximately 20%.

5:00pm  |  NW17-218

Demonstration of Prototype Mirror Langmuir Probe Control System Using a Red Pitaya Field Programmable Gate Array Board

William McCarthy

PSFC

High bandwidth, high spatial resolution measurements of electron temperature, density and plasma potential are valuable for resolving turbulence in the boundary plasma of tokamaks. While Langmuir probes can provide such measurements their temporal and spatial resolution is limited by the sweep rate for obtaining I-V characteristics or by the need to use multiple electrodes, each sampling a single plasma quantity at high bandwidth.

5:00pm  |  NW17-218

Can we truly isolate divertor operations from the core plasma?

Adam Kuang

PSFC

A set of experiments were performed in Alcator C-Mod during the 2016 campaign to attempt to better understand the impact of divertor conditions on scrape-off layer transport. Preliminary experimental and simulation results indicate that a combination of the closed divertor geometry in Alcator C-Mod, and magnetic shear enhanced polarization currents near the X-point effectively isolate divertor conditions from the main chamber and suggest a possible pathway forward.

5:00pm  |  NW17-218