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