PSFC Student Seminars

All Seminars are on Wednesday at 5:15pm, unless otherwise noted.
NW17-218, 175 Albany Street, Cambridge
For further information: info@psfc.mit.edu

Sep 20, 2022

Edge turbulence studies across confinement regime transitions at ASDEX Upgrade

Rachel Bielajew

MIT PSFC

Edge turbulence is thought to play an important role in tokamaks in the transition between the low confinement operating regime, L-mode, and high confinement operating regimes such as H-mode. High confinement regimes that are free of damaging Edge-Localized Modes, such as the “improved confinement regime” I-mode, are promising for future reactor operation.  However, open questions remain about the role of turbulence in regulating transport in I-mode, and changes in edge turbulence leading up to the I-mode and H-mode transitions. 

4:00pm  |  NW17-218

Dec 10, 2019

Non-relativistic pair plasma turbulence

Lucio Milanese

MIT

Achieving a more comprehensive understanding of electron-positron (pair) plasmas is important to interpret observations and explain the dynamics of astrophysical systems and phenomena such as the magnetosphere of pulsars, the accretion disks of black holes, gamma-ray bursts and astrophysical jets.  Magnetized electron-positron plasmas in the contexts of astrophysical jets and pulsar wind nebulae are thought to be in a turbulent state, as the large separation between the energy injection scale and the dissipation scale generates an extended turbulent inertial range. We present results from theoretical and numerical efforts aimed at elucidating the turbulent dynamics of strongly magnetized, low beta, sub-relativistic electron-positron plasmas. The key concepts of Kolmogorov energy cascade and critical balance will be introduced and discussed.   

5:00pm  |  NW17-218

Dec 3, 2019

Feasibility study of electron-scale electron temperature fluctuation diagnostics

Xiang Chen

MIT

Measurements of turbulent fluctuations of physical quantities such as density, temperature, electric field, play an important role in the study of transport. The electron-scale electron temperature fluctuations (denoted as T̃­­ee)are predicted to exist in fusion plasmas and they're also predicted to contribute significantly to heat loss in a fusion reactor. Diagnostics for T̃­­ee  is crucial for better understanding plasma transport and predicting the performance of plasmas in next-generation tokamaks like ITER and DEMO. However, to date, diagnostics for T̃­­ee  are still missing. We aim to use simulations to explore the feasibility of making measurements of T̃­­ee  in the core plasma of tokamaks and stellarators, which paves the way for further hardware installation of this diagnostic on a fusion device. The simulations are carried out using CGYRO, a gyrokinetic code for collisional plasmas developed by General Atomics, and the results are compared with the ones obtained with an old code GYRO. The good agreements between the two codes lay a solid foundation for further study.

5:00pm  |  NW17-218

Dec 3, 2019

Inverse transfer of magnetic energy through magnetic reconnection

Muni Zhou

MIT

A wide range of space and astrophysical systems, such as the solar corona, heliosheath and Weibel-produced magnetic field in supernova shocks, of which the dynamics are governed by turbulence and reconnection, can be conceptualized as an ensemble of interacting flux ropes. We investigate magnetic field dynamics in a system of parallel flux ropes as well as more generic magnetically-dominated turbulent systems, focusing on the inverse magnetic energy transfer.  An analytical model is introduced and shown to capture the evolution of the main quantities of interest, as borne out by our 2D and 3D reduced magnetohydrodynamics (RMHD) and 2D particle-in-cell simulations. Magnetic reconnection is identified as the key mechanism enabling the inverse transfer and setting its properties: magnetic energy decays as T̃­­-1, where  T̃­­ is time normalized to the reconnection timescale; and the field correlation length grows as T̃­­1/2.  Critical balance is shown (by magnetic structure functions) to govern the aspect ratio of the flux ropes in 3D RMHD simulations. This quantitative description of inverse energy transfer could improve our understanding of longstanding problems such as coronal heating, galactic magnetogenesis, and high-energy emission in gamma-ray bursts.

5:00pm  |  NW17-218

Nov 26, 2019

Quench dynamics of a HTS cable and potential quench detection systems

Erica Salazar

MIT

High temperature superconductors (HTS) are poised to revolutionize magnetic confinement fusion tokamaks by enabling new high-field tokamak designs that are smaller, cost less, and are faster to build than present low temperature superconductor (LTS) based devices.This presentation will discuss the steps required to 1) experimentally characterize the quench behavior of an HTS cable design 2) extrapolate the test data results using MATLAB to model the thermal hydraulic behavior and quench analysis within a high-field environment and 3) develop a novel quench detection system for the high-field HTS tokamak magnet system    

5:00pm  |  NW17-218

Nov 19, 2019

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

Nov 19, 2019

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

Nov 12, 2019

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

Nov 12, 2019

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

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