Speaker: William Young (General Fusion): Accurate temperature measurements are critical to establishing the behavior of General Fusion’s SPECTOR plasma injector, both before and during compression. As compression tests impose additional constraints on diagnostic access to the plasma, a two-color, filter-based soft x-ray temperature diagnostic has been implemented. The soft x-ray diagnostic has been validated against a Thomson scattering system on an uncompressed version of SPECTOR with more diagnostic access. The multipoint Thomson scattering diagnostic also provides up to a six point temperature and density profile, with the density measurements validated against a far infrared interferometer. Temperatures above 300 eV have been demonstrated to be sustained for over 500 microseconds in uncompressed plasmas. Optimization of soft x-ray filters is ongoing, in order to balance blocking of impurity line radiation with signal strength.
Speaker: Meritt Reynolds (General Fusion): General Fusion (GF) is working to build a magnetized target fusion (MTF) power plant based on compression of magnetically-confined plasma by liquid metal. GF is testing this compression concept by collapsing solid aluminum liners onto plasmas formed by coaxial helicity injection in a series of experiments called PCS (Plasma Compression, Small).
We simulate the PCS experiments using the finite-volume MHD code VAC. The single-fluid plasma model includes temperature-dependent resistivity and anisotropic heat transport. The time-dependent curvilinear mesh for MHD simulation is derived from LS-DYNA simulations of actual field tests of liner implosion.
In previous work the 3D MHD simulations reproduced the appearance of n=1 mode activity in experiments performed in negative D-shape geometry (MRT and PROSPECTOR machines) and predicted better stability during compression in positive D-shape geometry similar to most spherical tokamaks (SPECTOR machine). Here we discuss simulations of the recent SPECTOR experiments PCS13 and PCS14 and the upcoming experiment PCS15.
Comparison of simulated Mirnov and x-ray diagnostics with experimental measurements will be presented, showing that PCS14 compressed well to a linear compression ratio of 2.5:1. Simulation indicates that the electron temperature rose from from 200 eV to 300 eV and the plasma pressure increased by more than a factor of 10.
Speaker: Dr. Peter O'Shea (General Fusion): Magnetic reconstruction on laboratory plasmas is a standard tool at General Fusion. While development of a polarimeter progresses, our reconstructions on laboratory based plasma injectors rely solely on edge magnetic (“Bdot”) probes. On plasma experiments built for field compression (PCS) tests, the number and locations of Bdot probes is limited by mechanical constraints. Additional information about the magnetic structure of our plasmas, especially near the core, is needed. Fortunately we have been able to infer much about the q profiles in our Spector plasmas by using passive MHD spectroscopy. The coaxial helicity injection (CHI) process of forming our compact toroid (CT) plasmas naturally generates very hollow current profiles. This causes reverse shear magnetic configurations in our early plasmas. Central Ohmic heating naturally peaks the temperature and thus current profiles as our plasmas evolve in time. This peaking of the current profile leads to a simultaneous reduction of the core safety factor (q(0)) and a reduction in the reverse shear. As the central, non-monotonic q profile hits rational flux surfaces, we observe on both edge magnetic probes and soft X-ray diagnostics transient magnetic reconnection events (MRE’s) due to the double tearing mode. Modal analysis and observations of the changes in these MRE’s as we change the currents in our plasmas allows us to infer the q surfaces involved in each burst. Many plasma discharges have several MRE’s in succession allowing us to estimate the continuous evolution of the core q profile in our shots. This information greatly enhances our certainty of the overall q profile when combined with edge magnetic probes.
Speaker: Dr. Stephen Howard (General Fusion): Achieving net energy gain with a Magnetized Target Fusion (MTF) system requires the initial plasma state to satisfy a set of performance goals, the conceptually simplest of these include total particle inventory (~10^21 ions), sufficient magnetic flux (~0.3 Wb) to confine the plasma inventory without becoming MHD unstable, and initial energy confinement time several times longer than the compression time. Other requirements are more complex and flexible; initial temperature can be traded off against increasing maximum compression ratio, device size can be reduced by increasing density, and limits on confinement time can be dealt with by speeding up the compression. General Fusion (GF) is now constructing Plasma Injector 3 (PI3) to explore the physics of reactor-scale plasmas and to demonstrate performance goals on total inventory, magnetic flux, and energy confinement time. These goals are a >10x increase from previous MTF experiments completed by GF. Energy considerations lead us to design around an initial state of Rvessel = 1 m. PI3 will use fast coaxial helicity injection via a Marshall gun to create a spherical tokamak plasma, with no additional heating. MTF requires solenoid-free startup with no vertical field coils, and will rely on flux conservation by a metal wall. PI3 is a continuation of the SPECTOR sequence of devices, so increasing device radial size by 5x is expected to yield magnetic lifetime increase of 25x, while peak temperature of PI3 is expected to be similar (400-500 eV) to what has been achieved on SPECTOR. Physics investigations will study MHD activity and the resistive and convective evolution of current, temperature and density profiles. We seek to understand the confinement physics, radiative loss, thermal and particle transport, recycling and edge physics of PI3.
Speaker: Dr. Michel Laberge (General Fusion): General Fusion is developing Magnetized Target Fusion (MTF), using pneumatically driven liquid metal to compress a plasma. Prior to construction of a full scale pneumatic system, General Fusion is investigating the physics of compressing compact toroid plasmas using solid aluminum liners accelerated inward by a chemical driver to compress the plasma. Fourteen such tests have so far been conducted, and results from these experiments will be presented. The MHD code VAC (Versatile Advection Code) has been modified to work with moving boundary conditions in order to model these implosions, and simulation runs of these models will also be presented.
This poster was presented at the 2017 Exploratory Plasma and Fusion Research Workshop, and provides an overview of the physics objectives for the PI3 large plasma injector program at General Fusion.
This poster was presented at the 2017 Exploratory Plasma and Fusion Research Workshop, and provides an overview of Passive MHD Spectroscopy For Augmenting Magnetic Reconstructions on Spherical Tokamak Plasmas at General Fusion.