Quick Links

Presentation Abstract 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.

Presentation Abstract 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.

Presentation Abstract 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.