Graduate Program Events |
Events on Wednesday, April 14th, 2021
- Stellarator Beta Limits with Extended MHD Modeling Using NIMROD
- Time: 1:00 pm
- Place:
- Speaker: Torrin Bechtel, Physics PhD Graduate Student
- Abstract: The nonlinear, extended MHD code NIMROD is employed to simulate self-consistent stellarator behavior at high beta. Finite anisotropic thermal conduction allows for sustained pressure gradients within stochastic regions. The configuration under investigation is an l=2, M=10 torsatron with vacuum rotational transform near unity. Finite-beta plasmas are generated from vacuum fields using a volumetric heating source and temperature dependent resistivity. With realistic parameters the configuration is unstable to interchange, which acts to limit the achievable beta. Simulations performed in a single field period domain do not exhibit a complete crash from the instability, but otherwise closely match theorized linear and nonlinear interchange behavior. In more dissipative regimes where instability is suppressed, steady-state solutions are obtained. A conventional equilibrium beta limit is observed due to pressure induced stochastic magnetic field formation. The parametric dependence of the equilibrium limit is examined in detail. Equilibrium results are compared with several reduced models for effective collisional transport across stochastic magnetic fields and with the HINT code. Collisionality independent models also are investigated for more realistic parallel thermal conduction rates. Join Zoom Meeting Meeting ID: 913 2120 9265 Passcode: 298219
- Host: Chris Hegna, Faculty Advisor
- Overlapping Aluminum-Gate Quantum Dots for Valley-Orbit Based Qubits in Si/SiGe
- Time: 4:00 pm
- Place:
- Speaker: JP Dodson, Physics PhD Graduate Student
- Abstract: Variability in valley-orbit state splittings will be a problem for large-scale implementations of silicon-based quantum processors. Depending on the particular qubit architecture, the role of valley-orbit states varies; however, a common theme for nearly all silicon-based qubits is that valley-orbit splittings must be precisely engineered or have large in situ tunability. Here, we investigate overlapping aluminum-gate devices for valley-orbit based qubits in Si/SiGe which enable high in situ tunability of valley-orbit states. Spectroscopic measurements of low-lying one- and two-electron valley-orbit states are taken to determine the quantitative relationship between the valley, singlet-triplet and orbital splittings. By exploiting the dependence of valley-orbit state splittings on electrostatic confinement and electron number, we show progress towards single-shot readout in the (4,1)-(3,2) electron regime, allowing for in situ tunability of the qubit frequency and enhancement of the readout window.
- Host: Mark Eriksson, Faculty Advisor