Events at Physics
Events During the Week of March 4th through March 11th, 2018
- Plasma Physics (Physics/ECE/NE 922) Seminar
- MHD modeling of DIII-D QH-mode discharges and comparison to observations
- Time: 12:00 pm
- Place: 2241 Chamberlin Hall
- Speaker: Dr. Jacob King, Tech-X Corporation
- Abstract: It is desirable to have an ITER H-mode regime that is quiescent to edge-localized modes (ELMs). ELMs deposit large, localized, impulsive, surface heat loads that can damage the divertor. One such regime is quiesent H-mode (QH-mode) with edge harmonic oscillations or broadband MHD as observed on DIII-D, JET, JT-60U, and ASDEX-U [Burrell et al. Phys Plasmas 2012, Garofalo et al. Nucl Fusion 2012 and references within]. These ELM-free discharges have the edge-plasma confinement necessary for burning-plasma operation on ITER. QH-mode is characterized by perturbations with small toroidal-mode numbers (n '1-5) where measurements from beam-emission spectroscopy, electron-cyclotron emission, and magnetic probe diagnostics show density, temperature and magnetic oscillations. These measurements demonstrate that the perturbations are a saturated macroscopic mode localized within the edge pedestal region. The particle transport is enhanced compared to standard H-mode discharges with ELMs, leading to essentially steady-state
profiles in the pedestal region. Finally, the operation regime of the QH-mode is dependent on the rotation profile, and QH-mode discharges are produced with an applied torque through either coor counter-neutral-beam injection and/or neoclassical toroidal viscosity from plasma interaction with non-resonant magnetic fields.
Extended-MHD modeling of DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 2002] QH-mode discharges with nonlinear NIMROD [C. R. Sovinec et al., JCP 195, 355 2004] simulations saturates into a turbulent state, but does not saturate when the steady-state flow inferred from measurements is not included. This is consistent with the experimental observations of the quiescent regime on DIII-D. The simulation with flow develops into a saturated turbulent state where the n=1 and 2 toroidal modes become dominant through an inverse cascade. Each mode in the range of n=1-5 is dominant at a different time. Consistent with experimental observations during QH-mode, the simulated state leads to large particle transport relative to the thermal transport. Analysis shows that the amplitude and phase of the density and temperature perturbations differ resulting in greater fluctuation-induced convective particle transport relative to the convective thermal transport. Comparison to magnetic-coil measurements shows rotation frequencies differ between the simulation and experiment which indicates that more sophisticated extended-MHD two-fluid modeling is required.
This work was supported by the DOE Office of Science (Office of Fusion Energy Sciences
- Chaos & Complex Systems Seminar
- Wada basins and distributed fields of determination
- Time: 12:05 pm
- Place: 4274 Chamberlin (Refreshments will be served)
- Speaker: UW Department of Asian Languages and Cultures, Steve Ridgely
- Abstract: Wada Basins are spaces containing three or more subregions in which each boundary is shared by all subregions. This topological concept, attributed to Takeo Wada and described by his Kyoto University colleague Kunizō Yoneyama in 1917, has gained an afterlife through application to complex systems in which a “basin” of initial conditions might be said to exhibit the “Wada property.” The topological form of a Wada basin would seem to map determination across a distributed field such that indeterminacy would be inherent to the spatial form, well beyond a metaphor for systems about which we have insufficient information.
- Host: Clint Sprott
- "Physics Today" Undergrad Colloquium (Physics 301)
- Building clocks out of atoms and nanoscale probes out of diamond
- Time: 1:20 pm
- Place: 2241 Chamberlin Hall
- Speaker: Shimon Kolkowitz, UW Madison Department of Physics
- Host: Wesley Smith
- No events scheduled
- Astronomy Colloquium
- Probing Composition and Alignment of Interstellar Dust Grains via Stratospheric Observations of Dust Emission Polarization Spectra
- Time: 3:30 am
- Place: 4421 Sterling Hall, 3:30 PM Coffee and cookies, 3:45 PM Talk Begins
- Speaker: Giles Novak, Northwestern University and CIERA
- Abstract: Astronomers have known about interstellar dust particles for almost a century, and for most of this time we have also known that these spinning grains somehow become aligned with respect to the ambient interstellar magnetic field. However, our knowledge of the grains’ composition is very incomplete, as is our understanding of the grain alignment mechanism. Such information is needed if we are to (a) remove the contaminating dust signal from cosmological observations of gravity waves from the inflationary epoch, and (b) efficiently exploit polarimetry of aligned grains to study the role of magnetic fields in star/planet formation. By describing two recent observations, I will show how measurements of the polarization spectrum of dust emission can help fill in the gaps in our knowledge of grain composition and alignment. These two observations are (1) the first measurement of the submillimeter polarization spectrum of a translucent molecular cloud, that my collaborators and I obtained with a balloon-borne experiment flown over Antarctica, and (2) the first observation of a far-IR polarization spectrum that rises with increasing wavelength as predicted by models, made toward the nearby stellar nursery Rho Oph using the new polarimeter for the Stratospheric Observatory For Infrared Astronomy (SOFIA).
- R. G. Herb Condensed Matter Seminar
- Magnetic cooling and On-chip thermometry for nanoelectronics below 10 mK
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Mario Palma
- Abstract: Cooling of nanoelectronic devices below 1mK is a challenging task, since the thermal coupling with the dilution refrigerator becomes weak at low temperatures and electronic devices are extremely susceptible to external heat leaks such as microwave radiation and electrical noise. Despite these technological challenges, there is a completely new world of physics, which can be explored once low temperatures are achieved. To reach such low temperature we developed a parallel network of nuclear refrigerators, which combines magnetic nuclear refrigeration technique with transport setup, by cooling of single measurement leads. In order to evaluate the cooling capacity of the setup, we developed various on-chip thermometry to read temperature below 10 mK. In particular, we focused on normal metal-Insulator-superconductor tunnel junctions, which are used as primary and secondary thermometer down to 7 mK. Furthermore, we implemented nuclear refrigeration of Coulomb blockade thermometer, proving cooling of the device down to 2.8 mK, which is the lowest temperature ever reported so far in a nanoelectronic device.
- Host: Eriksson
- Spanish night at the planetarium
- Time: 6:30 pm
- Place: 201 S Gammon Rd. Madison, WI
- Speaker: WIPAC/ El Universo es Tuyo
- Abstract: Come enjoy an evening hosted by the planetarium and WIPAC through the program "El Universo es Tuyo" on Friday March 9th 2018 from 6:30-8:30pm. There will be many hands-on activities from local organizations as well as planetarium shows in Spanish. Check back for more info soon!