The current revival of theAmerican economy is being predicated on social distancing, notably the Six-FootRule of the CDC, which offers little protection from pathogen-bearing aerosoldroplets sufficiently small to be mixed through an indoor space. The importanceof indoor airborne transmission of COVID-19 is now widely recognized, but nosimple safety guideline has been proposed to protect against it. We here buildupon models of airborne disease transmission to derive a guideline that boundsthe ``cumulative exposure time", the product of the number of occupantsand their time in an enclosed space. The bound depends on the rates ofventilation and air filtration, dimensions of the room, breathing rate,respiratory activity and face mask use of its occupants, and infectiousness of therespiratory aerosols. By synthesizing data from indoor spreading events withrespiratory drop-size distributions, we estimate an infectious dose on theorder of ten aerosol-borne virions. Thenew coronavirus (SARS-CoV-2) is thus inferred to be an order of magnitude moreinfectious than its forerunner (SARS-CoV), consistent with the pandemic statusachieved by COVID-19. Case studies are presented for classrooms and nursinghomes, and an online app is provided to facilitate use of our guideline.Implications for contact tracing and quarantining are considered, appropriatecaveats enumerated. Particular consideration is given to respiratory jets, whichmay substantially elevate risk when facemasks are not worn. Finally, the guideline can be expressed as abound on the safe excess (exhaled) CO2 level in the room for a giventime, which enables real-time monitoring of transmission risk using cheapsensors, as well as optimization of HVAC systems to balance infection riskagainst energy use and cost.
See http://www.mit.edu/~bazant/COVID-19 for publications with John W. M. Bush and others, anonline app by Kasim Khan, and a massive open online course (MOOC), 10.S95xPhysics of COVID-19 Transmission on edX.
The ohmic breakdown of neutral gas molecules by applying the external toroidal electric field has been generally used over several decades to produce initial plasmas in a tokamak. However, the physical mechanism of the electron avalanche during the ohmic breakdown has not been clearly revealed yet due to the complex topology of time-varying electromagnetic fields. Although the classical Townsend avalanche theory has been widely adopted for modeling the ohmic breakdown, we found clear evidence from KSTAR experiments that the Townsend theory is not valid for the ohmic breakdown. Here, we present the first systematic ohmic breakdown theory, namely a turbulent ExB mixing avalanche , which addresses the crucial roles of the self-generated electric field within a complex electromagnetic topology. It was found that the ohmic breakdown is totally different from the Townsend avalanche as the strong self-electric field produced by the plasma space-charge drastically decreases the plasma growth rate and greatly enhances the plasma transport via turbulent ExB mixing effects. A state-of-art particle simulation  demonstrated the novel theory by successfully reproducing KSTAR experiments. A comprehensive understanding of the multi-dimensional plasma dynamics in the complex electromagnetic topology provides new physical insights on a design strategy of robust breakdown scenarios in tokamak fusion reactors such as ITER and beyond.
References:  Min-Gu Yoo, et al., Nature Communications 9 (2018) 3523  Min-Gu Yoo, et al., Computer Physics Communications 221 (2017) 143.
Stability, or instability, is the central theme for plasma physics. In this talk, I will prove and demonstrate using examples from plasma physics the following facts.
1) The only route for a conservative system to become unstable is through the resonance between a positive-action mode and a negative-action mode.
It is not the “free energy” that drives the system unstable.
2) For most stable systems, stability is a consequence of being symplectic (or G-unitary for complex systems), instead of the existence of
dissipation or damping.
3) Dissipation can destabilize a system by breaking the symplectic (or G-unitary) condition.
4) By not preserving the symplecticity, numerical and analytical models for conservative systems diverge quickly from the true dynamics.
 R. Zhang, H. Qin et al., arxiv:1801.01676 (2018)
 R. Zhang, H. Qin et al., Phys. Plasmas 23, 072111 (2016)
 J. Xiao, H. Qin & J. Liu, Plasma Sci. Tech. 20, 110501 (2018)
*Things in this context include conservative systems, numerical and analytical models thereof.
The DOE workshop on “Integrated Simulations” has identified a number of critical aspects in the integration of physics modules in a whole device model for tokamak simulations. Challenge includes not only the need for physics description, but also a need for hardware infrastructure, software integration and difficulties in integrating multi-scale coupling.
This seminar summarizes the conclusions from the integrated simulations workshop on MHD stability and disruptions, boundary physics and core transport, as well as a need for research on innovative workflows that enable the integration.