Plasma Stability in a Tokamak with Reactor Technologies Taking into Account the Pressure Pedestal

Studying stationary regimes with high plasma confinement in a tokamak with reactor technologies (TRT) [1] involves calculating the plasma stability taking into account the influence of the current density profiles and pressure gradient in the pedestal near the boundary. At the same time, the operating limits should be determined by the parameters of the pedestal, which, in particular, are set by the stability limit of the peeling–ballooning modes that trigger the peripheral disruption of edge localized modes (ELM). Using simulation of the quasi-equilibrium evolution of the plasma by the ASTRA and DINA codes, as well as a simulator of magnetohydrodynamic (MHD) modes localized at the boundary of the plasma torus based on the KINX code, stability calculations are performed for different plasma scenarios in the TRT with varying plasma density and temperature profiles, as well as the corresponding bootstrap current density in the pedestal region. At the same time, experimental scalings for the width of the pedestal are used. The obtained pressure values are below the limits for an ITER-like plasma due to the lower triangularity and higher aspect ratio of TRT plasma. For the same reason, the reversal of magnetic field shear in the pedestal occurs at a lower current density, which causes the instability of modes with low toroidal wave numbers and reduces the effect of diamagnetic stabilization.

Spatial distribution of the X-ray-emitting plasma of SS Cygni in quiescence and outburst

We present our analysis of the Suzaku data of SS Cygni (SS Cyg) from 2005 both in quiescence and outburst. A fluorescent iron Kα line bears significant information about the geometry of an X-ray-emitting hot plasma and a cold reflector, such as the surfaces of the white dwarf (WD) and the accretion disk (AD). Our reflection simulation has revealed that the X-ray-emitting hot plasma is located either very close to the WD surface in the boundary layer (BL), with an upper limit radial position of <1.004 times the white dwarf radius (RWD), or near the entrance of the BL where the optically thick AD is truncated at a distance of 1.14–1.27 RWD for the assumed WD mass of 1.19 M⊙ in quiescence. In the latter configuration, the plasma torus is located just above the inner edge of the AD. The result suggests that the accreting matter is heated up close to the maximum temperature immediately after the matter enters the BL. The matter probably expands precipitously at the entrance of the BL and leaves the disk plane to reach a height comparable to the radial distance of the plasma torus from the center of the WD. In outburst, on the other hand, our spectral analysis favors the picture that the optically thick disk reaches the WD surface. In addition, the plasma distributes above the disk like coronae, as suggested by a previous study, and the 90% upper limit of the coronae radial position is 1.2 RWD.

Io's auroral emissions via global hybrid plasma simulations

We tackle the Io's aurora source and topology by carrying out a set of global hybrid simulations of Io's interaction with the plasma torus under different model geometry and background conditions. Based on the simulated results, we compute the photon emission rates above the Io's surface and present the resulting images from a virtual telescope and topological maps showing the distribution of the emission sources across the moon's surface. This allows us to compare the structure of the aurora with the real observations and conclude on the different assumptions. We found a reasonable agreement with the real observations in the case of non-collisional background electron populations. From the comparison of the local magnetic field topology with model aurora structures, we also infer that an induced dipole feature is more probable to play a role in the interaction of Io with the Jovian magnetosphere. In addition we also examine the potential contribution of energetic electron beams, being observed in the Io's wake region, to the overall auroral emissions.

The plasma/atmosphere interaction at Io: revisiting the ion/neutral charge-exchange cross-sections and their ion-velocity dependency

<p>The Io Torus plasma is mostly composed of singly and multi-charged S and O ions. These ions interact with the neutrals of Io’s atmosphere (S, O, SO<sub>2</sub> and SO) through symmetrical (i.e. O<sup>+</sup> + O => O + O<sup>+</sup>) and asymmetrical (i.e. S<sup>++</sup> + O => S + O<sup>++</sup>) charge-exchanges. Charge-exchange cross-sections were estimated in Johnson & Strobel, 1982 and McGrath & Johnson, 1989 at 60 km/s (the plasma corotation velocity in Io’s frame), and are used in numerical simulations of the torus/neutral cloud interaction (i.e. Delamere and Bagenal, 2003).</p> <p>Dols et al., 2008 proposed numerical simulations of the multi-species chemistry interaction at Io using these cross-sections at 60 km/s. The plasma/atmosphere interaction at Io is strong and the flow velocity and ion temperature are drastically reduced close to Io (v < 10 km/s). Thus, velocity-dependent charge-exchange cross-sections are critical for such numerical simulations and their effect on the local plasma and neutral supply at Io should be explored.</p> <p>We propose to revisit the calculation of ion/neutral charge-exchange cross-sections following Johnson & Strobel, 1982’s approach for plasma velocities relevant to the local interaction at Io (V=10-120 km/s). More sophisticated calculations were proposed in McGrath & Johnson, 1989 but both publications offered very few details about their procedure.</p> <p>We will illustrate the effect of using velocity-depend charge-exchange cross-sections in numerical simulations of the multi-species plasma/atmosphere interaction at Io.</p> <p>More generally, this presentation aimed at providing an incentive for the community to expand the work of McGrath & Johnson, 1989.</p> <p> </p> <p><em>Johnson & Strobel, Charge-exchange in the Io torus and exosphere, JGR, 87,1982</em></p> <p><em>McGrath & Johnson, Charge exchange cross sections for the Io plasma torus, JGR, 94, 1989</em></p> <p><em>Delamere & Bagenal, Modeling variability of plasma conditions in the Io torus, JGR, 108, 2003</em></p> <p><em>Dols, Delamere, Bagenal, Kurth, Paterson, A multi-species chemistry model of Io’s local interaction with the plasma torus, JGR, 113, 2008</em></p>

How much Io material reaches Europa?

<p>The plasma interaction with Io’s atmosphere results in at least a ton per second of escaping neutrals. Most of these neutrals supply extended neutral clouds along Io's orbit  and eventually become ionized and accelerated to corotation with Jupiter, populating the Io plasma torus as well as spreading out to fill Jupiter’s vast magnetosphere. About half to two-thirds of the plasma torus ions charge-exchange with the extended neutral clouds  and leave the torus as energetic neutral atoms, passing Europa’s orbit. Energetic neutrals are also produced directly in the plasma-atmosphere interaction, escaping with sufficient speed to reach Europa’s orbit before being ionized. The iogenic ions that are accelerated to high energies in the middle magnetosphere ultimately move back inward, again crossing Europa’s orbit. We present estimates of the fluxes of these various iogenic populations and how much oxygen, sulfur and sodium might be hitting Europa.</p>

Modelling the electron density distribution in the Io Plasma Torus using Juno radio occultations

<p>The innermost galileian moon Io hosts an intense volcanic activity, which ejects about 10<sup>3</sup> kg/s of gas into Jupiter's magnetosphere. Here these neutrals are ionized by interaction with the background plasma and they are accelerated from keplerian velocity to corotation velocity thanks to Alfvén's theorem. This plasma cloud around the planet (the so-called Io Plasma Torus or IPT) slowly diffuses across Jupiter's magnetic field, but high electron densities (>1000-2000 cm<sup>-3</sup>) are found between 5-8 R<sub>J</sub>.</p><p>Juno is travelling along highly eccentric, polar orbits around the planet and flies very close to Jupiter's surface during each perijove. Thus, the radio links used for ground communication and radio science cross the IPT both in the uplink and the downlink leg. Being a dispersive medium, the torus introduces a different path delay on the X/X and Ka/Ka links established between the Ground Station and the spacecraft. Thus, the path delay can be extracted through a linear combination of the two links, and then quantitatively analyzed and fitted to different parametric models of the IPT.</p><p>In this work we have used almost all the available Juno radio occultations of the IPT in order to improve an already existing model by introducing both longitudinal and temporal variations of the electron density. To this end, we looked for the 2D Fourier expansion in longitude and time of the parameters of this model with the goal of minimizing the residuals of the fit and pointing out periodicities in the morphology of the torus.</p>