Modelling of parallel dynamics of a pellet-produced plasmoid

The parallel expansion of a dense, pellet-produced plasmoid is modelled with parameters relevant to pellet fuelling experiments in the Wendelstein7-X stellarator. Good agreement is found between the analytical theory and more detailed modelling. In particular, much of the energy deposited in the pellet by the ambient plasma is transferred to the pellet ions by the ambipolar electric field during the expansion. The validity of the hydrodynamic treatment of the plasmoid and the ambient plasma is discussed.

On the growth of the thermally modified non-resonant streaming instability

ABSTRACT The cosmic rays non-resonant streaming instability is believed to be the source of substantial magnetic field amplification. In this work, we investigate the effects of the ambient plasma temperature on the instability and derive analytical expressions of its growth rate in the hot, demagnetized regime of interaction. To study its non-linear evolution, we perform hybrid-PIC simulations for a wide range of temperatures. We find that in the cold limit, about two-thirds of the cosmic rays drift kinetic energy is converted into magnetic energy. Increasing the temperature of the ambient plasma can substantially reduce the growth rate and the magnitude of the saturated magnetic field.

A Double Hemispherical Probe for the Advancement of In Situ Plasma Measurements

<p>Langmuir probes are conductors of simple geometries (spheres, disks, cylinders, etc.) inserted into a plasma. By sweeping a voltage on the probe and measuring the current collected or emitted, a current-voltage (I-V) relationship can be found and interpreted to derive the density, temperature, and potential of the ambient plasma. Over the past 50 years, Langmuir probes have been flown on spacecraft missions for in-situ measurements of the local plasma environment. However, even after decades of use, there are still challenges in the analysis and interpretation of Langmuir probe measurements due to local plasmas created around the probe as a result of plasma interactions with the probe itself and spacecraft.</p><p>The Double Hemispherical Probe (DHP) is a directional Langmuir probe made of two hemispheres that are electrically isolated from each other and swept with a voltage together to get two separate I-V curves. The DHP uses the I-V curve differences between the two hemispheres to gain information of the asymmetry of the local plasma around the probe to retrieve the true ambient plasma parameters. Specifically, the DHP is intended to improve the plasma measurements in the following scenarios: i) Low-density plasmas; ii) flowing plasmas; iii) high-surface-emission environments; and iv) dust-rich plasmas. The following discusses the current progress of the DHP development.</p><p>Low-density plasmas create large Debye sheaths around the spacecraft that may engulf the Langmuir probe attached to a boom with a finite length. The potential drop in the sheath can change the characteristics of charged particles collected by the probe, causing mischaracterization of the ambient plasma. As expected, the I-V curves of both hemispheres match in the bulk plasma. It was found that as the DHP is moved ‘deeper’ into the sheath of the spacecraft, the currents of the two hemispheres diverge. The saturation current ratio of the hemispheres of the DHP was found to have monotonic relationships with the plasma characteristics measured in the sheath. A technique was created to retrieve the ambient plasma parameters.</p><p>In space ions generally have relative velocities with respect to the spacecraft due to flowing plasmas or fast-moving spacecraft, creating an ion wake behind the probe itself. This self-wake can cause issues in interpreting the I-V curves for both ion and electron species. The ion saturation current of either hemisphere of the DHP is dependent on the ion Mach number (the ratio of the ion flow speed to the thermal speed). Electrons are generally in the thermal state. However, depending on the ratio of the probe size to the Debye length, ambipolar electric fields can be created at the wake boundaries, causing the reduction of the electron density in the downstream side of the probe and its subsequent underestimation measured by traditional single Langmuir probes. It was shown that the DHP can identify this self-wake effect and properly measure the true ambient plasma parameters.    </p><p>Future work will explore the effects of high-surface-emission environments and dust-rich plasmas on DHP measurements and to develop techniques to resolve the true ambient plasma parameters in these environments. </p>

Element Abundances of Solar Energetic Particles and the Photosphere, the Corona, and the Solar Wind

From a turbulent history, the study of the abundances of elements in solar energetic particles (SEPs) has grown into an extensive field that probes the solar corona and physical processes of SEP acceleration and transport. Underlying SEPs are the abundances of the solar corona, which differ from photospheric abundances as a function of the first ionization potentials (FIPs) of the elements. The FIP-dependence of SEPs also differs from that of the solar wind; each has a different magnetic environment, where low-FIP ions and high-FIP neutral atoms rise toward the corona. Two major sources generate SEPs: The small “impulsive” SEP events are associated with magnetic reconnection in solar jets that produce 1000-fold enhancements from H to Pb as a function of mass-to-charge ratio A/Q, and also 1000-fold enhancements in 3He/4He that are produced by resonant wave-particle interactions. In large “gradual” events, SEPs are accelerated at shock waves that are driven out from the Sun by wide, fast coronal mass ejections (CMEs). A/Q dependence of ion transport allows us to estimate Q and hence the source plasma temperature T. Weaker shock waves favor the reacceleration of suprathermal ions accumulated from earlier impulsive SEP events, along with protons from the ambient plasma. In strong shocks, the ambient plasma dominates. Ions from impulsive sources have T ≈ 3 MK; those from ambient coronal plasma have T = 1 – 2 MK. These FIP- and A/Q-dependences explore complex new interactions in the corona and in SEP sources.