In the realm of energy and plasma physics, understanding the behavior of particles in extreme environments like the solar corona or within fusion reactors is crucial. Researchers Uddipan Banik and Amitava Bhattacharjee from the Princeton Plasma Physics Laboratory have delved into these phenomena, shedding light on the mechanisms behind non-thermal particle acceleration and temperature inversion in the solar corona. Their work, published in the journal Physical Review Letters, offers insights that could have practical applications for the energy sector, particularly in fusion energy research.
Banik and Bhattacharjee developed a self-consistent quasilinear theory (QLT) to explain the ubiquitous non-thermal power-law distribution functions observed in astrophysical and laboratory plasmas. Their theory describes how multiple particle species relax in electromagnetically driven kinetic plasmas. The researchers derived a Fokker-Planck equation that includes both drive diffusion and Balescu-Lenard diffusion and drag coefficients. This equation accounts for the heating of particles directly by the drive and indirectly by waves, as well as internal turbulence and Coulomb collisions.
The researchers found that under a super-Debye (but sub-Larmor) drive with a steep power-spectrum, both electron and ion distributions relax towards a universal attractor with a $v^{-5}$ $(E^{-2})$ tail, similar to a $κ= 1.5$ distribution. This phenomenon occurs due to Debye screening, where large-scale fields accelerate the unscreened, fast particles but not the screened, slow ones. The universality of this distribution can be broken by shallow power-spectra and incomplete relaxation. Importantly, collisions cannot decelerate suprathermal particles, rendering the high-velocity tail immune to Maxwellianization.
The implications of this research extend to understanding temperature inversion in the solar corona. The suprathermal particles generated in the solar corona by chromospheric convection, despite collisional losses, can escape the sun’s gravity—a process known as velocity filtration. This process inverts the temperature profile and raises it to approximately 10^6 K. The researchers’ analysis of velocity filtration with a $κ≈ 1.5-2$ distribution, inspired by QLT, provides a reasonable fit to the spectroscopic data of heavy ions and explains the abrupt temperature rise. This rise is a consequence of the divergence of pressure in the $κ→1.5$ limit.
For the energy sector, particularly fusion energy research, understanding these particle acceleration mechanisms and distribution functions is vital. It can help in designing more efficient and stable plasma confinement systems, which are essential for achieving sustainable nuclear fusion. The insights gained from this research could contribute to advancements in fusion energy technologies, bringing us closer to a future powered by clean, abundant fusion energy.
Source: Banik, U., & Bhattacharjee, A. (2023). Non-thermal particle acceleration in multi-species kinetic plasmas: universal power-law distribution functions and temperature inversion in the solar corona. Physical Review Letters.
This article is based on research available at arXiv.