Acceleration of charged particles to extremely large energies by a sub-Dreicer electric field

Abstract

Acceleration of a fraction of initially low-energy electrons in a cold, collisional plasma to energies orders of magnitude larger than thermal is shown to be possible with a sub-Dreicer electric field. Because such an electric field does not satisfy the runaway condition, any acceleration will be statistical. Random scattering collisions are probabilistic such that there is 63% chance that a particle collides after traveling one mean free path and a 37% chance of not colliding. If one considers only the electrons that do not collide on traversing a mean free path and also considers that the collisional mean free path scales quadratically with particle kinetic energy, one realizes that there will be a small fraction of electrons that never collide and are accelerated to increasingly high energy. Because the mean free path scales quadratically with kinetic energy, after each successfully traveled mean free path, continued acceleration becomes more likely. This model is applied to an MHDdriven plasma jet experiment at Caltech and it is shown that electrons are accelerated by an electric field associated with a fast magnetic reconnection event occurring as the jet breaks apart. This statistical acceleration model indicates that a fraction $1.3 Â 10 À7 of electrons with initial energy distributed according to a Maxwellian with T ¼ 2 eV will be accelerated to 6 keV in the Caltech experiment and then collide to produce the observed X-ray signal. It is shown that the statistical acceleration model provides a credible explanation for the production of solar energetic electrons.

Caltech Lab Experiments and the Insights They Provide Into Solar Corona Phenomena

“…(2018) observed a ∼1 μs burst of ∼6 keV X‐rays coincident with the RTI. This short X‐ray burst is interpreted as resulting from a small cohort of electrons being collisionlessly accelerated to the full LdI / dt voltage drop (Marshall & Bellan, 2019). Although the mean free path of the thermal electrons is only a few microns for the 2 eV temperature and high density of the jet which has a length of tens of cm, statistical analysis shows that a small cohort of electrons can nevertheless be collisionlessly accelerated to high energy.…”

“…These models are not only relevant to solar physics, but also to magnetospheric physics and to astrophysics which can have similar physics but at very different scales from the lab or the Sun. Examples of such theoretical models are the following: A model showing why flux ropes tend to be collimated (Bellan, 2003) A model for how accretion disks and astrophysical jets form a complete electric circuit that transfers angular momentum in a conservative way much like a generator transfers angular momentum via wires to a distant motor (Bellan, 2016a, 2018d) A model providing a time‐dependent analytic solution for an astrophysical jet (Bellan, 2020) A model for how energetic particles are created in the presence of sub‐Dreicer electric fields (Marshall & Bellan, 2019) A model providing an intuitive explanation for fast collisionless magnetic reconnection (Yoon & Bellan, 2017, 2019a) in the electron MHD context, that is, the context where the timescale is so fast that ions can be considered stationary, and the reconnection length scale is short compared to the ion skin depth, so Hall terms and electron inertia are important. A model showing that ions experience fast stochastic heating during fast collisionless reconnection (Yoon & Bellan, 2018, 2019b) A model showing how the reverse current associated with coronal mass ejection drives EUV fronts in the solar corona (Wongwaitayakornkul et al., 2019) …”

3D Numerical Simulation of Kink-driven Rayleigh–Taylor Instability Leading to Fast Magnetic Reconnection

“…While highly simplified, this particle simulation nevertheless indicates that high-energy X-rays could come from the electric field generated by the KDRT-induced magnetic reconnection; it thus gives insight into the acceleration process creating high-energy electrons. This description of the electron acceleration process is very simplified and macroscopic; a more physically realistic description that takes into account changes in collisionality of electrons as they accelerate was given in Marshall & Bellan (2019).…”

“…However, this model did not arrange for the lateral acceleration to be from a primary KI and there was no numerical simulation showing the RTI being driven by a KI. Similarly, a statistical model of electron acceleration from a reconnecting electric field was developed (Marshall & Bellan 2019), but there was no numerical verification using actual geometry.…”

Single-cycle THz signal accompanying laser wake in photoionized plasmas and plasma channels

“…e . Hence, the external voltage must be lower than the Dreicer's field E D = 5.6 × 10 −12 (n e0 /T e )λ ei Z V/m [25]. Electron drift along the cylindrical channel induces an azimuthal magnetic field,…”