Particles crossing separatrices are ubiquitous in plasmas. This work presents the first definitive observation of heating due to collisional separatrix crossing in a plasma with heating scaling as the square root the collision rate. This “classic” result was theoretically predicted by Galeev in 1969 [1], Rosenbluth in 1972 [2], Minick 2006 [3] and more recently Dubin[4]; but never observed directly in a plasma.

We observe this dissipative separatrix heating when a pure ion plasma column is forced back and forth across a partial trapping barrier. The barrier is an externally applied axisymmetric “squeeze” potential, imposed to create a velocity separatrix between trapped and passing particles. Trapped particles can only explore a limited section of phase space whereas passing particles can explore the entire phase space.
Weak collisions then cause separatrix crossings, leading to irreversible heating in each forcing cycle. The heating rate scales as the square root of the collision frequency times the oscillation rate, and thus is large for low-collisionality plasmas. 

The particle velocity distribution function is measured with Coherent Laser Induced Fluorescence, and shows passing and trapped particles having an out-of-phase response to the forced plasma oscillations, in quantitative agreement with recent collisionless adiabatic invariant analysis.

“Synthetic” collisions from externally applied electrostatic noise cause velocity diffusion and thus enhanced separatrix crossings, with resultant heating also observed to scale as square root of the synthetic collisions rate.

[1] A. A. Galeev,et. al., PRL, 22, 511 (1969)

[2] M.N. Rosenbluth, D.W. Ross, D.P. Kostomarov, Nuclear Fusion, 12, 3 (1972)

[3] H. Mynick, Physics of Plasmas, 13, 058102 (2006)

[4] D.H.E. Dubin, Physics of Plasmas, 24, 112120 (2017)

This talk discusses a new parametric instability mechanism caused by particles that are “weakly trapped” in the potential wells of a nonlinear wave[1]. The mechanism applies to low-collisionality plasmas supporting waves with near-acoustic dispersion relations such as ion sound waves, magnetized Langmuir waves, or Alfven waves. The theory is compared to particle in cell [PIC] simulations of Trivelpiece-Gould [TG] waves, as well as to experiments[2] on pure ion plasmas that observe parametric instability in TG standing waves. For TG waves, the standard parametric instability mechanism induced by wave-wave coupling is suppressed. The new mechanism predicts instability only if weakly trapped particles are present, at rates found to be in agreement with the simulations, and consistent with the experiments.

In the parametric instability studied here, a nonlinear “pump” wave is unstable to the growth of daughter waves with twice the wavelength and nearly the same phase velocity as the pump. This induces adjacent potential peaks in the wave to slowly approach one-another, receding from other pairs of peaks. Particles that are weakly trapped between approaching peaks, with kinetic energies just below the potential maxima, are heated by compression and escape the well, and then become retrapped on the other side of the approaching peaks, where they amplify the compression by pushing the peaks together.   Distributions with weakly trapped particle populations (appearing as phase space “holes”  or “rings” in the trapped particle distribution) often occur in nonlinear plasma waves and BGK states, and such distributions can be unstable to this new trapped particle mechanism.

[1] D. Dubin, Phys. Rev. Lett. 2018 (accepted).

[2] F. Anderegg, M. Affolter, A. Ashourvan, D. Dubin, F. Valentini and C. F. Driscoll, AIP Conf. Proc. 1668,  020001 (2015).

For practical purposes, laser-induced fluorescence (LIF) is frequently performed on metastable states that are produced directly from neutral gas particles and also from ions in other electronic states. Here rises an important question: when can Doppler-resolved LIF on metastable ions be used to infer the velocity distribution of ground-state ions, the majority ion population in many laboratory plasmas? Previous experimental results suggest that there are limitations of this laser diagnostic technique due to the finite lifetime of metastable ions. Simulations based on our newly developed Lagrangian model for LIF show that under circumstances where the metastable ion population is produced from direct ionization of neutrals, the velocity distribution measured using LIF will only faithfully represent processes which act on the ion dynamics in a time shorter than the metastable lifetime.¹ However, the LIF performed on the metastable population produced from pre-existing ions is not affected by metastable lifetime. Unlike the well-known optical pumping broadening in LIF measurements, this systematic error caused by the finite metastable lifetime has never been studied before in experiments. Understanding the behavior of each metastable population, whether produced from neutral particles or pre-existing ions, is important in LIF measurements as it provides a path for avoiding or correcting this new type of systematic error. It is a long-standing problem, however, to trace the production history of metastable ions. This paper presents the experimental measurement of ion metastable lifetime in a plasma as well as the relative fraction of metastables produced directly from neutral atoms as opposed to pre-existing ions. The technique relies on measuring the ionic wave response.

¹F. Chu and F. Skiff, Phys. Plasmas (1994–Present) 25, 013506 (2018).

Magnetic reconnection plays an important role in explosive phenomena in magnetized plasmas such as solar flares and substorms. Various waves and instabilities can be generated during reconnection and make a significant impact on the reconnection dynamics. Here, whistler and lower-hybrid waves during reconnection are studied with data from the Magnetospheric Multiscale (MMS) mission and the Magnetic Reconnection Experiment (MRX). In particular, the dispersion relation of the whistler mode near the magnetospheric (low-density) separatrix is measured for the first time. The measured dispersion relation shows that the whistler wave propagates nearly parallel to the magnetic field, which is consistent with a linear analysis. The linear analysis also confirms that the whistler wave is generated by temperature anisotropy in the electron tail population. The temperature anisotropy is caused by the loss of electrons with a high velocity parallel to the magnetic field to the exhaust region. The correlative behavior of the whistler wave and the lower-hybrid drift instability (LHDI) suggests that LHDI is responsible for the enhanced transport of high parallel velocity electrons to the exhaust. A statistical analysis of MRX data shows a positive correlation between whistler and LHDI activities. Finally, we have observed in both laboratory and space plasmas that the lower-hybrid drift wave (LHDW) is excited inside the current sheet during reconnection with a sizable guide field. LHDW propagates obliquely to the magnetic field. Moreover, LHDW induces density fluctuations that are in phase with electric field fluctuations. The excitation mechanism of LHDW is discussed via analysis of MRX and MMS data and a linear calculation. Initial results indicate that LHDW contributes to anomalous resistivity and electron heating in the current sheet. 

Arched magnetoplasma structures that carry electrical current ubiquitously exist in solar and heliospheric plasmas. Varieties of Alfvén waves and instabilities (e.g., fast waves, Kink and Kelvin-­Helmholtz instabilities) have been detected in recent space and ground based observations. Contemporary research on these topics is at the forefront of solar physics due to their remote diagnostic capabilities. After briefly reviewing the progress made in this area, results from a recently upgraded UCLA experiment on arched magnetized plasmas (plasma β ≈ 10^-3, Lundquist number ≈ 10²–10⁵, plasma radius/ion-gyroradius ≈ 20, B ≈ 1000 Gauss at footpoints) will be presented. The arched plasma was created using a lanthanum hexaboride (LaB₆) plasma source and it evolved in an ambient magnetoplasma produced by another LaB₆ source. The experiment runs continuously with a 0.5 Hz repetition rate. The plasma and wave parameters were recorded with a good resolution using movable Langmuir and three-axis magnetic-loop probes in 3D. Images of the plasma were recorded using a fast-CCD camera. In the upgraded experiment, the main focus is on the direct measurement of propagation and damping characteristics of fast waves and kink-mode oscillations. The kink-mode oscillations were observed as transverse oscillations across the symmetry plane of the arched plasma. The relative magnitudes of the parameters of the arched and ambient plasma were varied to simulate a variety of conditions relevant to coronal loops and solar prominences. Recent results reveal fascinating interplay among fast waves and global oscillations of the arched plasma. We also examine theoretical models of kink-modes and Alfvén waves in the presence of an electrical current in a magnetized plasma structure.

References: (1) Tripathi and Gekelman, Phys. Rev. Lett. 105, 075005 (2010); (2) Tripathi and Gekelman, Solar Phys. 286, 479 (2013)

Ball lightning (BL) is a fireball occasionally observed during thunderstorms [1], and it was recorded earliest by Aristotle. For centuries, BL had attracted great interests from scientists, including Musschenbrock, Arago, Faraday, Lodge, Tesla, Bohr, Kapitza and Ginzburg etc. About 100 models had been proposed for BL, but no consensus is reached about the nature of BL.

We will first introduce the characteristics and research history of BL, and then present a new BL theory [2]. Near the ground, lightning can produce a relativistic electron bunch, and the bunch excites an intense microwave pulse. This microwave is so strong to evacuate the ambient plasma and form a spherical plasma cavity. This formation process is demonstrated by particle-in-cell simulation. The microwave bubble model can explain many properties of BL, such as the occurrence site, relation to the lightning channels, appearance in aircraft, its shape, size, sound, spark, spectrum, motion, as well as the resulting injuries and damages. In particular, our theory is unique for a successful explanation of BL formation in aircraft.

Did someone ever detect radio signals with the same origin of BL? Yes, they are trans-ionospheric pulse pairs (TIPPs). TIPPs were first detected by a USA satellite in 1993 and are the most powerful natural radio sources on Earth. Using the BL-exciting mechanism, we quantitatively explain almost all the features of TIPPs [3]. Therefore, high-energy electrons from lighting can emit strong electromagnetic radiation, which is a fundamental assumption of our BL theory.

Finally, we point out further questions need to be answered in future and discuss experimental activities. Our work may drive the development of high-power microwave devices at an extreme level.

References:

[1] M. Stenhoff, Ball Lightning: An unsolved problem in atmospheric physics (Kluwer & Plenum, NY, 1999).

[2] H.-C. Wu, Sci. Rep. 6, 28263 (2016).

[3] H.-C. Wu, Geophys. Res. Lett. 44, 2597 (2017).