Arcjet Supplemental Diagnostics

This document proposes a new set of diagnostics designed to be implemented on the NASA Ames miniature Arcjet Research Chamber (mARC) for improved characterization of the flow. The diagnostics are grouped into three classes:higher cadence measurements, higher spatial resolution, and computer vision techniques for improved analysis of existing imaging. The goal is to better understand and quantify the following properties: flow statistics/uncertainty, temporal & spatial non-uniformity, flow temperature/enthalp

1D fast coded aperture camera

A fast (100 MHz) 1D coded aperture visible light camera has been developed as a prototype for imaging plasma experiments in the EUV/X-ray bands. The system uses printed patterns on transparency sheets as the masked aperture and an 80 channel photodiode array (9 V reverse bias) as the detector. In the low signal limit, the system has demonstrated 40-fold increase in throughput and a signal-to-noise gain of ≈7 over that of a pinhole camera of equivalent parameters. In its present iteration, the camera can only image visible light; however, the only modifications needed to make the system EUV/X-ray sensitive are to acquire appropriate EUV/X-ray photodiodes and to machine a metal masked aperture

Laboratory measurement of large‐amplitude whistler pulses generated by fast magnetic reconnection

We present observations of large‐amplitude (δB/B∼ 0.01) oblique whistler wave pulses generated by a spontaneous, 3‐D localized magnetic reconnection event in the Caltech jet experiment. The wave pulses are measured more than 50 ion skin depths from the reconnection location by a tetrahedron array of three‐axis B‐dot probes that mimic the pyramid flight formations of the Cluster and Magnetospheric Multiscale Mission spacecraft. Measurements of background parameters, wave polarization, and wave dispersion confirm that the pulses are whistler modes. These results demonstrate that localized impulsive reconnection events can generate large‐amplitude, oblique whistler wave pulses that propagate far outside the reconnection region. This provides a new pathway for the generation of magnetospheric whistler pulses and may help explain relativistic particle acceleration in phenomena such as solar flares that incorporate 3‐D localized impulsive magnetic reconnection

Laboratory measurement of large‐amplitude whistler pulses generated by fast magnetic reconnection

We present observations of large‐amplitude (δB/B∼ 0.01) oblique whistler wave pulses generated by a spontaneous, 3‐D localized magnetic reconnection event in the Caltech jet experiment. The wave pulses are measured more than 50 ion skin depths from the reconnection location by a tetrahedron array of three‐axis B‐dot probes that mimic the pyramid flight formations of the Cluster and Magnetospheric Multiscale Mission spacecraft. Measurements of background parameters, wave polarization, and wave dispersion confirm that the pulses are whistler modes. These results demonstrate that localized impulsive reconnection events can generate large‐amplitude, oblique whistler wave pulses that propagate far outside the reconnection region. This provides a new pathway for the generation of magnetospheric whistler pulses and may help explain relativistic particle acceleration in phenomena such as solar flares that incorporate 3‐D localized impulsive magnetic reconnection

Reverse Current Model for Coronal Mass Ejection Cavity Formation

We report here a new model for explaining the three-part structure of coronal mass ejections (CMEs). The model proposes that the cavity in a CME forms because a rising electric current in the core prominence induces an oppositely directed electric current in the background plasma; this eddy current is required to satisfy the frozen-in magnetic flux condition in the background plasma. The magnetic force between the inner-core electric current and the oppositely directed induced eddy current propels the background plasma away from the core, creating a cavity and a density pileup at the cavity edge. The cavity radius saturates when an inward restoring force from magnetic and hydrodynamic pressure in the region outside the cavity edge balances the outward magnetic force. The model is supported by (i) laboratory experiments showing the development of a cavity as a result of the repulsion of an induced reverse current by a rising inner-core flux-rope current, (ii) 3D numerical magnetohydrodynamic (MHD) simulations that reproduce the laboratory experiments in quantitative detail, and (iii) an analytic model that describes cavity formation as a result of the plasma containing the induced reverse current being repelled from the inner core. This analytic model has broad applicability because the predicted cavity widths are relatively independent of both the current injection mechanism and the injection timescale

Magnetically Induced Current Piston for Generating Extreme-ultraviolet Fronts in the Solar Corona

Single-pulse, globally propagating coronal fronts, called Extreme-ultraviolet (EUV) waves, were first observed in 1995 by the Extreme-ultraviolet Imaging Telescope and every observed EUV wave since has been associated with a coronal mass ejection (CME). The physical mechanism underlying these waves has been debated for two decades with wave or pseudo-wave theories being advocated. We propose a hybrid model where EUV waves are compressional fronts driven by a reverse electric current layer induced by the time-dependent CME core current. The reverse current layer flows in a direction opposite to the CME core current and is an eddy current layer necessary to maintain magnetic flux conservation above the layer. Repelled by the core current, the reverse current layer accelerates upward so it acts as a piston that drives a compressional perturbation in the coronal regions above. Given a sufficiently fast piston speed, the compressional perturbation becomes a shock that separates from the piston when the piston slows down. Since the model relates the motion of the EUV front to CME properties, the model provides a bound for the core current of an erupting CME. The model is supported and motivated by detailed results from both laboratory experiments and ideal 3D magnetohydrodynamic simulations. Overlaps and differences with other models and spacecraft observations are discussed

Preliminary Measurements of the Motion of Arcjet Current Channel Using Inductive Magnetic Probes

This paper covers the design and first measurements of non-perturbative, external inductive magnetic diagnostics for arcjet constrictors which can measure the motion of the arc current channel. These measurements of arc motion are motivated by previous simulations using the ARC Heater Simulator (ARCHeS), which predicted unsteady arc motion due to the magnetic kink instability. Measurements of the kink instability are relevant to characterizing motion of the enthalpy profile of the arcjet, the arcjet operational stability, and electrode damage due to associated arc detachment events. These first measurements indicate 4 mm oscillations at 0.5-2 kHz of the current profile

Apex Dips of Experimental Flux Ropes: Helix or Cusp?

We present a new theory for the presence of apex dips in certain experimental flux ropes. Previously such dips were thought to be projections of a helical loop axis generated by the kink instability. However, new evidence from experiments and simulations suggest that the feature is a 2D cusp rather than a 3D helix. The proposed mechanism for cusp formation is a density pileup region generated by nonlinear interaction of neutral gas cones emitted from fast-gas nozzles. The results indicate that density perturbations can result in large distortions of an erupting flux rope, even in the absence of significant pressure or gravitational forces. The density pileup at the apex also suppresses the m = 1 kink mode by acting as a stationary node. Consequently, more accurate density profiles should be considered when attempting to model the stability and shape of solar and astrophysical flux ropes

Determination of a macro- to micro-scale progression leading to a magnetized plasma disruption

We report the observations of a plasma jet evolving through a macro- to micro-scale progression sequence. This leads to a fast magnetic reconnection that results in the jet breaking off from its originating electrode and forming a force-free state. A sausage-like pinching occurs first and squeezes an initially fat, short magnetized jet so that it becomes thin. The thin jet then becomes kink unstable. The lengthening of the jet by the kinking thins the jet even more since the kink is an incompressible instability. When the jet radius becomes comparable to the ion-skin depth, Hall and electron inertial physics become important and establish the environment for a fast magnetic reconnection. This fast reconnection occurs, disrupting the jet and establishing a force-free state. X-ray bursts and whistler waves, evidence of a magnetic reconnection, are observed when the plasma jet breaks off from the electrode. This experimentally observed sequence of successive thinning from pinching followed by kinking is reproduced in a three-dimensional ideal Magnetohydrodynamic (MHD) numerical simulation. The results of the experiment and the numerical simulation, together demonstrate a viable path from macro-scale MHD physics to micro-scale non-MHD physics where fast reconnection occurs

Determination of a macro- to micro-scale progression leading to a magnetized plasma disruption

We report the observations of a plasma jet evolving through a macro- to micro-scale progression sequence. This leads to a fast magnetic reconnection that results in the jet breaking off from its originating electrode and forming a force-free state. A sausage-like pinching occurs first and squeezes an initially fat, short magnetized jet so that it becomes thin. The thin jet then becomes kink unstable. The lengthening of the jet by the kinking thins the jet even more since the kink is an incompressible instability. When the jet radius becomes comparable to the ion-skin depth, Hall and electron inertial physics become important and establish the environment for a fast magnetic reconnection. This fast reconnection occurs, disrupting the jet and establishing a force-free state. X-ray bursts and whistler waves, evidence of a magnetic reconnection, are observed when the plasma jet breaks off from the electrode. This experimentally observed sequence of successive thinning from pinching followed by kinking is reproduced in a three-dimensional ideal Magnetohydrodynamic (MHD) numerical simulation. The results of the experiment and the numerical simulation, together demonstrate a viable path from macro-scale MHD physics to micro-scale non-MHD physics where fast reconnection occurs