As models describing CMEs mature, there is gradual convergence towards a stan- dard model. The solar community has already accepted that tether cutting and ux cancellation are the same process, applied to the corona and the photosphere, respec- tively. While supporters of break-out and tether-cutting still argue about whether reconnection occurs above or below the highly-sheared eld region, both processes use magnetic reconnection to create the ux rope and drive the eruption. This has motivated the categorizing of breakout as a kind of external tether cutting [29].
Loss of equilibrium type models also are converging as scientists agree on the fundamental forces involved. The Van Tend & Kuperus circuit model represents an
implementation of catastrophe model [29] in cylindrical geometry. In both circuit and catastrophe models, the system evolves along a series of equilibria until the driving mechanism (usually an increase in current) pushes the system to a regime without an equilibrium and the system erupts. The torus instability and catastrophe models are now accepted by their respective proponents [105] as describing the same critical threshold from two dierent perspectives. While catastrophe species a pre-eruptive evolution and avoids consideration of unstable equilibria away from the critical thresh- old, torus instability does not specify the pre-eruptive evolution and focuses on the family of unstable equilibria. Since both models are based on the same force balance equation, they produce an onset of eruption at the same point [105].
The kink instability and certain aspect of ux injection remain distinct from the other models. The kink instability is not an eruption mechanism and is a parallel plasma process caused by the current. Flux injection distinguishes itself from other solar models by permitting energy transfer during the eruption process. Laboratory experiments can tie both the kink instability and ux injection models to existing solar models. Laboratory plasma loops can kink and erupt since no simplifying as- sumptions are made. Energy can also be injected into laboratory plasmas by an external power supply and the kink instability has been observed and quantied in laboratory experiments [129, 130].
While scientists still debate about whether or not solar eruptions contain pre- existing ux ropes, it is prudent to note that the descriptions from both sides look remarkably similar. The ux cancellation end goal (Fig. C.3) looks like a current carrying ux rope emerging from the photosphere [131]. It is to no surprise that plenty of observational evidence can be produced to back each side. Nevertheless, the quest for scientic understanding continues: nature is self-consistent, and so the solar physics community will inevitably converge on a standard CME model.
Appendix D
Operational Details
One striking feature of the Caltech experiment is the tens of Megawatts of power during the plasma life-time. This energy injection rate is greater than the Caltech on-site power generation capability and would be prohibitively expensive to sustain.
Fortunately, the plasma only exists for a few microseconds so the energy usage is negligible. Megawatts-scale experiments can be powered by standard 120 V outlets through pulse-power techniques. These technique take advantage of long charging times to build up energy within a capacitor. This energy is released by fast switches over extremely short time scales, producing tremendous power output.
This chapter discusses the operational details of plasma breakdown, strapping eld, and diagnostics. The focus will be on describing hardware though relevant theory will also be presented.
D.1 Experimental setup
A representation of the experimental setup can be found in Fig. D.1. The vacuum chamber axis denes the z-axis of the coordinate system. The cathode and anode dene the x-y plane, with the gap separating cathode from anode dening the origin.
The bias coils (purple) generate arched magnetic elds similar to a horseshoe mag- net. Fast gas valves pu gas through the center of the bias coils, creating spatially non-uniform gas distributions with higher densities near the nozzle. The electrodes, bias coils, and gas system make up the plasma gun, which is used to form plasmas
SideAviewA0Zoomed. EndAview
Plasma atAbreakdown
StrappingA coils
y x VacuumAchamber Cathode
Anode Cathode
y z Anode
StrappingA coils MainA
Bank
Lintrinsic
LExtra
59AμF
0.77AF StrappingA
Bank
BiasAcoil BiasAcoil
Plasma MagneticAprobe clusters Gas
Valves Gas
Gas
Figure D.1: Representative side and end view of experimental set-up.
(red). High voltage applied to the electrodes by a 59µF capacitor ionizes gas to form an arched plasma much smaller than the vacuum chamber (Fig. D.1 End view). The capacitor is typically charged to 2.5-5 kV driving 30-70 kA of current which ows in the y direction at the plasma loop apex. Additional inductance (Lextra) can be added to the intrinsic inductance of the system (Lintrinsic) to slow down the current pulse.
A 0.77 F capacitor bank powers two 7.6 cm diameter strapping eld coils (blue) mounted 9.5 cm in front of the electrode. The strapping coils each have 11 turns and are placed in a coaxial conguration inside the chamber to produce strapping eld in the xdirection, so that theJy×Bxstrap force inhibits plasma loop expansion whereJy is the electric current density in the plasma loop.
The timing is programmed to a set of function generators which coordinate the entire events across millisecond and microsecond time scales. Once programmed, each experimental shot is mostly automated and the user is in charge of manually starting the capacitor charging process.