Historical research guide · 0966 · 2009
Formation of episodic magnetically driven radiatively cooled plasma jets in the laboratory
F. Suzuki-Vidal · S. V. Lebedev · A. Ciardi · S. N. Bland · J. P. Chittenden · G. N. Hall · A. J. Harvey-Thompson · A. Marocchino · Cheng Ning · C. Stehlé · Adam Frank · Eric G. Blackman · S. C. Bott · T. P. Ray
In one minute
Astronomers see jets from young stars that arrive in bursts — knots and shocks strung along the flow, as though the engine switched on and off. Francisco Suzuki-Vidal, Sergey Lebedev and colleagues at Imperial College, with collaborators in Paris, Rochester, San Diego and Dublin, made one in a laboratory. On the MAGPIE machine a million amps rises in 250 billionths of a second through a thin aluminium foil; the toroidal magnetic field underneath lifts a magnetic bubble and pinches a jet along its axis — the magnetic tower that theorists propose for the real thing. The new ingredient is the foil. Where the group’s earlier wire arrays gave one eruption, the foil gives four or five in a row about 30 nanoseconds apart, because plasma keeps closing the gap at the base and the current reconnects there. Each bubble travels faster than the last, up to 325 kilometres a second, and runs into the wreckage of the one before it, building shocks. The laboratory now has a jet that repeats itself.
Connection
Chapter 9 is about plasma that organises itself into a structure and holds it, and this is that at its most demanding: a magnetically pinched jet inside its own bubble, made and remade several times inside a single shot. Chapter 13 is where the laboratory and the sky are put on one footing — the same dimensionless numbers, the same reconnection at the base of the flow, the same knots downstream.
From the source
Magnetically driven, radiatively cooled plasma jets can be produced in a laboratory by a one megaamp, 250 nanosecond current pulse on the MAGPIE pulsed power facility. The jet is driven by the pressure of a toroidal magnetic field in a magnetic tower configuration: a magnetically collimated jet on the axis of a magnetic bubble that is confined by the ambient medium — the same picture Lynden-Bell proposed for jets from protostars to neutron stars.Abstract; Introduction, paragraphs 2 and 3
Replacing the radial wire array of the earlier experiments with a radial aluminium foil 6 to 6.5 microns thick, held between a 3.1 millimetre cathode and a 60 millimetre outer electrode, turns a single eruption into a repeating one: four or five successive magnetic tower outflows in one shot, emerging with a periodicity of about 30 nanoseconds and continuing for the duration of the current pulse.Abstract; Experimental setup; Formation of episodic magnetic tower jets, with Figure 3
The mechanism of the repetition is reconnection of current at the base of the cavity. The Lorentz force ablates the foil near the cathode and opens a radial gap of about 0.3 to 0.7 millimetres through which Poynting flux enters; plasma expanding from the cathode and the remaining foil closes that gap, the force on the closing plasma pushes it upward, and a new magnetic tower forms. The foil’s mass distribution, rising with radius rather than constant as in a wire array, is what makes the gap small enough to close quickly.Formation of episodic magnetic tower jets, closing paragraph
The cavities are measured, not inferred. Each bubble inflates radially at 50 to 60 kilometres per second and axially at 130 to 200, and each successive one is faster, reaching 325 kilometres per second for the third — consistent with the earlier episodes having swept the ambient plasma out of the way. Every episode is accompanied by a burst of soft x-rays, at photon energies of 200 to 300 electronvolts and above 800, correlated with the formation of the new jet, which says each episode begins with plasma pinching on the axis of the cavity.Formation of episodic magnetic tower jets, with Figure 4
With argon injected above the foil at an initial number density of ten to the seventeenth or eighteenth per cubic centimetre, the flow builds a shock structure of its own: a conical shock at about 60 degrees from the axis moving at 60 kilometres per second, which later splits into two oblique shocks; a faster, roughly spherical bow shock at 110 kilometres per second ahead of the hydrodynamic precursor jet; and a Mach stem where the two meet. The later magnetic bubbles, at around 300 kilometres per second, then overtake all of it. The ambient gas does not change the roughly 30 nanosecond periodicity of the jets.Jet propagation in an ambient gas, with Figure 5
What to watch: the authors report first measurements of trapped toroidal magnetic field inside the towers of about 0.5 tesla, implying a magnetic Reynolds number consistent with a jet temperature near 200 electronvolts from x-ray spectroscopy, and first estimates of the Poynting flux from an inductive probe across the gap. Some plasma parameters are yet to be measured accurately, so the case that the dimensionless numbers match those of jet launching from young stellar objects rests for now on the similarity to the earlier radial wire array experiments. The comparison the authors want tested is between current reconnection at the base of the laboratory jet and the accretion and reconnection thought to drive episodic outflows from young stars.Future directions
The way in
https://doi.org/10.1007/978-90-481-9999-0_4How to cite it
F. Suzuki-Vidal, S. V. Lebedev, A. Ciardi, S. N. Bland, J. P. Chittenden, G. N. Hall, A. J. Harvey-Thompson, A. Marocchino, Cheng Ning, C. Stehlé, Adam Frank, Eric G. Blackman, S. C. Bott, T. P. Ray (2009) Formation of episodic magnetically driven radiatively cooled plasma jets in the laboratory doi:10.1007/978-90-481-9999-0_4