Investigation of Plasma Evolution in a Coaxial Small-Gap Magnetically Insulated Transmission Line

Иванов, В. В.; Laca, P.J.; Bauer, Bruno; Presura, R.; Sotnikov, V. I.; Astanovitskiy, A. L.; Galloudec, B. Le; Glassman, Jack; Wirtz, R. A.

doi:10.1109/tps.2004.835491

Cited by 26 publications

(7 citation statements)

References 11 publications

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“…The temperature range where all of the spectral features could have been observed simultaneously was 2-2.5 eV. This is consistent with previous electrode-plasma temperature estimates at these linear current densities (0.1-1 MA=cm) [57][58][59].…”

Section: B Density and Temperaturesupporting

confidence: 91%

Experimental study of current loss and plasma formation in the Z machine post-hole convolute

Gómez

Gilgenbach

Cuneo

et al. 2017

Phys. Rev. Accel. Beams

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The Z pulsed-power generator at Sandia National Laboratories drives high energy density physics experiments with load currents of up to 26 MA. Z utilizes a double post-hole convolute to combine the current from four parallel magnetically insulated transmission lines into a single transmission line just upstream of the load. Current loss is observed in most experiments and is traditionally attributed to inefficient convolute performance. The apparent loss current varies substantially for z-pinch loads with different inductance histories; however, a similar convolute impedance history is observed for all load types. This paper details direct spectroscopic measurements of plasma density, temperature, and apparent and actual plasma closure velocities within the convolute. Spectral measurements indicate a correlation between impedance collapse and plasma formation in the convolute. Absorption features in the spectra show the convolute plasma consists primarily of hydrogen, which likely forms from desorbed electrode contaminant species such as H 2 O, H 2 , and hydrocarbons. Plasma densities increase from 1 × 10 16 cm −3 (level of detectability) just before peak current to over 1 × 10 17 cm −3 at stagnation (tens of ns later). The density seems to be highest near the cathode surface, with an apparent cathode to anode plasma velocity in the range of 35-50 cm=μs. Similar plasma conditions and convolute impedance histories are observed in experiments with high and low losses, suggesting that losses are driven largely by load dynamics, which determine the voltage on the convolute.

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Section: B Density and Temperaturesupporting

confidence: 91%

Experimental study of current loss and plasma formation in the Z machine post-hole convolute

Gómez

Gilgenbach

Cuneo

et al. 2017

Phys. Rev. Accel. Beams

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“…The plasma (once transitioned from the injected neutral particles) is a fully ionized, electron-proton population with 10 14 -10 17 cm ÿ3 number density at 3 eV (consistent with available data in Refs. [17,18]). A single simulation is given in Table I for this case with a relatively high plasma desorption rate (D 0:25 ml=ns), demonstrating the dramatic loss of coupled load current that can result from cathode plasma desorption, independent of any anode effects.…”

Section: Simulation Resultsmentioning

confidence: 99%

“…Experiments with coaxial MITLs, however, have been carried out to specifically measure the impact of electrode plasmas [15][16][17][18][19]. For example, at relevant current levels and voltages of order 1 MA and 1 MV, plasma evolution from the cathode was observed on time scales of approximately 100 ns, with densities of order 10 16 cm ÿ3 and temperatures of order 3 eV [17,18]. This experiment used coaxial MITLs with anode-cathode (AK) gaps of order 2 mm and carefully prepared (cleaned) electrode surfaces.…”

Section: Introductionmentioning

confidence: 99%

Plasma evolution and dynamics in high-power vacuum-transmission-line post-hole convolutes

Rose

Welch

Hughes

et al. 2008

Phys. Rev. ST Accel. Beams

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Vacuum-post-hole convolutes are used in pulsed high-power generators to join several magnetically insulated transmission lines (MITL) in parallel. Such convolutes add the output currents of the MITLs, and deliver the combined current to a single MITL that, in turn, delivers the current to a load. Magnetic insulation of electron flow, established upstream of the convolute region, is lost at the convolute due to symmetry breaking and the formation of magnetic nulls, resulting in some current losses. At very highpower operating levels and long pulse durations, the expansion of electrode plasmas into the MITL of such devices is considered likely. This work examines the evolution and dynamics of cathode plasmas in the double-post-hole convolutes used on the Z accelerator [R. B. Spielman et al., Phys. Plasmas 5, 2105]. Three-dimensional particle-in-cell (PIC) simulations that model the entire radial extent of the Z accelerator convolute -from the parallel-plate transmission-line power feeds to the z-pinch load regionare used to determine electron losses in the convolute. The results of the simulations demonstrate that significant current losses (1.5 MA out of a total system current of 18.5 MA), which are comparable to the losses observed experimentally, could be caused by the expansion of cathode plasmas in the convolute regions.

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“…At a radius of $20 cm, the MITLs transition from tapered lines to have a constant 1 cm AK gap. It is necessary to avoid very small gaps for radii <20 cm due to electrode plasma formation and drift which can eventually result in AK gap closure [29][30][31][32]. The transition from a tapered, constant impedance gap to a uniform radial gap results in an increasing vacuum impedance that causes sheath ''lift off'' and drives quasiperiodic structures (vortices) in the electron flow [17,21,33].…”

Section: Introductionmentioning

confidence: 99%

Steady-state modeling of current loss in a post-hole convolute driven by high power magnetically insulated transmission lines

Madrid

Rose

Welch

et al. 2013

Phys. Rev. ST Accel. Beams

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Quasiequilibrium power flow in two radial magnetically insulated transmission lines (MITLs) coupled to a vacuum post-hole convolute is studied at 50 TW-200 TW using three-dimensional particle-in-cell simulations The voltages assumed for this study result in electron emission from all cathode surfaces. Electrons emitted from the MITL cathodes upstream of the convolute cause a portion of the MITL current to be carried by an electron sheath. Under the simplifying assumptions made by the simulations, it is found that the transition from the two MITLs to the convolute results in the loss of most of the sheath current to anode structures. The loss is quantified as a function of radius and correlated with Poynting vector stream lines which would be followed by individual electrons. For a fixed MITLconvolute geometry, the current loss, defined to be the difference between the total (i.e. anode) current in the system upstream of the convolute and the current delivered to the load, increases with both operating voltage and load impedance. It is also found that in the absence of ion emission, the convolute is efficient when the load impedance is much less than the impedance of the two parallel MITLs. The effects of space-charge-limited (SCL) ion emission from anode surfaces are considered for several specific cases. Ion emission from anode surfaces in the convolute is found to increase the current loss by a factor of 2-3. When SCL ion emission is allowed from anode surfaces in the MITLs upstream of the convolute, substantially higher current losses are obtained. Note that the results reported here are valid given the spatial resolution used for the simulations.

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Investigation of Plasma Evolution in a Coaxial Small-Gap Magnetically Insulated Transmission Line

Cited by 26 publications

References 11 publications

Experimental study of current loss and plasma formation in the Z machine post-hole convolute

Experimental study of current loss and plasma formation in the Z machine post-hole convolute

Plasma evolution and dynamics in high-power vacuum-transmission-line post-hole convolutes

Steady-state modeling of current loss in a post-hole convolute driven by high power magnetically insulated transmission lines

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