Small Gap Experiments in Magnetically Insulated Transmission Lines

Self Cite

We describe herein a system of self-magnetically insulated vacuum transmission lines (MITLs) that operated successfully at 20 MA, 3 MV, and 55 TW. The system delivered the electromagnetic-power pulse generated by the Z accelerator to a physics-package load on over 1700 Z shots. The system included four levels that were electrically in parallel. Each level consisted of a water flare, vacuum-insulator stack, vacuum flare, and 1.3-m-radius conical outer MITL. The outputs of the four outer MITLs were connected in parallel by a 7.6-cm-radius 12-post double-post-hole vacuum convolute. The convolute added the currents of the four outer MITLs, and delivered the combined current to a single 6-cm-long inner MITL. The inner MITL delivered the current to the load. The total initial inductance of the stack-MITL system was 11 nH. A 300-element transmission-line-circuit model of the system has been developed using the TL code. The model accounts for the following: (i) impedance and electrical length of each of the 300 circuit elements, (ii) electron emission from MITL-cathode surfaces wherever the electric field has previously exceeded a constant threshold value, (iii) Child-Langmuir electron loss in the MITLs before magnetic insulation is established, (iv) MITL-flow-electron loss after insulation, assuming either collisionless or collisional electron flow, (v) MITL-gap closure, (vi) energy loss to MITL conductors operated at high lineal current densities, (vii) time-dependent self-consistent inductance of an imploding z-pinch load, and (viii) load resistance, which is assumed to be constant. Simulations performed with the TL model demonstrate that the nominal geometric outer-MITL-system impedance that optimizes overall performance is a factor of $3 greater than the convolute-load impedance, which is consistent with an analytic model of an idealized MITL-load system. Power-flow measurements demonstrate that, until peak current, the Z stack-MITL system performed as expected. TL calculations of the peak electromagnetic power at the stack, stack energy, stack voltage, outer-MITL current, and load current, as well as the pinch-implosion time, agree with measurements to within 5%. After peak current, TL calculations and measurements diverge, which appears to be due in part to the idealized pinch model assumed by TL. The results presented suggest that the design of the Z accelerator's stack-MITL system, and the TL model, can serve as starting points for the design of stack-MITL systems of future superpower accelerators.

Section: Gap Closurementioning

confidence: 99%

55-TW magnetically insulated transmission-line system: Design, simulations, and performance

Stygar¹,

Corcoran²,

Ives³

et al. 2009

Self Cite

“…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%

“…First, negative ion production in a cathode plasma [29,40,41] can lead to anode plasma formation due to unmagnetized negative ions crossing the AK gap and striking the anode. Second, charge-exchange between neutrals and plasma ions can rapidly increase the plasma layer thicknesses and lead to AK gap closure from the anode electrode [42,43].…”

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

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.

“…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].…”

Section: Introductionmentioning

confidence: 99%

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

Rose

Welch

Hughes

et al. 2008

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.