Quantitative analysis of plasma ablation using inverse wire array Z pinches

Harvey‐Thompson, A. J.; Лебедев, С. В.; Bland, S. N.; Chittenden, J. P.; Hall, G. N.; Marocchino, A.; Suzuki-Vidal, F.; Bott, S. C.; Palmer, J. B. A.; Ning, Cheng

doi:10.1063/1.3077305

Cited by 51 publications

(47 citation statements)

References 26 publications

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“…From the end of the current pre-pulse and until ∼140-160 ns (depending on wire diameter used) the generator current remains in the inverse wire array. During this time the wires of the inverse array behave in the same way as wires in standard, imploding wire arrays [9]. The wires remain stationary at their initial positions and ablate plasma which flows radially outwards.…”

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confidence: 81%

Suppression of the Ablation Phase in Wire Array Z Pinches Using a Tailored Current Prepulse

et al. 2011

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A new wire array configuration has been used to create thin shell-like implosions in a cylindrical array for the first time. The setup introduces a ∼5 kA, ∼25 ns current prepulse followed by a ∼140 ns current-free interval before the application of the main (∼1 MA) current pulse. The prepulse volumetrically heats the wires which expand to ∼1 mm diameter leaving no dense wire core and without development of instabilities. The main current pulse then ionises all the array mass resulting in suppression of the ablation phase, an accelerating implosion and no trailing mass. Rayleigh-Taylor instability growth in the imploding plasma is inferred to be seeded by µm-scale perturbations on the surface of the wires. The absence of wire cores is found to be the critical factor in altering the implosion dynamics.Fast z-pinch plasma implosions driven by multi megaAmpere currents are very efficient at converting stored electrical energy into X-rays. Achieving high x-ray powers by releasing this energy in a short time requires a high degree of azimuthal symmetry of the imploding plasma and a low level of initial axial perturbations seeding the growth of magnetic Rayleigh-Taylor (MRT) instabilities. The highest x-ray powers in z-pinch implosions were achieved using cylindrical arrays made of large numbers of fine metallic wires (280-300 TW, >2 MJ at ∼20% efficiency on the Z facility at SNL [1] [2]). Experimental studies of wire array z-pinches have shown [3] [4] [5] thateven for large number of wires the arrays do not form a plasma shell imploding as a 2-D object. Instead, the wires remain as discrete, compact objects for the first 60-80% of the implosion and during this time only the coronal plasma, continuously ablated from the stationary cores, are accelerated towards the axis by the J×B force [4]. This ablation phase determines the initial conditions for the implosion in two ways. Firstly, the ablated plasma fills the interior of the array which contributes to the mitigation of the growth of the MRT instability. Secondly, the quasi-periodic modulation of the ablation rate along each individual wire occurring at the 'natural' wavelength (∼250 μm for W and ∼500 μm for Al) is responsible for the large level of axial perturbations at the start of the implosion phase. The existence of the ablation phase is observed in all known wire array z-pinch experiments (currents between ∼1 MA and 26 MA, and implosion times between ∼50 ns and ∼800 ns) and occurs because current is initially carried by the small amount of plasma formed on the wire surfaces, and not by the wire cores themselves.In this letter we report on experiments where, for the first time, the ablation phase in wire array z-pinches was suppressed, including dramatic suppression of the axial perturbations in the wires and suppression of the precursor plasma flow. This was achieved by using a short duration low-level current pre-pulse (∼25 ns, ∼5 kA) followed by a ∼140 ns current-free interval before the main (∼1 MA, 100 ns) current pulse driving the implosion is applied. Th...

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confidence: 81%

Suppression of the Ablation Phase in Wire Array Z Pinches Using a Tailored Current Prepulse

et al. 2011

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“…Previous work has shown that the ablation rate along the length of each wire core is modulated at a well-defined wavelength (k $ 0.25 mm for W), 21 and that this leads to a modulation of the ablation stream density (typical amplitude $50%). 22 Apart from these regular modulations in density, the overall dynamics along the array length are uniform, and thus, our axial average should give a good handle on the overall array dynamics.…”

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confidence: 99%

Shock-less interactions of ablation streams in tungsten wire array z-pinches

et al. 2013

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Shock-less dynamics were observed during the ablation phase in tungsten wire array experiments carried out on the 1.4 MA, 240 ns MAGPIE generator at Imperial College London. This behaviour contrasts with the shock structures which were seen to dominate in previous experiments on aluminium arrays [Swadling et al., Phys. Plasmas 20, 022705 (2013)]. In this paper, we present experimental results and make comparisons both with calculations of the expected mean free paths for collisions between the ablation streams and with previously published Thomson scattering measurements of the plasma parameters in these arrays [Harvey-Thompson et al., Phys. Plasmas 19, 056303 (2012)]. V C 2013 AIP Publishing LLC. [http://dx

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“…Aside from the short resistive phase, the voltage across the inverse array wires (measured at the top of the central cathode rod) remains very low until the explosion of the array. 25 When the explosion begins, the impedance (resistance and inductance) of the wires, and therefore the voltage across them, rapidly increases. This characteristic voltage history makes the inverse array a promising, compact tool for rapidly switching current into a load.…”

Section: Experimental Setup and Overview Of Two-stage Wire Array mentioning

confidence: 99%

“…B. Overview of the operation of a two-stage wire array z-pinch Inverse arrays have been shown to exhibit the same ablation dynamics as standard wire array z-pinches, with the exception that the ablation flow is directed radially outwards. 25,26 At the beginning of the generator current pulse, energy is deposited into the array wires via resistive heating. This "resistive phase" lasts for $20 ns and ends when plasma forms on the surface of the wires.…”

Section: Experimental Setup and Overview Of Two-stage Wire Array mentioning

confidence: 99%

“…The width of the voltage pulse ($15 ns) defines the timespan for energy deposition within the load array, despite the fact that a small amount of current continues to flow in the load, driven by inductive current division heavily biased toward the low inductance inverse array. 23,25 Energy deposition was determined from the experimental data by numerical computation of the following integral:…”

Section: Preconditioning Of the Load Array And Energy Depositionmentioning

confidence: 99%

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Characterisation of the current switch mechanism in two-stage wire array Z-pinches

et al. 2015

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In this paper, we describe the operation of a two-stage wire array z-pinch driven by the 1.4 MA, 240 ns rise-time Magpie pulsed-power device at Imperial College London. In this setup, an inverse wire array acts as a fast current switch, delivering a current pre-pulse into a cylindrical load wire array, before rapidly switching the majority of the generator current into the load after a 100–150 ns dwell time. A detailed analysis of the evolution of the load array during the pre-pulse is presented. Measurements of the load resistivity and energy deposition suggest significant bulk heating of the array mass occurs. The ∼5 kA pre-pulse delivers ∼0.8 J of energy to the load, leaving it in a mixed, predominantly liquid-vapour state. The main current switch occurs as the inverse array begins to explode and plasma expands into the load region. Electrical and imaging diagnostics indicate that the main current switch may evolve in part as a plasma flow switch, driven by the expansion of a magnetic cavity and plasma bubble along the length of the load array. Analysis of implosion trajectories suggests that approximately 1 MA switches into the load in 100 ns, corresponding to a doubling of the generator dI/dt. Potential scaling of the device to higher current machines is discussed.

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Quantitative analysis of plasma ablation using inverse wire array Z pinches

Cited by 51 publications

References 26 publications

Suppression of the Ablation Phase in Wire Array Z Pinches Using a Tailored Current Prepulse

Suppression of the Ablation Phase in Wire Array Z Pinches Using a Tailored Current Prepulse

Shock-less interactions of ablation streams in tungsten wire array z-pinches

Characterisation of the current switch mechanism in two-stage wire array Z-pinches

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