Pulsed Power Plasmas

 Introduction

 Pulsed Power Drivers

 X-pinches

 Wire arrays

 Pulsed Power ICF

 Laboratory Astrophysics

Wire Array Z-Pinches

Wire array z-pinches comprise a cylindrical arrangement of metal wires (typically Al or W) driven by a fast-rising current drive. Wire diameters are typically 5 to 50 microns in diameter and are strung between two electrodes. On smaller generators, arrays can use as few 8 wires, and larger generators can use several hundred.

For most experiments the drive current is 100 kA to several MA and rises to a maximum in around 100ns. This axial current creates an azimuthal magnetic field which causes the array to collpase onto its axis, creating a high power soft x-ray burst. The largest pulsed power generator in the world is based at Sandia National Laboratories, and produces 1.8 MJ of x-ray energy at a power of 280 TW, and is a potential driver for both Inertial Confinement Fusion (ICF) and Inertial Fusion Energy (IFE) [1]. This has recently been refurbished to provide a peak current of 26 MA.
Wire array z-pinches are a complex plasma system, and produce a wide range of plasma parameters throughout their evolution. The experiment can typically be broken down into 4 identifiable stages:

1) Wire inititation
2) Ablation phase
3) Implosion phase
4) Stagnation and x-ray generation

The high voltage from the pulsed power generator (100kV to several MV) initiates the array by rapidly causing electrical breakdown at the wire surface, forming a plasma in a few nanoseconds. At this point each wire forms an heterogeneous plamsma structure referred to as the core-corona model. A cold dense wire core is surrounded by a hot low density coronal plasma. Due to the greater volume and higher ionisation (an hence conductivity) of the corona, it is this plasma which carried much of the current and hence is accelerated towards the array axis. During the ablation phase, which can last for up to 80% of the experiment, the cores remain stationary and replenish the coronal material as it is removed by the Lorentz force [2].The rate at which mass is ablated from the wires typically scales as I² [3], and this determines the density of both the ablated plasma streams and subsequent structures formed interior to the array as these ‘jets’ converge onto the array axis [4, 5].

The stream densities produced in experiments have ion densities in the range of 1x10¹⁴ – 5x10¹⁷ cm^-3, typically with temperatures 5-15eV and Mach numbers between 3 and 5. Streams at current of 1 MA are typically collisional on the order of the array diameter (~8 mm), and form shock structures observable in emission images around objects place in their path, such as occurs in nested wire array experiments [5].

Eventually enough mass has been ablated from the wires that they begin to break, and this triggers the implosion phase. The JxB force accelerates much of the array mass towards the axis at velocities of 300 km s^-1, as can be deduced from the change in array diameter observed on radial streak camera images

The magnetic piston snow ploughs up the pre-fill plasma which serves to stabilize the implosion surface. The stagnation of this mass at the array axis converts kinetic energy into thermal energy and this, along with additional heating from the plasma compression by the magnetic field, is radiated as a short (< 10ns) high power x-ray pulse. A time slice at stagnation form a 3D Magneto-Hydrodynamic simulation using the GORGON code [7], and example x-ray data is given below from the Z machine at Sandia National Laboratories (Ref [1])

[1] M.E. Cuneo et al, Plasma Phys Control. Fusion, 48, R1 (2006)
[2] S.V.Lebedev et al, Phys. Plasmas, 6, 2016, (1999)
[3] S. Lebedev et al, Phys. Plasmas, 8, 3734, (2001)
[4] S.C.Bott et al, Phys Rev E, 74, 046403 (2006)
[5] S.C.Bott et al, IEEE Trans. Plasma Sci, 35, 165 (2007)
[6] D.J.Ampleford et al, Phys. Rev Lett., to be submitted
[7] J.P.Chittenden et al, Plasma Phys Control. Fusion, 46, B457 (2004)
[8] R.B.Spielman et al, Phys. Plasmas, 5, 2105, (1998)