The Z-pinch, perhaps the oldest subject in plasma physics, has achieved a remarkable renaissance in recent years, following a few decades of neglect due to its basically unstable MHD character. Using wire arrays, a significant transition at high wire number led to a great improvement in both compression and uniformity of the Z-pinch. Resulting from this the Z-accelerator at Sandia at 20 MA in 100 ns has produced a powerful, short pulse, soft x-ray source >230 TW for 4.5 ns) at a high efficiency of ~15%. This has applications to inertial confinement fusion. Several hohlraum designs have been tested. The vacuum hohlraum has demonstrated the control of symmetry of irradiation on a capsule, while the dynamic hohlraum at a higher radiation temperature of 230 eV has compressed a capsule from 2 mm to 0.8 mm diameter with a neutron yield >3 × 1011 thermal DD neutrons, a record for any capsule implosion. World record ion temperatures of >200 keV have recently been measured in a stainless-steel plasma designed for Kα emission at stagnation, due, it was predicted, to ion-viscous heating associated with the dissipation of fast-growing short wavelength nonlinear MHD instabilities. Direct fusion experiments using deuterium gas-puffs have yielded 3.9 × 1013 neutrons with only 5% asymmetry, suggesting for the first time a mainly thermal source. The physics of wire-array implosions is a dominant theme. It is concerned with the transformation of wires to liquid-vapour expanding cores; then the generation of a surrounding plasma corona which carries most of the current, with inward flowing low magnetic Reynolds number jets correlated with axial instabilities on each wire; later an almost constant velocity, snowplough-like implosion occurs during which gaps appear in the cores, leading to stagnation on the axis, and the production of the main soft-x-ray pulse. These studies have been pursued also with smaller facilities in other laboratories around the world. At Imperial College, conical and radial wire arrays have led to highly collimated tungsten plasma jets with a Mach number of >20, allowing laboratory astrophysics experiments to be undertaken. These highlights will be underpinned in this review with the basic physics of Z-pinches including stability, kinetic effects, and finally its applications.

A review of the dense Z-pinch

We have developed an accelerator architecture that can serve as the basis of the design of petawatt-class $z$-pinch drivers. The architecture has been applied to the design of two $z$-pinch accelerators, each of which can be contained within a 104-m-diameter cylindrical tank. One accelerator is driven by slow ($\ensuremath{\sim}1\text{ }\ensuremath{\mu}\mathrm{s}$) Marx generators, which are a mature technology but which necessitate significant pulse compression to achieve the short pulses ($\ensuremath{\ll}1\text{ }\ensuremath{\mu}\mathrm{s}$) required to drive $z$ pinches. The other is powered by linear transformer drivers (LTDs), which are less mature but produce much shorter pulses than conventional Marxes. Consequently, an LTD-driven accelerator promises to be (at a given pinch current and implosion time) more efficient and reliable. The Marx-driven accelerator produces a peak electrical power of 500 TW and includes the following components: (i) 300 Marx generators that comprise a total of $1.8\ifmmode\times\else\texttimes\fi{}{10}^{4}$ capacitors, store 98 MJ, and erect to 5 MV; (ii) 600 water-dielectric triplate intermediate-store transmission lines, which also serve as pulse-forming lines; (iii) 600 5-MV laser-triggered gas switches; (iv) three monolithic radial-transmission-line impedance transformers, with triplate geometries and exponential impedance profiles; (v) a 6-level 5.5-m-diameter 15-MV vacuum insulator stack; (vi) six magnetically insulated vacuum transmission lines (MITLs); and (vii) a triple-post-hole vacuum convolute that adds the output currents of the six MITLs, and delivers the combined current to a $z$-pinch load. The accelerator delivers an effective peak current of 52 MA to a 10-mm-length $z$ pinch that implodes in 95 ns, and 57 MA to a pinch that implodes in 120 ns. The LTD-driven accelerator includes monolithic radial transformers and a MITL system similar to those described above, but does not include intermediate-store transmission lines, multimegavolt gas switches, or a laser trigger system. Instead, this accelerator is driven by 210 LTD modules that include a total of $1\ifmmode\times\else\texttimes\fi{}{10}^{6}$ capacitors and $5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ 200-kV electrically triggered gas switches. The LTD accelerator stores 182 MJ and produces a peak electrical power of 1000 TW. The accelerator delivers an effective peak current of 68 MA to a pinch that implodes in 95 ns, and 75 MA to a pinch that implodes in 120 ns. Conceptually straightforward upgrades to these designs would deliver even higher pinch currents and faster implosions.

/pdf/architecture-of-petawatt-class-z-pinch-accelerators-eqi07hh1a7.pdf

Architecture of petawatt-class z-pinch accelerators.

High current pulsed-power generators efficiently store and deliver magnetic energy to z-pinch targets. We review applications of magnetically driven implosions (MDIs) to inertial confinement fusion. Previous research on MDIs of wire-array z-pinches for radiation-driven indirect-drive target designs is summarized. Indirect-drive designs are compared with new targets that are imploded by direct application of magnetic pressure produced by the pulsed-power current pulse. We describe target design elements such as larger absorbed energy, magnetized and pre-heated fuel, and cryogenic fuel layers that may relax fusion requirements. These elements are embodied in the magnetized liner inertial fusion (MagLIF) concept [Slutz “Pulsed-power-driven cylindrical liner implosions of laser pre-heated fuel magnetized with an axial field,” Phys. Plasmas, 17, 056303 (2010), and Stephen A. Slutz and Roger A. Vesey, “High-Gain Magnetized Inertial Fusion,” Phys. Rev. Lett., 108, 025003 (2012)]. MagLIF is in the class of magneto-inertial fusion targets. In MagLIF, the large drive currents produce an azimuthal magnetic field that compresses cylindrical liners containing pre-heated and axially pre-magnetized fusion fuel. Scientific breakeven may be achievable on the Z facility with this concept. Simulations of MagLIF with deuterium-tritium fuel indicate that the fusion energy yield can exceed the energy invested in heating the fuel at a peak drive current of about 27 MA. Scientific breakeven does not require alpha particle self-heating and is therefore not equivalent to ignition. Capabilities to perform these experiments will be developed on Z starting in 2013. These simulations and predictions must be validated against a series of experiments over the next five years. Near-term experiments are planned at drive currents of 16 MA with D2 fuel. MagLIF increases the efficiency of coupling energy (=target absorbed energy/driver stored energy) to targets by 10-150X relative to indirect-drive targets. MagLIF also increases the absolute energy absorbed by the target by 10-50X relative to indirect-drive targets. These increases could lead to higher fusion gains and yields. Single-shot high yields are of great utility to national security missions. Higher efficiency and higher gains may also translate into more compelling (lower cost and complexity) fusion reactor designs. We will discuss the broad goals of the emerging research on the MagLIF concept and identify some of the challenges. We will also summarize advances in pulsed-power technology and pulsed-power driver architectures that double the efficiency of the driver.

Magnetically Driven Implosions for Inertial Confinement Fusion at Sandia National Laboratories

Most of the modern high-current high-voltage pulsed power generators require several stages of pulse conditioning (pulse forming) to convert the multimicrosecond pulses of the Marx generator output to the 40-300-ns pulses required by a number of applications including X-ray radiography, pulsed high-current linear accelerators, Z-pinch, isentropic compression, and inertial fusion energy drivers. This makes the devices large, cumbersome to operate, and expensive. Sandia, in collaboration with a number of other institutions, is developing a new paradigm in pulsed power technology: the linear transformer driver (LTD) technology. This technological approach can provide very compact devices that can deliver very fast high-current and high-voltage pulses. The output pulse rise time and width can be easily tailored to the specific application needs. Trains of a large number of high-current pulses can be produced with variable interpulse separation from nanoseconds to milliseconds. Most importantly, these devices can be rep-rated to frequencies only limited by the capacitor specifications (usually 10 Hz). Their footprint, as compared with current day pulsed power accelerators, is considerably smaller since LTD do not require large oil and deionized water tanks. This makes them ideally suited for applications that require portability. In this paper, we present Sandia National Laboratories' broad spectrum of developmental effort to design construct and extensively validate the LTD pulsed power technology.

High-Current Linear Transformer Driver Development at Sandia National Laboratories

We have developed two new gas switches that are designed to be used with linear transformer drivers The switches, which can be DC charged to 200 kV and triggered with less than a 2-ns $1\mathrm{\text{\ensuremath{-}}}\ensuremath{\sigma}$ jitter, have overall inductances ranging from 69 to 85 nH When transferring 400 J of energy per shot, the switches have lifetimes in excess of 5000 shots The two switches are insulated with 130--270 PSIA of air and are submerged in transformer oil during operation These switches should allow development of linear transformer drivers that are more compact and have higher peak power

https://link.aps.org/pdf/10.1103/PhysRevSTAB.13.080401

New low inductance gas switches for linear transformer drivers

The design of a 6.5-MV linear transformer driver (LTD) for flash-radiography experiments is presented. The design is based on a previously tested 1-MV LTD and is predicted to be capable of producing diode voltages of 6.5 MV for a 50-Omega radiographic-diode load. Several fault modes are identified, and circuit simulations are used to determine their effect on the output pulse and other components. For all the identified fault modes, the peak load voltage is reduced by less than 5%

Design, Simulation, and Fault Analysis of a 6.5-MV LTD for Flash X-Ray Radiography

Most of the modern high-current high-voltage pulsed power generators require several stages of pulse conditioning (pulse forming) to convert the multi-microsecond pulses of the Marx generator output to the 40–300 ns pulse required by a number of applications including x-ray radiography, pulsed high current linear accelerators, Z-pinch, Isentropic Compression (ICE), and Inertial Fusion Energy (IFE) drivers. This makes the devices large, cumbersome to operate, and expensive. Sandia, in collaboration with a number of other institutions, is developing a new paradigm in pulsed power technology; the Linear Transformer Driver (LTD) technology. This technological approach can provide very compact devices that can deliver very fast high current and high voltage pulses. The output pulse rise time and width can be easily tailored to the specific application needs. Trains of a large number of high current pulses can be produced with variable inter-pulse separation from nanoseconds to milliseconds. Most importantly, these devices can be rep-rated to frequencies only limited by the capacitor specifications (usually is 10Hz). Their footprint as compared with current day pulsed power accelerators is considerably smaller since LTD do not require large oil and de-ionized water tanks. This makes them ideally fit for applications that require portability. In the present paper we present Sandia Laboratory's broad spectrum of developmental effort to design construct and extensively validate the LTD pulsed power technology.

Linear Transformer Driver (LTD) development at Sandia national laboratory

The RITS accelerator was upgraded from three pulse forming lines (PFL) and inductive voltage adder (IVA) cells to six PFLs and IVA cells during 2005 with radiographic diode experiments beginning in 2006 [1], [2]. The original RITS-6 accelerator configuration is capable of operation at voltages greater than 10 MV with a 80-Ω magnetically-insulated transmission line (MITL). Low impedance diodes such as the self-magnetic pinch (SMP) diode are not matched well to the high impedance MITL and decrease the diode voltage to about 5 MV. A low impedance MITL has been designed and tested with an operating impedance of about 40 Ω at the output end. Experimental results with both MITL impedances will be presented. The output pulse shape of the RITS-6 accelerator can be modified to emulate other accelerators and to investigate the impact of various pulse features on radiographic diode performance. Simulations and experimental results are discussed below.

D.M. VanDeValde

Papers

High-Current Linear Transformer Driver Development at Sandia National Laboratories

Design, Simulation, and Fault Analysis of a 6.5-MV LTD for Flash X-Ray Radiography

Linear Transformer Driver (LTD) development at Sandia national laboratory

RITS-6 Output Pulse Modifications