A review of the dense Z-pinch

Plasma Phys. Control. Fusion 53 (2011) 093001
Topical Review
This alleviates the need for complex liquid–vacuum geometry, as the pulsed power energy is
coupled to the Z-pinch by direct contact. It also eases the vacuum requirements in the chamber
which will have a 10 Torr pressure background of argon. However the inner gap of the RTL
must operate in high vacuum to prevent electric breakdown, and a sliding seal arrangement has
been devised to allow the pre-pumped RTLs to be inserted and connected to the pulsed power
source, without opening the full chamber to high vacuum between each shot. After that the
RTL will be destroyed, and a new RTL inserted. Therefore the RTL can be built either of frozen
flibe which will melt in the coolant, or of a material immiscible in the molten salt which could
then be separated and recycled. To alleviate shock generation arising from the pulsed energy
absorption the flibe is in the form of curtains, with voids between, and the bubbling pool at
the bottom of the chamber is two phase. Aspects of the nuclear technology involved are being
studied in separate experiments. One of these considers low mass recyclable transmission lines
theoretically and experimentally to determine the ability of the electrodes to carry the current
required as a function of electrode thickness [465]. It was concluded that a transmission line
of only a few tens of kilograms of material could carry the large currents require for fusion.
6.5. Technology of pulsed power; longer pulses
The further development of pulsed power to achieve more powerful x-ray sources and ignition
of indirect drive ICF requires a greater understanding of the present technological approach
and its limitations, together with the development of alternative techniques such as linear
transformer drivers (LTDs) pioneered at Tomsk. In brief LTD accelerating cavities contain the
pulse-forming capacitors. These are switched at low voltage, inductively adding the pulses and
using soft iron-core isolation. Higher currents can be obtained by employing many capacitors
in parallel, while higher voltage can be achieved by inductively adding the output voltage of
several cavities in series.
In addition the scaling of the Z-pinch itself to higher currents and the trade-off of longer
current pulses with lower voltages should be explored.
A three-dimensional e.m. model of pulsed power has been developed recently by Rose
et al [466]. It has been applied to the new ZR-accelerator at Sandia which is designed to give
27 MA. This represents a major computational step compared with the earlier development of
the Z-accelerator [467]. Extrapolating to new designs however assumes a knowledge of the
scaling laws of the Z-pinch itself. Empirical scaling relations have been developed by Stygar
et al [468], but these ignore the fact that the radiated energy can be three or four times the
kinetic energy of the implosion, which is discussed in sections 5.8 and 7.2. Indeed the analysis
assumes competition between ablation-dominated and RT-dominated pinches, each of which
has its own scaling law, and concludes that a decrease in implosion time is preferential.
In contrast, experiments were undertaken ten years ago by Deeney et al [386] and Douglas
et al [469] to see if x-ray powers could be maintained with a longer current rise-time (∼250 ns),
implying a lower voltage and fewer insulator problems. These were surprisingly successful.
Characteristics of wire-array implosions on the longer 1 µs time scale are being carried out on
the SPHINX machine at CEA, Gramat [470]. This leads to a longer x-ray pulse, dominated by
a large plasma bubble at the cathode and the associated zippering along the axis. However there
has been a dramatic improvement in the performance of SPHINX by employing a microsecond
prepulse [471]. Previously the long x-ray pulse [470] was a result of a cathode bubble (see
section 5.10) which collapsed onto the axis first, starting the main x-ray pulse. During the
prepulse the wires are heated, but after ∼4 µs at 400 V there occurs plasma breakdown,
presumably of vaporized low Z impurities, dropping the voltage to 40 V. The current at this time
is 1750 or 8 A per wire and the deposited energy is far below that needed to cause melting of
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