According to Fourier’s law,
a temperature difference across
a material results in a linear temperature profile and a thermal conductance
that decreases inversely proportional to the system length. These
are the hallmarks of diffusive heat flow. Here, we report heat flow
in ultrathin (25 nm) GaP nanowires in the absence of a temperature
gradient within the wire and find that the heat conductance is independent
of wire length. These observations deviate from Fourier’s law
and are direct proof of ballistic heat flow, persisting for wire lengths
up to at least 15 μm at room temperature. When doubling the
wire diameter, a remarkably sudden transition to diffusive heat flow
is observed. The ballistic heat flow in the ultrathin wires can be
modeled within Landauer’s formalism by ballistic phonons with
an extraordinarily long mean free path.
A 6.1-MV, 0.79-MA laser-triggered gas switch (LTGS) is used to synchronize the 36 modules of the Z machine at Sandia National Laboratories. Each module includes one switch, which serves as the last command-fired switch of the module, and hence is used to determine the time at which each module electrically closes relative to the other modules. The switch is $81-cm in length, $45-cm in diameter, and is immersed in mineral oil. The outer switch envelope consists of six corrugated monomer-cast acrylic insulators and five contoured stainless-steel rings. The trigger electrodes are fabricated from copperinfused tungsten. The switch is pressurized with several atmospheres of sulfur hexafluoride (SF 6 ), which is turbulently purged within 2 seconds after every shot. Each switch is powered from a 6-MV, 0.78-MJ Marx generator which pulse charges a 24-nF intermediate-store water capacitor in 1:4-s. Closure of the switch allows power to flow into pulse-forming transmission lines. The power pulse is subsequently compressed by water switches, which results in a total accelerator output power in excess of 70-TW. A previous version of the LTGS performed exceptionally at a 5.4-MV, 0.7-MA level on an engineering test module used for switch development. It exhibited a 1-jitter of $5 ns, a prefire and flashover rate less than 0.1%, and a lifetime in excess of 150 shots. When installed on the Z accelerator, however, the switch exhibited a prefire probability of $3%, a flashover probability of $7%, and a 15-ns jitter. The difference in performance is attributed to several factors such as higher total charge transfer, exposure to more debris, and more stressful dynamic mechanical loading upon machine discharge. Under these conditions, the replacement lifetime was less than ten shots. Since refurbishment of Z in October 2007, there have been three LTGS design iterations to improve the performance at 6.1-MV. The most recent design exhibits a prefire rate of less than 0.1%, a flashover rate of $0:2%, a single switch jitter of $6-ns, and a lifetime of greater than 75 shots. Modifications to achieve the performance improvement are detailed in this article.
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