Ranchero is an explosively driven magnetic flux-compression generator that has been developed, over the last four years, as a versatile power source for high energy density physics experiments. It is coaxial, and comprises a 15 cm-diameter armature and a 30-cm stator, each aluminum. The length may be varied to suit the demands of each experiment; thus far, lengths of 0.43 m and 1.4 m have been used. The stator is filled and driven by a high performance cast explosive, and the ultimate performance of the device is limited by the smoothness of the armature expansion. The armature explosive is initiated on axis by PETN hemispheres, spaced at intervals of about 18 mm and 24.5 mm; each is simultaneously detonated by a slapper detonator system. Calculations of armature expansion predicted ripples less than 0.2 mm, and this was confirmed in early experiments. Yet, ripples approaching tens of millimeters were observed in some more recent experiments. The authors discuss the possible origins of the se large ripples, and the methods the authors have used to correct them.
An experimental series is planned to implode a dense heavy liner to a velocity in excess of 1 cm/microsecond (10 mm/microsecond) using a RANCHERO coaxial explosive flux compression generator. The goal of this study is to choose the liner mass and starting radius that will deliver the greatest amount of kinetic energy to a target at 1 cm final radius. In this study we used the 1D-MHD simulation code RA YEN to search for the proper initial conditions. The results will be used as a starting point for 2-D simulations and preliminary designs for the first experiments planned in the 2009/2010 time frame. The preliminary results indicate that a liner velocity of 1.25 cm/microsecond and a kinetic energy of greater than 4 megajoules may be possible.
In the late 1990s, Los Alamos pursued a coaxial flux compression generator (FCG) concept that was described in several publications under the name 'Ranchero.' These FCGs were designed to be cost effective high current generators, and a variety of configurations were tested. The Ranchero armature is a 152 mm diameter aluminum cylinder with a 6 mm thick wall. The high explosive (HE) is detonated simultaneously on axis, and as the armature expands a factor of two, the wall thins to H" mm. At the final 300 mm diameter, the circumference is over 900 mm, and this should allow currents to be generated in the 90 MA range. No tests significantly over 50 MA have been performed but an experiment is planned. We have recently begun using Ranchero devices for a new application and we continue to improve the design. In this paper we describe recent tests of Ranchero and its subsystems. The load for our new application is an imploding aluminum liner that would deform due to the magnetic pressure applied during the initial flux loading. It will, however, implode properly when powered only during the H"9?s Ranchero flux compression time. This gives rise to a new system with explOSively formed fuse (EFF) opening switches and an integral closing switch that isolates the load. A capacitor bank delivers 2.8 MA to the Ranchero circuit in H"5?s. During this time, four parallel 63.5 mm wide EFFs, external to the coaxial system, complete the circuit. After armature motion begins, insulation which initially isolates the load is severed, connecting the load to the FCG in parallel with the EFFs. External HE charges are initiated on each of the EFFs to produce a resistance rise timed to not precede closure of the load isolation switch. The EFFs achieve significant resistance, and the flux remaining in the 191 nH generator and 3 nH transmission line is compressed to generate 30.85 MA in a H"2.5 nH static load. On three tests, the EFF system has operated flawlessly, and only H"00kA is driven back into the EFFs during peak voltage of the generator output. A test incorporating a 19.5 nH dual liner dynamic load has also been completed, and these results are also presented. Ranchero generators have been operated with armatures from 43 cm to 1.4 m long, corresponding to initial inductances from 56 to 191 nH. MHD code modeling gives better agreement with experiments using modules 43 cm long than the 1.4 m modules, and these results will also be presented.
"Megagauss VIII was held in connection with the conference "Physical Phenomena at High Magnetic Fields - III" (PPHMF-III) in order to encourage and facilitate cross-links between the two scientific communities"--p. xiii.
The authors are currently developing a high explosive pulsed power system concept that they call Ranchero. Ranchero systems consist of series-parallel combinations of simultaneously initiated coaxial magnetic flux compression generators, and are intended to operate in the range from 50 MA to a few hundred MA currents. One example of a Ranchero system is shown here. The coaxial modules lend themselves to extracting the current output either from one end or along the generator midplane. They have previously published design considerations related to the different module configurations, and in this paper they concentrate on the system that they will use for their first imploding liner tests. A single module with end output. The module is 1.4-m long and expands the armature by a factor of two to reach the 30-cm OD stator. The first heavy liner implosion experiments will be conducted in the range of 40--50 MA currents. Electrical tests, to date, have employed high explosive (HE) charges 43-cm long. They have performed tests and related 1D MHD calculations at the 45-MA current level with small loads. From these results, they determine that they can deliver currents of approximately 50 MA to loads of 8 nH.
The Ranchero Magnetic Flux Compression Generator (FCG) has been used to create current pulses in the 10-100 MA range for driving both "static" low inductance (0.5 nH) loads1 for generator demonstration purposes and high inductance (10-20 nH) imploding liner loads2 for ultimate use in physics experiments at very high energy density. Simulations of the standard Ranchero generator have recently shown that it had a design issue that could lead to flux trapping in the generator, and a non- robust predictability in its use in high energy density experiments. A re-examination of the design concept for the standard Ranchero generator, prompted by the possible appearance of an aneurism at the output glide plane, has lead to a new generation of Ranchero generators designated the RancheroS (for swooped). This generator has removed the problematic output glide plane and replaced it with a region of constantly increasing diameter in the output end of the FCG cavity in which the armature is driven outward under the influence of an additional HE load not present in the original Ranchero. The resultant RancheroS generator, to be tested in LA43S-L13, probably in early FY17, has a significantly increased initial inductance and may be able to drive a somewhat higher load inductance than the standard Ranchero. This report will use the Eulerian AMR code Roxane to study the ability of the new design to drive static loads, with a goal of providing a database corresponding to the load inductances for which the generator might be used and the anticipated peak currents such loads might produce in physics experiments. Such a database, combined with a simple analytic model of an ideal generator, where d(LI)/dt = 0, and supplemented by earlier estimates of losses in actual use of the standard Ranchero, scaled to estimate the increase in losses due to the longer current carrying perimeter in the RancheroS, can then be used to bound the expectations for the current drive one may apply to any load assembly in future experiments.
The Eulerian AMR rad-hydro-MHD code Roxane was used to investigate modifications to existing designs of the new RancheroS class of Magnetic Flux Compression Generators (FCGs) which might allow some members of this FCG family to exceed 100 MA driving a 10 nH static load. This report details the results of that study and proposes a specific generator modification which seems to satisfy both the peak current and desired risetime for the current pulse into the load. The details of the study and necessary modifications are presented. For details of the LA43S RancheroS FCG design and predictions for the first use of the generator refer to the relevant publications.
A key element in the design of a coaxial generator system is the simplicity of the geometry. The clean cylindrical geometry allows us a reasonable chance at modeling RANCHERO performance using our 1D and 2D MHD modeling codes. The results of numerical simulations have been compare to several tests of the RANCHERO system in a variety of configurations. Recent comparisons of 1D calculations with the REOT-2 data have been extremely good and suggest that the generator is behaving in a very 1D like nature until reaching 90-95% of peak current. Differences between calculated current and measured performance during the last 3 mm (out of 70 mm) of flux compression may be a consequence of either the EOS for SF6, 2D effects, or both. This study will examine the existing models and attempt to provide a robust integrated model which can then be used to drive design studies, pre- and post-shot analysis, and predict performance parameters for slight variations of the base design of RANCHE RO.
