F. Lassalle

Use of microsecond current prepulse for dramatic improvements of wire array Z-pinch implosion

The Sphinx machine [F. Lassalle et al., “Status on the SPHINX machine based on the 1microsecond LTD technology”] based on microsecond linear transformer driver (LTD) technology is used to implode an aluminium wire array with an outer diameter up to 140mm and maximum current from 3.5to5MA. 700to800ns implosion Z-pinch experiments are performed on this driver essentially with aluminium. Best results obtained before the improvement described in this paper were 1–3TW radial total power, 100–300kJ total yield, and 20–30kJ energy above 1keV. An auxiliary generator was added to the Sphinx machine in order to allow a multi microsecond current to be injected through the wire array load before the start of the main current. Amplitude and duration of this current prepulse are adjustable, with maxima ∼10kA and 50μs. This prepulse dramatically changes the ablation phase leading to an improvement of the axial homogeneity of both the implosion and the final radiating column. Total power was multiplied by a factor of 6, ...

Gas-Puff-on-Wire-Array

Results of the experiments carried out on the GIT-12 generator at the current level of 3.5 MA and the Z-pinch implosion times from 700 ns to 1.1 mus are reported. A gas-puff-on-wire-array (triple-shell) load configuration with the outer double gas puff (neon) and the inner wire array (aluminum) was used in the experiments. In the course of the research, implosion dynamics of the triple-shell Z-pinch was studied, and the radiation yield in the spectral range of neon and aluminum K-shell X-rays has been measured. Optimization of the inner wire array parameters (masses and initial diameters) aimed at obtaining the maximum aluminum K-shell radiation yield has been carried out.

Z

Some compact pulsed‐current generators are described here. They allow the generation of isentropic compression loading of metals and other materials. The range of pressures achievable is 30–300 kbar for the Explosive Switch Compact Generator (ESCG). This generator consists of a RLC circuit that discharges in a strip (also called plate)‐line insulated by dielectric foils. Placed on the end of this strip‐line, some material samples can be studied under dynamical loading. Typical dimensions of the tested samples are 1 mm thick, 8mm diameter. A current of 700 kA to 1.6 MA allows some 30–300 kbar ramp pressures generated with a 500ns rise time. The switch used in the ESCG is a linear‐wave‐explosive switch. 0D circuit and MHD simulations are discussed and compared with experimental results. Diagnostics based on current‐voltage and free‐surface velocity measurements are presented. Finally, the generation of isentropic compression principles and simulations discussed above are used to analyse the potential of a n...

-Pinch Experiments on the GIT-12 Generator at Microsecond Implosion Times

SPHINX is a 6 MA, 1-μs Linear Transformer Driver (LTD) operated by the CEA Gramat (France) and primarily used for imploding Z-pinch loads for radiation effects studies. Among the options that are currently being evaluated to improve the generator performances are an upgrade to a 20 MA, 1-μs LTD machine and various power amplification schemes, including a compact Dynamic Load Current Multiplier (DLCM). A method for performing magnetic ramp compression experiments, without modifying the generator operation scheme, was developed using the DLCM to shape the initial current pulse in order to obtain the desired load current profile. In this paper, we discuss the overall configuration that was selected for these experiments, including the choice of a coaxial cylindrical geometry for the load and its return current electrode. We present both 3-D Magneto-hydrodynamic and 1D Lagrangian hydrodynamic simulations which helped guide the design of the experimental configuration. Initial results obtained over a set of experiments on an aluminium cylindrical liner, ramp-compressed to a peak pressure of 23 GPa, are presented and analyzed. Details of the electrical and laser Doppler interferometer setups used to monitor and diagnose the ramp compression experiments are provided. In particular, the configuration used to field both homodyne and heterodyne velocimetry diagnostics in the reduced access available within the liners interior is described. Current profiles measured at various critical locations across the system, particularly the load current, enabled a comprehensive tracking of the current circulation and demonstrate adequate pulse shaping by the DLCM. The liner inner free surface velocity measurements obtained from the heterodyne velocimeter agree with the hydrocode results obtained using the measured load current as the input. An extensive hydrodynamic analysis is carried out to examine information such as pressure and particle velocity history profiles or magnetic diffusion across the liner. The potential of the technique in terms of applications and achievable ramp pressure levels lies in the prospects for improving the DLCM efficiency through the use of a closing switch (currently under development), reducing the load dimensions and optimizing the diagnostics.

SYRINX Project: HPP Generators Devoted to Isentropic Compression Experiments

Some compact pulsed-current generators are described here. They allow the generation of isentropic compression loading of metals and other materials. The range of pressures achievable is 30-300 kbar for the explosive switch compact generator (ESCG). This generator consists of a RLC circuit that discharges in a strip (also called plate)-line insulated by a dielectric foil. Placed on the end of this strip-line, some material samples can be studied under dynamical loading. Typical dimensions of the tested samples are 1 mm thick, 8 mm diameter. A current of 700 kA to 1.7 MA allows some 30-300 kbar ramp pressures generated with a 500 ns rise time. The switch used in the ESCG is a linear-wave-explosive switch. OD circuit and MHD simulations are discussed and confronted to experimental results. Diagnostics based on current-voltage and free-surface velocity measurements are presented. Finally, the generation of isentropic compression principles and simulations discussed above are used to analyse the potential of a new compact generator for which development is contracted to ITHPP a French company. This generator should allow us to explore the 100 kbar-1 Mbar isentropic compression regime in order to study material behavior under dynamic loading for a large range of material and geometry samples.

Microsecond ramp compression of a metallic liner driven by a 5 MA current on the SPHINX machine using a dynamic load current multiplier pulse shaping

Results of the experiments carried out on the GIT‐12 generator at the current level of 3.5 MA and the Z‐pinch implosion times from 700 ns to 1.1 μs are presented. A multi‐shell (triple‐shell) load configuration with the outer gas puffs (neon) and the inner wire array (aluminum) was used in the experiments. In the course of the research, implosion dynamics of the triple‐shell z‐pinch was studied, and the radiation yield in the spectral range of neon and aluminum K‐lines have been measured. Optimization of the inner wire array parameters aimed at obtaining the maximum aluminum K‐shell radiation yield has been carried out. As a result of optimization of the gas‐puff‐on‐wire‐array Z‐pinch load, the aluminum K‐shell radiation yield (hv> 1.55 keV) up to 4 kJ/cm in the radiation pulse with FWHM less than 30 ns has been obtained. Comparison of the experimental results with the results of preliminary 1D RMHD simulations allows a conclusion that at least 2/3 of the generator current is switched from a gas puff to a...

Syrinx project: compact pulse-current generators devoted to material study under isentropic compression loading

Summary form only given. The current multiplier (CM) concept was proposed to increase the driver-to-load energy transfer efficiency. The suggested CM requires additional volumes with high self-inductance (magnetic flux extruders) connected through vacuum convolutes prior to the load and they extrude the magnetic flux toward the load magnifying the load current. We present the design criteria allowing to achieve high extruder self-inductance at low parasitic inductances added to generator and load in the modified circuit. Two configurations of this new device with one extruder are considered for GIT 12 microsecond MA generator having inductance of ~100 nH. The extruder inductance was either a large vacuum volume or a smaller volume with magnetic core. The discussed design procedure allowed to define optimum coreless and cored CM hardware configurations at conservative values of the AK gaps in CM vacuum lines (15-25 mm). The optimum coreless CM had 80 cm height and 170 cm diameter. Operating on a 8 nH inductive load in experimental tests on GIT 12 it allowed to increase the load current from 4.7 MA @ 1.7 mus without CM to ~6 MA @ 1.5 mus2. A more compact cored CM configuration with 36 cm height and 85 cm diameter operating at a 4.6 nH inductive load allowed further load current increase up to ~8 MA @ 1.7 mus. Further experimental tests with a Ne gas-puff z-pinch load showed that the peak load current at ~1 mus was increased from 3.5 MA (no CM) to 5.2 MA and that the energy-delivered to load at implosion was increased from ~170 kJ to ~330 kJ. No considerable energy losses in the CM vacuum gaps and CM convolute were recorded. Therefore, it is experimentally confirmed that the proposed new device is applicable for improving characteristics of existing and future pulse-power generators.

Experiments with a Gas‐Puff‐On‐Wire‐Array Load on the GIT‐12 Generator for Al K‐shell Radiation Production at Microsecond Implosion Times

SPHINX is a microsecond linear transformer driver located at Atomic Energy Comission (CEA) Gramat (France), which can deliver a current pulse of 6 MA within 800 ns in a Z -pinch load. Using the concept of the dynamic load current multiplier (DLCM), which was proposed by Chuvatin, we expect to increase the load current above 6 MA, while decreasing its rise time to ~ 300 ns. The DLCM developed by the CEA Gramat and International Technologies for High Pulsed Power (ITHPP) is a compact system made up of concentric electrodes (autotransformer), a dynamic flux extruder (cylindrical wire array), a vacuum convolute (eight post-hole rods), and a closing switch (compact vacuum surface switch). The latter is a key component of the system, which is used to prevent the current from flowing into the load until the inductance builds up due to the implosion of the wire array. This paper presents the design and testing of the DLCM surface switch, resulting from both electrostatic simulations and experiments on the SPHINX generator. These studies, carried out either with or without load (open circuit), were valuable for a first experimental evaluation of the DLCM scheme in a microsecond regime and provided detailed information on the surface switch behavior.

Design and Experimental Validation of Two Current Multiplier Configurations on a Microsecond MA Generator

Axial radiation flux from an aluminum Z-pinch on SPHINX machine is used to drive 5 mm diameter, 7 mm height hohlraums. 2006 results demonstrated that >200 GW power in 10 ns could be achieved creating >35 eV radiation temperatures. A 20-30 ns delay between the axial power starting time and the temperature rise had, however, to be explained. In this paper, analysis of this behaviour based on 2D r-z hydroradiative simulations is shown. A mobile source with plasma starting from 3 to 10 mm upstream of the holhlraum and with ~30 cm/mus velocity is an essential feature. This hypothesis of a mobile source is also inferred by Z-pinch zipper effect measurements and by pictures of Z-pinch implosion which show edge effects on the same spatial scale. Improvement of the configuration is considered, like using a conical pedestal to limit edge effects on the Z-pinch or using a power improved Z-pinch source. First experimental results with tins improved configuration are shown.

Design and Testing of a Surface Switch for the Dynamic Load Current Multiplier on the SPHINX Microsecond LTD

SPHINX is a microsecond linear transformer driver LTD, used essentially for implosion of Z-pinch loads in direct drive mode. It can deliver a 6-MA current pulse within 800 ns into a Z-pinch load. The dynamic load current multiplier concept enables the current pulse to be modified by increasing its amplitude while reducing its rise time before being delivered to the load. This compact system is made up of concentric electrodes (autotransformer), a dynamic flux extruder (cylindrical wire array), a vacuum convolute (eight postholes), and a vacuum closing switch, which is the key component of the system. Several different schemes are investigated for designing a vacuum switch suitable for operating the dynamic load current multiplier on the SPHINX generator for various applications, including isentropic compression experiments and Z-pinch radiation effects studies. In particular, the design of a compact vacuum surface switch and a multichannel vacuum switch, located upstream of the load are studied. Electrostatic simulations supporting the switch designs are presented along with test bed experiments. Initial results from shots on the SPHINX driver are also presented.