A. Morell

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, ...

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

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.

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 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.

2D r-z Analysis of Hohlraum Experiments on Sphinx Machine using Axial Radiation from a Wire Array Z-pinch

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.

Investigation of Switch Designs for the Dynamic Load Current Multiplier Scheme on the SPHYNX Microsecond Linear Transformer Driver

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.

PPPS-2013: Microsecond ramp compression of a metallic liner driven by a shaped 5 MA current on the SPHINX machine

Summary form only given. SPHINX is a 6MA, 1-μs Linear Transformer Driver operated by the CEA Gramat (France) and primarily used for imploding Z-pinch loads for radiation effects studies. A method for performing magnetic ramp compression experiments, without modifying the generator operation scheme, is developed using a compact pulse shaping system. This system, a Dynamic Load Current Multiplier (DLCM), is inserted in vacuum between the convolute and the load. We present the overall experimental configuration chosen for these experiments, based on electrical and hydrodynamic simulations. Initial results obtained over a set of experiments on an aluminum cylindrical liner, ramp-compressed to a peak pressure of 230 kbar, are presented and analyzed. Detailed features of the electrical and Photonic Doppler Velocimetry (PDV) setups used to monitor and diagnose the ramp compression experiments are provided. Current profiles measured at various critical locations throughout the system, particularly the load current, are in good agreement with simulated current profiles. They enabled a comprehensive tracking of the current circulation and demonstrate adequate pulse shaping by the DLCM. Also, the liner inner free surface velocity measurements agree with the hydrocode results obtained using the measured load current as the input. 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) and optimizing the load dimensions.

Microsecond conical wire array experiments as a source of plasma jets relevant to laboratory astrophysics experiments

Summary form only given. OEDIPE is a compact 950 kA 1.2 µs rise time Linear Transformer Driver generator used as a test-bed for the higher current SPHINX machine. Tungsten conical wire array experiments have been performed on this test-bed in order to produce plasma jets relevant to laboratory astrophysics experiments. Initial experiments have demonstrated that jets produced on a microsecond time-scale reach up to 10 cm length and last over the entire duration of the current discharge.

Spectral characterization of al and ar K-shell z-pinch source on Sphinx

X-ray spectroscopy is a key diagnostic for characterizing plasma radiation Sources. A wide range of spectroscopic techniques is used on the Sphinx machine to study the spectral emission from aluminum and argon z-pinch loads. Depending on the desired time or spectral resolution, either dispersive crystal-based, time-integrated and time-resolved methods or non-dispersive techniques, using a combination of relevant x-ray detectors and filters, were developed. In particular, a defocusing crystal-based spectrometer which is time-integrated, but spatially-resolved, is utilized to capture the X emission either on films, or on image plates, at 1.7 keV or 3.1 keV. As an upgrade of this configuration, pin diodes were added to provide continuous time resolution on the main K-lines or K continuum. The output of this technique is cross-compared to the results obtained using a set of detectors/filters of different thicknesses and absorption K-edges.

Numerical analysis of hohlraum experiments using direct axial flux done on the sphinx (6MA, 800NS) z-pinch machine

In this paper, we present recent analysis that allowed us to explain the full history of the temperature pulse shape and highlights the importance of the interaction between the axial plasma jet and the hohlraum wall in expansion, which artificially increases the temperature seen by diagnostics. Experimental/numerical results confrontation will be shown. Conclusion on this kind of vaccum hohlraum configuration is given.

Analysis of single array experiments on sphinx machine in 6 MA-800 ns regime for radiation effects and hedp studies

The Sphinx machine1 is a 6 MA, 1 µs driver based on the LTD technology, used for Z-pinch experiments. Despite the long implosion time (600 to 900 ns), Sphinx wire arrays with large radius (≫40 mm) have already shown to behave and to be controlled by the same physical processes as faster Z-pinches.