Jean Larour

Theoretical and experimental behavior of a compact magnetically insulated line oscillator electromagnetic structure

The magnetically insulated line oscillator (MILO) is a high power microwave source capable of delivering output powers greater than 1GW at a frequency of several gigahertz. The device is a crossed electric and magnetic field oscillator without any external structure to establish the static magnetic field, required to guide the electron beam, thus making it compact. We report on a compact MILO device based on the U.S. Air-Force design where all dimensions are reduced by a factor of 2. All the key points are reviewed using the Electromagnetic-PIC (Particle-In-Cell) code MAGIC in two-dimensional geometry. The operating frequency of such a device is 2.44GHz, leading to an output power slightly above 1GW for an operating voltage of 500kV and a maximum current of 45kA. The output power can be increased up to 2GW by optimizing the output coupling and reducing the beam loading effect. The analysis of the compact electromagnetic structure is based on the calculation of the external Q factor (Qext). The Qext coeffi...

Compact submicrosecond, high current generator for wire explosion experiments

The PIAF generator was designed for low total energy and high energy density experiments with liners, X-pinch or fiber Z-pinch loads. These studies are of interest for such applications as surface and material science, microscopy of biological specimens, lithography of x-ray sensitive resists, and x-ray backlighting of pulsed-power plasmas. The generator is based on an RLC circuit that includes six NWL 180 nF–50 kV capacitors that store up to 1.3 kJ. The capacitors are connected in parallel to a single multispark switch designed to operate at atmospheric pressure. The switch allows reaching a time delay between the trigger pulse and the current pulse of less than 80 ns and has jitter of 6 ns. The total inductance without a load compartment was optimized to be as low as 16 nH, which leads to extremely low impedance of ∼0.12 Ω. A 40 kV initial voltage provides 250 kA maximum current in a 6 nH inductive load with a 180 ns current rise time. PIAF has dimensions of 660×660×490 mm and weight of less than 100 kg...

Evidence of the 3π∕4 interaction mode in a compact magnetically insulated line oscillator process

The electron beam generated in crossed-field devices such as the magnetron and the magnetically insulated line oscillator (MILO), which is closely related to the linear magnetron, interacts with a slow wave structure (SWS) in a configuration where the rf electromagnetic field is 180° out of phase from one cell to the next inside the periodic structure (the so-called π mode). We present here an analysis of a compact MILO device based on the U.S. Air Force geometry, where all dimensions are reduced by a factor of 2. By considering the geometrical parameters of the SWS, we establish precise criteria to maximize the microwave output power up to 3GW and to optimize the electronic efficiency up to 32%. To do so, the dispersion diagram is studied in such a way as to force the 3π∕4 mode in which the compact MILO oscillates. We report a discussion on the electromagnetic coupling in order to conclude on the output power stability of this interaction regime.

Experimental study of X-pinch in a submicrosecond regime

X-pinch plasmas are known as point-like X-ray sources with the potential application for backlighting diagnostics. X-pinches are commonly driven by large pulsed power generators delivering high-voltage pulses with short rise-times, typically under 100 ns. When operating with slower capacitor banks, X-pinches become less reliable because of the arising X-ray pulse jitter and because of the appearance of two or more X-ray bursts coming from multiple hot spots. In this paper, a very compact inductance-capacitance (LC) generator with the current rise-time of 200 ns was used to drive molybdenum and tungsten wire X-pinches. A single peak, small shot-to-shot jitter emission of X-rays was obtained. Time-integrated penumbral imaging recorded the X-ray source dimension of less than 15 /spl mu/m in the spectral region above 2.4 keV. The total yield of more than 80 mJ was registered with the radiation pulse duration as short as 1.5 ns. The appearance of single- or multiple-source core structures is discussed in correlation with used wire material and X-pinch torsion angle. The results confirm the possibility of using an X-pinch driven by a fast compact capacitor bank for backlighting applications.

A compact ultrafast capillary discharge for EUV projection lithography

A novel type of ultrafast capillary discharge device was operated in Xenon at low pressure to obtain Extreme UV (EUV) radiation complying with the demands for future microlithography. After being triggered by means of the hollow cathode effect, the discharge starts on-axis and is then heated up effectively to a maximum temperature of 30 eV within a nanosecond timescale using a stored energy of 0.5 J. During a pulse length of 5 ns concerning the EUV emission, the transient plasma inside the capillary whose aspect ratio is much more than 10 emits mainly between 10 and 20 nm. The spectrum consists of Xe VII to Xe x line radiation.

3-D PIC Numerical Investigations of a Novel Concept of Multistage Axial Vircator for Enhanced Microwave Generation

The enhancement of power conversion efficiency of a classical axial VIRtual CAthode oscillaTOR (vircator) by introducing one or more reflectors beyond the anode in the cylindrical waveguide is numerically investigated. The targeted microwave (MW) output frequency lies in the S-band at around 3 GHz for an operation in TM01 mode. Powered by a 511-kV voltage signal for a duration of 45 ns, the design under consideration operates with an injected electron beam of mean voltage and mean current of around 508 kV and 19 kA, respectively. Full-wave 3-D modeling is performed using well-tested electromagnetic particle-in-cell codes such as Computer Simulation Technology Particle Studio and Magic. Simple rules for designing and installing the reflectors are given. The number of reflectors required to maximize the efficiency is discussed. The power conversion efficiency is shown to be improved over a classical axial vircator design by a factor of 12.8. A maximum mean output power of about 1.26 GW is delivered off-axis in the S-band at around 3 GHz, with an efficiency of nearly 13%. Besides, it is also shown that increasing the number of reflectors allows switching the operation mode from TM01 to TE11 along with a shift of MWfrequency from the S- to the L-band. A five- or six-reflector configuration is predicted to generate MW at both 2.86 and 1.4 GHz with conversion efficiencies ranging from 3.5% to 6.6%. A vircator including seven reflectors is expected to operate in TE11 mode at 1.4 GHz with an efficiency of about 8%.

Modeling of the L-shell copper X-pinch plasma produced by the compact generator of Ecole polytechnique using pattern recognition

Principal component analysis is applied and compared with the line ratios of special Ne-like transitions for investigating the electron beam effects on the L-shell Cu synthetic spectra. The database for the principal component extraction is created over a non Local Thermodynamic Equilibrium (non-LTE) collisional radiative L-shell Copper model. The extracted principal components are used as a database for Artificial Neural Network in order to estimate the plasma electron temperature density and beam fractions from a representative time-integrated spatially resolved L-shell Cu X-pinch plasma spectrum. The spectrum is produced by the explosion of 25-μm Cu wires on a compact LC (40 kV 200 kA and 200 ns) generator. The modeled plasma electron temperatures are about Te ∼ 150 eV and Ne = 5 × 1019 cm−3 in the presence of the fraction of the beams with f ∼ 0.05 and a centered energy of ∼10 keV.

Improved Design of a Multistage Axial Vircator With Reflectors for Enhanced Performances

The basic design of an axial virtual cathode oscillator (vircator) with axial extraction operating in TM01 mode is modified by introducing thin conducting disks, also called reflectors, into the cylindrical waveguide. The operation principal of this novel type of device relies on the formation of a series of virtual cathodes, located at the center of adjacent quasi-cavities. The behavior of this new type of multistage vircator is numerically investigated using CST Particle Studio 3-D particle-in-cell code. Progressively decreasing the radii of the reflectors installed upstream in the tube allows the mitigation of spurious modes. Tapering the radii of the reflectors turns out to be crucial in focusing the electron beam on axis in the downstream region and maximizing the TM01 power conversion efficiency. This novel architecture enables a five-reflector vircator operating with an injected electron beam of 508-kV mean voltage and 19-kA mean current to deliver up to 2-GW mean power sustained only by the TM01 mode in the S-band with a power conversion efficiency close to 21%.

Vulnerability of optical detection systems to megajoule class laser radiative environment

The Laser MegaJoule (LMJ) facility will host inertial confinement fusion experiments in order to achieve ignition by imploding a Deuterium-Tritium filled microballoon [1]. In this context an X-ray imaging system is necessary to diagnose the core size and the shape of the target in the 10-100 keV band. Such a diagnostic will be composed of two parts: an X-ray optical system and a detection assembly. The survivability of each element of this diagnostic has to be ensured within the mixed pulse consisting of X-rays, gamma rays and 14 MeV neutrons created by fusion reactions. The design of this diagnostic will take into account optics and detectors vulnerability to neutron yield of at least 1016. In this work, we will present the main results of our vulnerability studies and of our hardening-by-system and hardening-by- design studies.

Characterization of Zn X-Ray Laser at PALS Centre, Its Applications in Dense Plasma Probing and Astrophysics

This report presents the results from experiments at PALS Centre using a Zn X-ray laser with the pulse length of 0.15 ns and the wavelength of 21.2 nm, working in single or double pass regime with the output energy of 0.4 mJ or 4 mJ per pulse, respectively. The current X-ray laser was experimentally examined to obtain its temporal coherence and spectral width using a path-difference interferometer. The double pass regime shows that QSS plasma based source-amplifier is promising for “short” fs soft X-ray pulses. The X-ray laser is commonly used for user’s experiments. Its advantages can be shown in applications such as probing of dense plasmas (up to 2.5×1024 cm−3) or single shot experiments (4×1014 photons/pulse). The simple technique based on Talbot effect was used to obtain the gradients of electron densities of line plasmas produced under conditions corresponding to XRL’s amplifiers operating in TCE and QSS regime. To investigate radiative shock wave in laboratory is challenging in aspects of the optimization of experimental parameters. Due to the high electron density (1022 cm−3) produced in the gas medium propagated by the shock wave, the velocity of the shock wave, and the absorption losses on optical elements, it is necessary to use the energetic single shot probe.