P. Modin

Self-Contained, Hand-Portable, and Repetitive Ultrawideband Radiation Source

This paper presents the design and experimental results of a hand-portable, self-contained, and repetitive radiation source of high-power ultrawideband (UWB) pulses. This source consists of a deployed UWB antenna driven by a high-pulsed power (HPP) generator and powered by a self-contained 50-kV rapid charger at repetition rates up to 100 Hz. By changing the HPP generator, two different electric wave shapes with two different frequency spectra can be generated. Each HPP generator is based on the use of a repetitive Marx generator. On the one hand, a 200-kV/1.4-J Marx generator is associated with a coaxial pulse-forming stage consisting of two highly pressurized spark gap switches and a Blumlein line, which produces bipolar pulses. Its main characteristics are an output voltage of +100 kV/-100 kV and a pulsewidth of 1.5 ns. On the other hand, we have developed a second eight-stage Marx generator, where the pulse-forming stage is the last stage of the structure. It delivers pulses in the 150-kV/1-J range, with a fall time of 300 ps and an 850-ps pulse duration. Electrical signals are radiated by a deployed Valentine antenna. It is a new traveling wave antenna that is designed to radiate high-voltage repetitive pulses with the challenge of high gain and low dispersion in an extremely restricted volume. The design of a rapid charging power supply is also presented, meeting stringent package constraints while still enabling high repetition rates. It has already demonstrated its capability of charging, from a dc power battery, a 5-nF capacitance up to 50 kV in 5 ms at a 100-Hz repetition rate for some bursts of thousand pulses. The autonomy is more than 35 000 shots (depending on the number of battery packs inside). Electric field measurements were performed on the whole package to determine the figure of merit (the maximum value of far-field peak electric field strength multiplied by the distance) of the UWB source in each configuration (bipolar and monopolar sets). The figure of merit measured is 200 kV for both.

A Compact THz Source: 100/200 GHz Operation of a Cylindrical Smith–Purcell Free-Electron Laser

We report first operation in the terahertz regime of a cylindrical grating Smith-Purcell free-electron laser. Propagation of an annular electron beam in proximity to a cylindrical grating causes strong electron bunching due to a beam-surface wave interaction. Electromagnetic radiation results from the bunching (fundamental) and, at bunch harmonics, the Smith-Purcell effect. In the experiment, over 2.5 kW was generated at 100 GHz (fundamental) and over 100 W at 200 GHz (Smith-Purcell). The results illustrate the potential of this configuration for generation of high-power terahertz radiation.

Ultra Wideband Antennas for High Pulsed Power Applications

1.1 UWB antennas in the field of high pulsed power For the last few years, the generation of high-power electromagnetic waves has been one of the major applications of high pulsed power (HPP). It has aroused great interest in the scientific community since it is at the origin of several technological advances. Several kinds of high power radiation sources have been created. There currently appears to be a strong inclination towards compact and autonomous sources of high power microwaves (HPM) (Cadilhon et al., 2010; Pecastaing et al., 2009). The systems discussed here always consist of an electrical high pulsed power generator combined with an antenna. The HPP generator consists of a primary energy source, a power-amplification system and a pulse forming stage. It sends the energy to a suitable antenna. When this radiating element has good electromagnetic characteristics over a wide band of frequency and high dielectric strength, it is possible to generate high power electromagnetic waves in the form of pulses. The frequency band of the wave that is radiated can cover a very broad spectrum of over one decade in frequency. In this case, the technique is of undoubted interest for a wide variety of civil and military applications. Such applications can include, for example, ultra-wideband (UWB) pulse radars to detect buried mines or to rescue buried people, the production of nuclear electromagnetic pulse (NEMP) simulators for electromagnetic compatibility and vulnerability tests on electronic and IT equipment, and UWB communications systems and electromagnetic jamming, the principle of which consists of focusing high-power electromagnetic waves on an identified target to compromise the target’s mission by disrupting or destroying its electronic components. Over the years, the evolution of the R&D program for the development of HPM sources has evidenced the technological difficulties intrinsic to each elementary unit and to each of the physical parameters considered. Depending on the wave form chosen, there is in fact a very wide range of possibilities for the generation of microwave power. The only real question is

Observation of copious emission at the fundamental frequency by a smith-purcell free-electron laser with sidewalls

Summary form only given. A three-dimensional simulation using the Particle-in-cell code “MAGIC” predicted that for a grating with sidewalls, copious emission of coherent Smith-Purcell (SP) radiation at the fundamental frequency of the evanescent surface wave would occur 1. This may be contrasted with the two-dimensional theory proposed by Andrews and Brau (AB) some years ago 2, for which emission could only occur at harmonics of that frequency. In order to validate this prediction we have performed a demonstration experiment in the microwave domain, using the same set-up as in our previous confirmation of the AB theory 3, except that the grating has sidewalls only four cm apart. We observed a forward-moving beam of SP radiation at the predicted frequency after reflection by a plane mirror, since the true emission is in the backward direction, which makes it hard to observe. Agreement with the PIC simulations is satisfactory.

Wideband Patch Antenna for HPM Applications

This paper presents the design, fabrication, and characterization of a compact wideband antenna for high-power microwave applications. The antennas proposed are array of high-power wideband patches with high compactness and less than λ/10 thick. The concept developed can be fed by high-voltage signals (up to 60 kV) in repetitive operation. Two designs are produced at central frequencies of 350 MHz and 1 GHz. Their relative bandwidth is larger than 40% at 350 MHz and 25% at 1 GHz for S11 <; - 10 dB, respectively. The arrays studied produce a gain of more than 14 dB.

Repetitive auto-triggered Marx generator as the driver of an ultrawideband source

Traditional uses of the Marx generator have been limited to energy storage and delivering systems, such as charging capacitors or pulse forming lines. However, low energy, compact, high peak power Marx generators can be used as repetitive drivers for many applications. This paper presents the design, the realisation and experimental tests of a repetitive auto-triggered Marx generator expected to be the driver of a broadband radiation system. This whole system consists of a pulsed power source, i.e. a pulse forming line charged by a Marx bank and an UWB antenna array. Design of the Marx generator were planned to reach a voltage level of up to 400kV, a 200Hz repetition rate and a good reproducibility. In this way, the generator is supplied with a high voltage pulsed power supply; charging and discharging circuits were made of home-designed inductors. Furthermore, we focus on the first stage of this Marx generator in which a new simple auto-triggered spark-gap was integrated. The Marx is then combined to a forming line and a peaking spark gap to deliver rectangular output pulses with rise-times closed to 250ps.

Compact and portable, repetitive high peak power generator for an ultra-wideband source

The development of an autonomous, repetitive, pulsed power generator is presented. This work is a coordinate effort between CEA, Pau University and Technix to develop a tightly integrated unit, including a battery pack, an intermediate dc/dc converter, a high voltage dc/dc converter, the control system and a high PRF Marx generator. Pau University has designed the Marx generator. They have built a 170 mm diameter, 330 mm length Marx generator capable of delivering 200 kV pulses into 50 Ω impedance with tens nanosecond rise times and a 100 Hz repetition rate, enabling it to drive a pulse forming line and peaking switches. The French Atomic Energy Commission has worked closely with the French company Technix in developing a rapid charging power supply to meet stringent package constraints and still permit high pulse repetition rates. This system has already demonstrated the ability to charge, from DC battery power, a 5 nF capacitance up to 50 kV in 5 ms in a burst of one thousand pulses with 100 Hz repetition rate, delivering a peak power of 3.2 kW. The autonomy is more than 35000 shots or 35 bursts. This generator is equipped with a microcontroller which is remote at a distance up to 75 m with an optical fiber interface. Details of this repetitive peak power generator are presented in this paper. Results of preliminary tests are also included.

High Repetition-Rate Operation of a Compact Buncher for Microwaves

Recently, we have designed and built a new microwave tube; it has been given the name “Arletron.” It works in S-band, and we describe in this paper its operation. A 200-keV 1-kA annular electron beam (e-beam) is strongly bunched in a simple assembly of two pillbox cavities. The bunching occurs at the frequency of the -mode resonance of the system. An extracting cavity, inserted just downstream from the buncher, has been used to extract and to convert the kinetic energy of the beam into an RF wave that is then radiated by a conical antenna. The Arletron is a short and compact high-power microwave source whose repetition rate depends on the pulsed power system; operation at 100 Hz has been routinely achieved. The e-beam does not intercept grids or electrodes before being collected. MAGIC simulations predict that an Arletron can be, in principle, operated from S-band to C-band (2-6 GHz). Although a magnetic field is needed to propagate the beam, it is low enough to be produced by permanent magnets.

Study of the homogeneity of an electron beam using Cerenkov emission

The RKA is a single shot generator which is capable of producing 500 keV, 30 kA, 100 ns electron beams. The beam, emitted by field emission in a diode, is propagated and focused on a target. To improve the understanding of its interaction with a given material, the beam must be characterized in terms of homogeneity, energy and angular dispersion. In this work, we present the results of an experiment where Cerenkov emission is the physical process we use to characterize the beam homogeneity. Two cameras are installed: a high speed gated camera and a streak camera. The code GEANT4 allows us to simulate the Cerenkov emission and the main interactions of electrons/photons inside the target. The initial conditions of the electrons in GEANT4 are provided by “MAGIC” simulations. This particle-in-cell code computes beam emission in the diode and its transportation towards the target. Extensive simulations have allowed us to optimize the full system, in particular in terms of anode thickness and material. The experimental results have confirmed these predictions. In addition, we present experimental tests of several cathode materials as well as the first results of beam propagation in gaseous Argon at a low pressure (order mbar).

Reaching high frequencies in a Smith-Purcell FEL with a multi-channel grating

Summary form only given. Three-dimensional simulations and experiments have shown that, for a grating equipped with sidewalls, copious emission of coherent Smith-Purcell (SP) radiation at the fundamental frequency of the evanescent surface wave is possible. One may reduce the wavelength by rescaling all the parameters of the grating, but this reduces the radiated power by the square of the scale factor. Both 3D MAGIC simulations and the theory of antenna arrays suggest that by laterally juxtaposing N copies of the N-fold reduced grating, the overall power reduction is only 1/N. Furthermore, the radiation is concentrated into a solid angle of size 1/N compared to that of a single reduced grating. An experiment at 32 GHz partially confirms these predictions for N = 7. Preliminary simulations indicate that with a single 20-fold reduced channel coherent Smith-Purcell radiation at 100 GHz is produced. An experiment to confirm that a 20-channel grating will produce a well-collimated radiation pattern with adequate power levels is being prepared.