Forrest J. Agee

Nuclear activation techniques for measuring direct and backscattered components of intense bremsstrahlung pulses

Abstract High-voltage electron accelerators used for bremsstrahlung generation can produce intense pulses of radiation with different endpoint energies. The energy spectrum can be changed by varying the charging voltage or by softening the photons with Compton scattering in a low atomic number material. The high dose rate and the flexible spectrum capabilities of the Aurora accelerator have been used to investigate the potential for measuring the bremsstrahlung spectrum by photoactivation of nuclear isomeric states. Recent success in calibrating lower intensity sources has shown that gram-sized targets of isotopes, such as 115In, can be used to sample the incident X-rays at several discrete gateway energies. When irradiated at these energies the targets are excited to metastable states with lifetimes suitable for conventional counting after the flash.

New capabilities for the Aurora flash X-ray machine

Abstract A series of recent upgrades enable the Aurora flash X-ray machine to provide a wide variety of different radiation pulses. The original design gives a single γ-ray pulse with 50 ns risetime and 135 ns width that irradiates a volume of around 1 m3 with up to 50 krad (Si). The upgrades allow for two pulses with arbitrary separation in time, each with half the output. The pulse widths can be shortened to around 30 ns (with reduced energy), and the risetime can be reduced to 10 ns. These upgrades are discussed in some detail. Other upgrades are described elsewhere: the moderate energy bremsstrahlung option is described by J.A. Anderson et al. [Nucl. Instr. and Meth. B40/41 (1989) 1189]; the high power microwave option is described by G.A. Huttlin et al. [IEEE Trans. Plasma Sci. PS-18 (1990) 618 and other papers in this series].

Flash x-ray sources powered by Blumlein pulsers: review and prospect for x rays with 100-ps switching

The flash x-ray systems developed at the University of Texas at Dallas (UTD) center around two critical subassemblies: (1) a Blumlein pulsed power source, and (2) an x-ray diode properly designed and matched to the pulse forming line. The pulse generator consists of either a single or several traxial Blumleins. For multiple lines, Blumleins are stacked in series at one end and charged in parallel and synchronously commutated with a single switching element at the other end. Extensive characterizations of these Blumlein pulsers have been performed over the past several years. Results indicate that they are capable of producing high power waveforms with risetimes and repetition rates in the range of 0.1-50 ns and 1-300 Hz, respectively, using a conventional thyratron, spark gap, or photoconductive switch. Blumlein pulsers switched by a thyratron or a spark gap have been used to drive x-ray diode loads with different characteristics and discharge geometries and high dose rates of x-rays with pulse durations in the range 3-20 ns have been obtained. In this report the technology and characteristics of these Blumlein based flash x-ray devices are reviewed. Prospects for producing ultra-fast x-ray pulses utilizing photoconductively-switched Blumlein devices are discussed.

High power photoconductive semiconductor switches treated with amorphic diamond coatings

Our recent efforts have resulted in implementation and demonstration of several intense photoconductively switched stacked Blumlein pulsers producing high power output pulses with risetimes as fast as 200 ps. A single GaAs photoconductive switch triggered with a low power laser diode array commutates these devices. During the avalanche-mode photoconductive switching of these pulsers at high powers, current filamentation associated with the high gain GaAs switches produces such high current density that switches are damaged near the metal-semiconductor interface and the lifetime is limited. This report presents progress toward improving the switch operation and lifetime by advanced treatments with the amorphic diamond coatings.

Development of Ultra-Wideband Pulsers at the University of Texas at Dallas

The generic concept for ultra-fast pulsers at the University of Texas at Dallas (UTD) employs a Blumlein based pulse forming system commutated by a fast switching device. Characterization studies of these pulsers have been extensively performed. The pulser design has been adapted to enable it to reliably produce powers as great as 100 MW, in nanosecond pulses with rise times on the order of 200 ps. These devices have compact line geometries and are commutated by an avalanche GaAs photoconductive semiconductor switch (PCSS) triggered with a low power laser diode array. Significant lifetime improvements for PCSS have been achieved by advanced switch treatments with amorphic diamond coatings also developed at UTD. This report presents the progress in the development of these pulsers for the ultra-wideband (UWB) applications.

Progress in Development and Characterization of Photoconductively-Switched Stacked Blumlein Devices Producing High Power Nanosecond Pulses

To fulfill the demand for pulsed power sources producing several hundred kV pulses at moderately high repetition rates, the University of Texas at Dallas (UTD) first introduced and implemented a new approach to combine the functions of pulse shaping and voltage multiplication using stacked Blumleins. This yielded the development of pulsers which consisted of several triaxial Blumleins stacked in series at one end. The lines were charged in parallel and synchronously commutated with a single switch at the other end. This allowed switching to take place at a low charging voltage relative to the pulser output voltage.1, 2, 3, 4

Design, Fabrication and Performance of a Diamond Treated High Power Photoconductive Switch

In this work, the semiconductor properties of amorphic diamond have been employed to improve the GaAs photoconductive semiconductor switch longevity by coating the switch cathode or anode areas or both. If the switch cathode is coated, the tunneling of electrons from amorphic diamond to GaAs during the off‐state stage of PCSS operation provides pre‐avalanche sites that diffuse conduction current upon switch activation. On the other hand, diamond coating of the switch anode results in increased hold‐off characteristics and longer switch lifetimes by blocking the leakage current. In this case the rectifying behavior of the amorphic diamond/GaAs heterojunction operating under reverse bias condition restricts the conduction until very high fields are reached.

High-power stacked Blumlein pulsers commutated by a photoconductive switch treated with amorphic diamond

Photoconductive switching of the stacked Blumlein pulsers, developed at the Univ. of Texas at Dallas (UTD), currently produces high power, nanosecond pulses with risetimes on the order of 200 ps. The device has a compact geometry and is commutated by a single GaAs photoconductive semiconductor switch (PCSS) triggered by a low power laser diode array. Filamentation of the conductivity associated with high gain GaAs switches produces such high current density that the switches are damages near the metal-semiconductor interface and the lifetime is limited. The semiconductor properties of amorphic diamond can be employed to improve the PCSS longevity by coating the switch cathode or anode areas or both. For example if the switch cathode is coated, the tunneling of electrons from amorphic diamond to GaAs during the off-state stage of PCSS operation provides pre-avalanche sites that diffuse conduction current upon switch activation. This report presents the progress toward improving the high gain switch operation and lifetime by advanced treatments with amorphic diamond coatings. A significant improvement in switch lifetime is demonstrated by testing the diamond-coated switch performance in a stacked Blumlein prototype pulser.

High power microwave modulator application for 10–100 kA, 100 kV thyratrons☆

Abstract The technical issues confronting the development of thyratron-switched, high power modulators for high power microwave systems are described.