Recently developed techniques for producing high current, pulsed ion beams have suggested the possibility of a pulsed implanter technology. Potential advantages of such an approach include lower capital costs, and higher average beam currents for applications requiring metal ions which are difficult to produce in conventional dc sources. We report here the development of a prototype 80 kV pulsed implanter. This implanter runs at 32 pulses per second, 2 A (titanium) peak ion current, a 6 μs accelerating pulse length, and ~100 μA titanium particle current. Significant currents have been demonstrated for titanium, copper and carbon; ions from any conducting solid can be used in the ion source. The characteristics of the source are discussed along with implanter operating data. Our results suggest that the accelerating gap runs in a primarily source dominated mode. The absolute beam currents and charge state are characterized by surface analysis of implanted samples. Techniques for scaling this technology to high (> 20 mA) average currents are discussed.

Repetitively pulsed metal ion beams for ion implantation

Even after 50 years of development, narrowband high-power microwave (HPM) source technologies remain the focus of much research due to intense interest in innovative applications of HPMs in fields such as directed energy, space propulsion, and high-power radar. A few decades ago, the main aim of investigations in this field was to enhance the output power of a single HPM source to tens or hundreds of gigawatts, but this goal has proven difficult due to physical limitations. Therefore, recent research into HPM sources has focused on five main targets: phase locking and power combination, high power efficiency, compact sources with a low or no external magnetic field, high pulse energy, and high-power millimeter-wave generation. Progress made in these aspects of narrowband HPM sources over the last decade is analyzed and summarized in this paper. There is no single type of HPM source capable of excellent performance in all five aspects. Specifically, high pulse energy cannot be achieved together with high power efficiency. The physical difficulties of high power generation in the millimeter wave band are discussed. Semiconductor-based HPM sources and metamaterial (MTM) vacuum electron devices (VEDs) are also commented on here. Semiconductor devices have the advantage of smart frequency agility, but they have low power density and high cost. MTM VEDs have the potential to be high power efficiency HPM sources in the low frequency band. Moreover, problems relating to narrowband HPM source lifetime and stability, which are the important determinants of the real-world applicability of these sources, are also discussed.

Progress in narrowband high-power microwave sources

High power electron beam generation studies were carried out in a planar diode configuration to investigate the effect of the accelerating gap, diode voltage, and anode-cathode materials on the electrode plasma expansion. The diode voltage has been varied from 145–428 kV, whereas the current density has been varied from 208–2215 A/cm2 with 100 ns pulse duration. It was found that the diode voltage and current follow the bipolar space-charge limited flow model. The anode and cathode plasma expansion velocities were calculated using the perveance data. The plasma expands at 11 cm/μs for 34 mm anode-cathode gap and the plasma velocity decreases for smaller gaps. It was found that the plasma expansion velocity increases significantly due to the cathode edge contribution and the edge contribution is particularly important during the beginning and the end of the accelerating pulse when the diode voltage and the corresponding electric field are comparatively low. It was also observed that the diode current incre...

Plasma expansion and fast gap closure in a high power electron beam diode

High power microwave (HPM) generation studies were carried out in KALI-5000 pulse power system. The intense relativistic electron beam was utilized to generate HPMs using a coaxial virtual cathode oscillator. The typical electron beam parameters were 350 kV, 25 kA, and 100 ns, with a few hundreds of ampere per centimeter square current density. Microwaves were generated with graphite and polymer velvet cathode at various diode voltage, current, and accelerating gaps. A horn antenna setup with diode detector and attenuators was used to measure the microwave power. It was observed that the microwave power increases with the diode voltage and current and reduces with the accelerating gap. It was found that both the peak power and width of the microwave pulse is larger for the velvet cathode compared to the graphite cathode. In a coaxial vircator, velvet cathode is superior to the graphite cathode due to its shorter turn on time and better electron beam uniformity.

High power microwave generation from coaxial virtual cathode oscillator using graphite and velvet cathodes

This paper presents the design and development of a repetitive Marx generator rated at 1 kJ, 300 kV, 12 kA, and 10 Hz and which is suitable to drive the load directly. Bipolar charging of a Marx generator scheme has been adopted to get the following: 1) faster rise time and 2) relatively low charging voltage. The faster rise time is due to the reduced number of spark gaps down to six for 12 in-series capacitors (0.15 μF and 50 kV) of a six-stage bipolar-charging Marx generator. Interstage inductive charging further enhances the efficiency of the overall system as compared to resistive charging. The generator is characterized by an aqueous resistive load for both polarities at the output voltage. Output voltages of the same magnitude have been achieved for both polarities. The matched impedance is found to be 25 Ω for which a suitable reflex triode is designed. Intense electron beams and high-power microwaves have been generated using this Marx generator and reflex triode system. Experimental results demonstrating the Marx generator and the reflex triode operation are presented and discussed.

Development and Characterization of Repetitive 1-kJ Marx-Generator-Driven Reflex Triode System for High-Power Microwave Generation

The research results of the explosive field emission cathode plasma expansion velocity and the initial emission area in the planar diode configuration with cathodes made of graphite, stainless steel, polymer velvet, carbon coated, and carbon fiber (needle type) cathodes are presented. The experiments have been performed at the electron accelerator LIA-200 (200 kV, 100 ns, and 4 kA). The diode voltage has been varied from 28–225 kV, whereas the current density has been varied from 86–928 A/cm2 with 100 ns pulse duration. The experimentally obtained electron beam diode perveance has been compared with the 1 dimensional Child-Langmuir- law. It was found that initially only a part of the cathode take part in the emission process. The plasma expands at 1.7–5.2 cm/μs for 4 mm anode-cathode gap for various cathode materials. It was found that the plasma expansion velocity increases with the decrease in the cathode diameter. At the beginning of the accelerating pulse, the entire cathode area participates in the e...

Emission properties of explosive field emission cathodes

Flash x-ray generation studies have been carried out in KALI-5000 Pulse power system. The intense relativistic electron beam has been bombarded on a tantalum target at anode to produce flash x-ray via bremsstrahlung conversion. The typical electron beam parameter was 360 kV, 18 kA, and 100 ns, with a few hundreds of A/cm(2) current density. The x-ray dose has been measured with calcium sulfate:dysposium (CaSO(4):Dy) thermoluminescent dosimeter and the axial dose distribution has been characterized. It has been observed that the on axis dose falls of with distance approximately 1/x(n), where n varies from 1.8 to 1.85. A maximum on axis dose of 46 mrad has been measured at 1 m distance from the source. A plastic scintillator with optical fiber coupled to a photomultiplier tube has been developed to measure the x-ray pulse width. The typical x-ray pulse width varied from 50 to 80 ns.

Generation and dose distribution measurement of flash x-ray in KALI-5000 system

Intense relativistic electron beam generation studies were carried out in a planar diode configuration to investigate the effect of the plasma expansion on the impedance characteristics of the diode. The typical electron beam parameter was 280kV, 18kA, 100ns, with a few hundreds of A∕cm2 current density. The diode voltage and current waveforms were analyzed with the bipolar space-charge limited flow model. The anode and cathode plasma expansion velocities were calculated using the perveance data. The plasma expands at 9.5cm∕μs for 31mm anode-cathode gap and the plasma velocity decreases for smaller gap. It was found that the electron emission is more uniform for 25mm anode-cathode gap as compared to 31mm gap. Effect of a nylon sheet on the anode plate has been studied with respect to the impedance characteristic of the diode. It was found that the diode closes at a faster rate in the presence of nylon sheet on anode. Nylon materials are released into the diode region due to the electron beam bombardment a...

Rakhee Menon

Papers

High power microwave generation from coaxial virtual cathode oscillator using graphite and velvet cathodes

Development and Characterization of Repetitive 1-kJ Marx-Generator-Driven Reflex Triode System for High-Power Microwave Generation

Emission properties of explosive field emission cathodes

Generation and dose distribution measurement of flash x-ray in KALI-5000 system

Impedance collapse and beam generation in a high power planar diode