Rakhee Menon

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

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.

Emission properties of explosive field emission cathodes

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...

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

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.

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

Summary form only given. The high intensity pulsed electron beam of KALI-5000 system is used to develop a flash X-ray source. The KALI-5000 system is capable of generating intense relativistic electron beam (IREB) of 1MeV, 60 kA and 100 ns when connected to impedance matched electron beam diode. This system is operated in 6 GW, 100 ns duration to produce the Bremsstrahlung flash X-ray on a tantalum target of planner anode-cathode geometry. The axial and the radial dose distribution are characterized using CaSO4: Dy TLDs (thermoluminescent dosimeter) as well as using the optical densitometer to measure the dose distribution of the exposed X-ray film. The measured dose distribution by TLDs and the optical densitometer plus X-ray film matches within plusmn5%.

Oscillation Frequency of a Reflex-Triode Virtual Cathode Oscillator

The most fundamental issues in virtual cathode oscillator (vircator)-based high-power microwave devices are efficiency improvement and oscillation frequency control. We present a study of the time-dependent behavior of the oscillation frequency of a reflex-triode vircator for various anode-cathode (AK) gaps. The typical electron-beam parameters were 200 kV, 4 kA, and 300 ns, with a current density of a few hundreds of amperes per square centimeter. Time-dependent frequency analysis is applied to the output signal of the vircator to investigate the time evolution of the emitted frequency spectrum for various AK gaps. We find that the dominant frequency peak is less affected by shot-to-shot fluctuations of diode voltage and current than the other frequency components. In addition, the time of occurrence of the dominant and secondary frequency peaks varies on a shot-to-shot basis. The highest microwave power is emitted when all the power is delivered into a single frequency with minimal mode hopping.

Pulsewidth Variation of an Axial Vircator

The KALI-1000 pulse power system has been used to generate single-pulse nanosecond-duration high-power microwaves from an axial virtual cathode oscillator. The typical electron beam parameters were 250 kV, 15 kA, and 100 ns, with a current density of a few hundreds of amperes per square centimeter. The shot-to-shot variation of the microwave pulsewidth was studied for various anode-cathode gaps and two different cathode materials (velvet and graphite). Results indicated that the average microwave pulsewidth for a diode with a graphite cathode increased as the anode-cathode gap was increased. The one-way analysis of variance was employed to examine the statistical correlation between the diode voltage, current, perveance, and the microwave pulsewidth for various diode gaps and for two different cathode materials. It was shown that the microwave pulsewidth variations are not statistically correlated with the diode voltage, current, and perveance.

Prepulse Suppression on a High-Power Electron Beam Accelerator Using a Dielectric Cathode Holder

Intense gigawatt relativistic electron beam generation studies were carried out with a Perspex cathode holder. Purpose of the dielectric cathode holder was to minimize the prepulse voltage across the diode. The typical electron beam parameter was 180 kV, 26 kA, 100 ns, with a few hundreds of ampere per square centimeter current density. It was found that corrugated Perspex of length ges35 mm can eliminate the prepulse voltage but affects the rise time of the diode voltage. 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 5.1 cm/mus for 18-mm anode-cathode gap and the plasma velocity increases for larger gap. Inserting a dielectric at the cathode holder could be a very effective method to reduce prepulse voltage at the electron beam diode, but it increases the rise time of the diode voltage and reduces the effective electron beam pulsewidth. The prepulse voltage reduces significantly (les10%) when an inductance is added to the charging circuit of the Blumlein line.

Explosive Emission Properties of Cathode Materials in Relativistic Electron Beam Generation

Relativistic electron beam generation studies have been carried out in LIA-400 system through explosive electron emission for various cathode materials. This paper presents the emission properties of different cathode materials at peak diode voltages varying from 10 to 220 kV and at peak current levels from 0.5 to 2.2 kA in a single pulse duration of 160-180 ns. The cathode materials used are graphite, stainless steel, and red polymer velvet. The perveance data calculated from experimental waveforms are compared with 1-D Child Langmuir formula to obtain the cathode plasma expansion velocity for various cathode materials. Various diode parameters are subject to shot to shot variation analysis. Velvet cathode proves to be the best electron emitter because of its lower plasma expansion velocity and least shot to shot variability.

Smaller spot size Flash X-ray generation from KALI 5000 system

A Flash X-ray diode has been designed and developed for generation of single shot, nanosecond duration x-rays from KALI-5000 Pulse Power System (0.8–1 MV, 40 kA, 100 ns). The typical electron beam parameter was 500 kV, 8 kA, 100 ns, with a few hundreds of A/cm current density. The spot size for different diode configurations varied from 4 mm to 2 mm. X-ray radiograph of various metal objects were taken on X-ray films. The diode voltage and current waveforms were analyzed with the space-charge limited flow model. The anode and cathode plasma expansion velocities were calculated using the perveance data and varies from 2.5–3.3 cm/μs. Time integrated x-ray dose was measured using pen dosemeter and the on-axis dose was 10 mR at 1m distance from the window. The radiograph of stainless steel objects shows good resolution.

Generation of short duration high power microwave using KALI - 5000

A pulse power system (1 MV, 5 0kA, and 100 ns) based on dual polarity charged Marx generator and Blumlein pulse forming line (PFL) was built to generate high power microwaves. This paper describes the inherent prepulse present with the Blumlein configuration and the mitigation strategies adopted. Further, a qualitative attempt to verify the generation of high power microwave output on the system is presented.