Pulsed RF Sources Research Articles

A 2 kW S-band RF source for multipactor research utilizing GaN HEMTs

A high power, pulsed RF source designated for use in multipactor research is described. Four gallium nitride high electron mobility transistors from Cree/Wolfspeed, capable of 700 W in long pulse mode (500 W rated output), are combined to achieve a maximum rated output of 2.8 kW with a pulse length of ∼100 µs. Custom splitters/combiners are used due to the power levels considered in addition to a custom power and sequencing control system to ensure the proper biasing and sequencing of the relatively delicate depletion mode GaN devices. With high efficiency and small size, gallium nitride devices present a good solution for lab based sources, and this paper aims to provide information helpful in the construction of such a source. The multipactor phenomenon itself is studied within a high impedance waveguide section—achieved with a tapered impedance transformer—placed in a WR284 traveling wave ring resonator, which increases the effective power up to a factor of 20, or ∼40 kW.

Implementation of High Power Microwave Pulse Compressor

High-frequency power amplifiers can be equipped with complementary pulse-compressor systems to fulfill the advanced cavity power requirements. Microwave pulse compression is a power extraction method based on pulsewidth reduction and electronic triggering. The cavity is fed by pulsed-RF source, the resonance is evolved at $t\geq t_{0}$ , $t_{0}=\textrm {cavity}$ filling time. Established standing waves or resonance condition is brokendown by triggered plasma switch programmed with adjustable delay and pulsewidth. This causes the stored energy to be transmitted as shorter output RF-pulse. The implementation of a microwave pulse compression system requires careful measurements and structural modifications to be able to tune design parameters for optimal setup. In the scope of this paper, the experimental results are obtained for different waveguide lengths and various load configurations (to exploit new opportunities). All connections, iris configuration, and precise adjustment of plasma triggering have been set up cautiously. The cavity gain is increased up to 23 dB with trigatron driving improvements. High power requirements and demands are investigated for many applications, particularly for biomedical science projects.

Spatially dispersive nonlinear transmission line experimental performance analysis

Spatially dispersive nonlinear transmission lines (NLTLs) have attracted interest as pulsed RF sources. The characteristics of these sources need to be further evaluated and understood to optimize their design. This paper presents the performance of a spatially dispersive nickel-zinc (NiZn) NLTL for two experiments. The first experiment evaluated the RF power and frequency generated as a function of the primary dielectric medium for SF 6 compared to the previously implemented Royco 602 transformer oil. The second experiment evaluated the RF power and frequency generated as a function of the ferrites' loss tangents, initial permeabilities, and dimensions. The NLTL's performance is presented for each experiment across a range of operational input voltage levels and ferrite bias conditions. Experimental primary dielectric performance results demonstrate approximately 4X increase in RF power at frequencies above 1.2 GHz for replacement of oil with SF 6 . Experimental ferrite performance results show output frequencies from 0.9 to 1.39 GHz and peak RF powers >2X higher for the higher permeability and larger volume ferrite.

Design and development of 3 kW pulsed solid-state RF power amplifier using two LDMOS in parallel configuration at 1 MHz

A 3 kW and 1 MHz pulsed solid state RF amplifier has been designed and developed to drive TH581 tetrode tube based RF amplifier to realise 100 kW pulsed RF source. The driver amplifier is operated with 1 ms pulse width at 50 Hz repetition rate. The 3 kW RF power is obtained by operating two Gemini pair LDMOS transistors (VDD=50 V) in parallel configuration at device itself and having single ended topology with common lumped element based input and output matching network. The amplifier delivers 3 kW RF power when fed by 25 dBm RF input drive from the signal generator. This scheme eliminates the need of RF combiner and divider which results in low loss, high efficiency, lesser number of components, simple and compact design.

Development of an RF coaxial resonator for investigating multipactor discharges on metal and dielectric surfaces

Multipactor discharge is a phenomenon in which electrons impact one or more material surfaces in resonance with an alternating electric field. The discharge can occur for a wide range of frequencies, from the MHz range to tens of GHz, and in wide array of geometries if the impacted surface has a secondary electron emission (SEE) yield larger than one. The discharge can take place on a single surface or between two surfaces. A novel coaxial resonator to investigate two-surface multipactor discharges on metal and dielectric surfaces in the gap region under high vacuum (HV) conditions (10−8 mbar) has been developed and tested at LAL. The RF coaxial resonator is ∼19 cm in length with an outer diameter of 11 cm. A pulsed RF source delivers up to 30 W average power over a wide frequency range 650–900 MHz to the RF resonator. The incident and reflected RF signals are monitored by calibrated RF diodes. An electron probe provides temporal measurements of the multipacting electron current with respect to the RF power. These experiments were successful in identifying multipacting and allowed us the evaluation of a home made sputtered titanium nitride (TiN) thin layers as a Multipactor suppressor.

Effect of source frequency and pulsing on the SiO2 etching characteristics of dual-frequency capacitive coupled plasma

A SiO2 layer masked with an amorphous carbon layer (ACL) has been etched in an Ar/C4F8 gas mixture with dual frequency capacitively coupled plasmas under variable frequency (13.56–60 MHz)/pulsed rf source power and 2 MHz continuous wave (CW) rf bias power, the effects of the frequency and pulsing of the source rf power on the SiO2 etch characteristics were investigated. By pulsing the rf power, an increased SiO2 etch selectivity was observed with decreasing SiO2 etch rate. However, when the rf power frequency was increased, not only a higher SiO2 etch rate but also higher SiO2 etch selectivity was observed for both CW and pulse modes. A higher CF2/F ratio and lower electron temperature were observed for both a higher source frequency mode and a pulsed plasma mode. Therefore, when the C 1s binding states of the etched SiO2 surfaces were investigated using X-ray photoelectron spectroscopy (XPS), the increase of C–Fx bonding on the SiO2 surface was observed for a higher source frequency operation similar to a pulsed plasma condition indicating the increase of SiO2 etch selectivity over the ACL. The increase of the SiO2 etch rate with increasing etch selectivity for the higher source frequency operation appears to be related to the increase of the total plasma density with increasing CF2/F ratio in the plasma. The SiO2 etch profile was also improved not only by using the pulsed plasma but also by increasing the source frequency.

Laboratory studies of the dynamic of resonance cones formation in magnetized plasmas

The paper is devoted to experimental studies of formation of resonance cones in magnetized plasmas by pulsed RF source in the lower-hybrid (whistler) and the upper-hybrid frequency ranges. It is shown that in both frequency ranges, resonance cones exhibit similar dynamics after switching-on the RF source: at first, wide maxima of radiation are formed in non-resonance directions, which then become narrower, with their direction approaching the resonance one. While the resonance cones are being formed, one observes a fine structure in the form of secondary radiation maxima. It is shown that the characteristic formation time of stationary resonance cones is determined by the minimal value of the group velocity of the quasi-electrostatic waves excited by the antenna. In the low-temperature plasma, this value is limited in the lower-hybrid frequency range by the spatial spectrum of the emitting antenna and in the upper-hybrid range, by the effects of spatial plasma dispersion.

Lasertron: A Pulsed rf Source Using a Laser-Triggered Photocathode

A new pulsed rf source, the “lasertron”, is being developed as a possible rf power source for future electron-positron linear colliders. In a series of systematic studies, a prototype lasertron was fabricated and tested. A peak power of 80 kW was attained at a 2.856 GHz rf frequency with a 1 µs time duration. This paper describes the results of experimentation on the lasertron, including the development of the photocathode and the laser system. Test results are compared with the analysis of beam dynamics in the lasertron.

Quasi-steady-state current drive with pulsed radiofrequency sources

The problem of driving quasi-steady-state currents in tokamak plasmas with pulsed RF sources is examined in the regime , where τRF is the length of the RF pulse, is the electron collision time, TRF is the period between RF pulses, and TL/R is the L over R time of the tokamak plasma. This regime is appropriate to the proposed Microwave Tokamak Experiment (MTX) in which a pulsed free electron laser will be used to drive current in Alcator C. It is found that δI/Ī ≈ TRF/TL/R ≪ 1, where δI/Ī is the fractional variation in the plasma current. The production of runaway electrons can be a problem in the collisional limit, νe TRF ≫ 1. This will be avoided in the proposed MTX experiment if the electron temperature exceeds 1.5 keV.