Continuous RF Power Research Articles

Experimental investigation on magnetic field strength providing thrust saturation in a magnetic nozzle radiofrequency plasma thruster

Magnetic field strength applied to a magnetic nozzle radiofrequency (rf) plasma thruster having a 10.5 cm diameter source tube is increased up to about 3 kG by pulsing the solenoid current. A target plate is installed at 30 cm downstream of the source and an impulse bit exerted to the target is measured to assess the thrust, where the thrust balance measurement was impossible due to the interaction between the pulsed magnetic fields and the eddy currents on surroundings. Since the diameter of the plasma plume at the target location is larger than the target diameter, a comparison between the thrust balance and target measurements under continuous magnetic field and rf power is performed prior to the pulsed magnetic field experiments, showing that about 65 percent of the plasma momentum is exerted to the target plate. Saturation of the impulse bit, being equivalent to the force multiplied by the rf pulse width, is clearly observed when increasing the magnetic field strength. The magnetic field providing the force saturation is found to be changed by the source diameter, which is qualitatively explained by considering a change in the plasma loss to the source wall in a thruster model containing the particle balance, power balance, and one-dimensional magnetic nozzle models. It is suggested that the magnetic field strength required for optimizing the force, i.e. the thrust, can be reduced when enlarging the source tube diameter.

Microfluidically Reconfigurable mm-Wave Slow Wave Phase Shifter With Integrated Actuation

A microfluidically reconfigurable slow wave phase shifter (MRPS) with integrated actuation is introduced. MRPS is based on a selectively metalized plate (SMP) repositionable within a microfluidic channel placed in close proximity to a microstrip line. SMP repositioning creates a variable capacitive loading to alter the speed of the propagating wave. The device exhibits < 2 dB insertion loss (IL) and a reconfiguration time of 50 ms. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$|S_{21}|$ </tex-math></inline-formula> , <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$|S_{11}|$ </tex-math></inline-formula> , and phase performances are characterized to be stable with respect to gravity by no more than 0.16 dB, 3.02 dB, and 7.73° variations, respectively. Vibration test shows 0.08 dB in IL, 1.72 dB in return loss (RL), and 4.23° in phase variations. The device is expected to handle 5.2 W of continuous RF power.

IFMIF-DONES RF System

The IFMIF-DONES accelerator cavities require around 5.7 MW of continuous RF power at 175 MHz to deliver a 125 mA and 40 MeV deuteron beam at the lithium target. The IFMIF-DONES RF System is composed of 56 RF Stations able to provide up to 200 kW RF power at 175 MHz in CW, delivering a grand total of 7.4 MW.Such a massive system requires an ambitious approach to be able to comply with the accelerator requirements with a high efficiency and a high availability. The newest advancements in the field of RF systems for accelerators, which are already implemented independently in some facilities, are integrated for the first time in a single system from the early design phases. The result of this approach is the RF Station, which is a totally independent and compact RF module containing all the components needed to feed, control, and protect the accelerator cavities. The amplification technology chosen is the Laterally-Diffused Metal-Oxide Semiconductor (LDMOS) transistor, providing a good efficiency and high robustness. The proposed RF power combination of up to 160 transistors is based on resonant cavity combiner, allowing a single-step combination that highly reduces the losses with respect to more traditional corporate combining schemes. In this proposal, the RF generation relies on a fully digital Low-Level RF (LLRF) providing accurate amplitude/phase control and high flexibility to implement all the required functions for a proper cavity operation. The RF Station has been designed maximizing the modularity and the reliability. In order to achieve the required modularity, the LDMOS transistors have been grouped in Amplifier Modules (AMDs) containing four of them but keeping independent outputs. Furthermore, the AC/DC power converters have also been grouped in modules feeding a common DC bus. The RF Station will have redundant modules to be able to operate properly in any condition, even with some of them failed.This innovative proposal for the IFMIF-DONES RF System design and detailed information on the RF Stations architecture are presented in this paper.

Electron heating and density production in microwave-assisted helicon plasmas

Microwaves are injected into argon and helium helicon plasmas at 6 to 20 mTorr neutral pressure, 1.2 kW pulsed microwave power, up to 500 W continuous RF power, and up to 1 kG magnetic fields, with the objective of heating the tail of the electron energy distribution function (EEDF) and populating ion metastable states. Langmuir probes are used to measure the EEDF and optical emission spectroscopy is used to monitor ion emission. The injection of microwave power in argon helicon plasmas is shown to heat the high energy tail of the EEDF without increasing the plasma density. Argon ion emission is shown to increase by a factor of 4. Injection of microwaves into a helium helicon plasma is shown to cool the bulk of the of the EEDF and increase the plasma density. Previously absent helium ion emission lines are observed with the injection of microwaves. All the microwave results are shown to be independent of RF power within the limits of the system.

High Power Combline Filter for Deep Space Applications

An S-band, compact, high power filter, for use in the Mars Orbiter Mission (MOM) of Indian Space Research Organization (ISRO), has been designed and tested for multipaction. The telemetry, tracking, and commanding (TT&amp;C) transponder of MOM is required to handle continuous RF power of 200 W in the telemetry path besides simultaneously maintaining an isolation of greater than 145 dBc to its sensitive telecommand path. This is accomplished with the help of a complex diplexer, requiring high power, high rejection transmit path filter, and a low power receive path filter. To reduce the complexity in the multipaction-free design and testing, the transmit path filter of the diplexer is split into a low rejection filter integral to the diplexer and an external high rejection filter. This paper highlights the design and space qualification phases of this high rejection filter. Multipaction test results with 6 dB margin are also presented. Major concerns of this filter design are isolation, insertion loss, and multipaction. Mission performance of the on-board filter is normal.

Regulated High-Voltage Power Supply (RHVPS): Integration, Operation, and Test Results With LHCD System of SST-1

A regulated high-voltage power supply (RHVPS) and its results are presented for its initial integration and operation with lower hybrid current drive (LHCD) system. Experimental results are shown to show the ability of RHVPS as a practical source of electrical power for LHCD. Klystron is operated at 3.7 GHz. with a continuous wave RF power level of 500 kW with various subsystems. RHVPS has proven its capability for feeding power to klystron with a safe and good level of required quality. Up to 65 kV and a load current in excess of 18 A was imparted to klystron with a trapezoidal pulse of voltage. Repeatability of operation is shown by successive shot results. RHVPS is explained with its scheme and control system. Basic parameters are fulfilled with a dummy load testing of RHVPS. Test results are illustrated for basic parameters with resistive dummy load. LHCD system operation parameters and experimental parameters of RHVPS for its operation are discussed. A regulation on flat top level of voltage is as per the requirement of LHCD system. Initial integration of RHVPS has been successful and future plans are mentioned for full-fledged SST-1 operation.

Investigation of Silver Migration Impacts on Microwave Systems Fabricated on LTCC Substrate Under High-Power RF Excitation and High Temperature and Humidity Conditions.

The demand for cost effective LTCC systems has led to increasing use of silver based thick film conductors for inner layers of RF modules. There exists some level of concern about the reliability of silver as a conducting material for electronic circuit packaging. This arises from the tendency of silver to migrate under the influence of electric fields when subjected to certain favorable environment conditions. However, previous research has shown that if silver is used in a properly designed LTCC system on inner layers, the LTCC dielectric forms a hermetic seal around the circuit pattern, which leads to a highly reliable circuit and a low cost solution. This paper describes the results of an investigation to characterize and quantify the impact of silver migration on real-life microwave systems fabricated with DuPont GreenTape™ 951 LTCC material system with silver metallization. A buried multilayer filter was chosen from an actual production design in a high-reliability application, and was tested at 15W of continuous RF power at 1.5 GHz under 85°C/85% temperature and relative humidity. The chosen test conditions were meant to simulate actual operating conditions encountered by the circuit in real-world, high reliability applications. The focus of this work is to collect useful operational information under environmental stresses encountered in typical applications. To this effect, unlike previously reported work [1 ], [2 ], [3 ] by proponents of silver, the work described in this paper is not forcing silver to migrate, but rather tries to evaluate the performance of an actual application circuit fabricated with silver under high-reliability test conditions and high power, high frequency RF excitation. Material characterization data as well as RF performance data – in terms of S parameters and power delivered – indicates no evidence of detrimental effects or performance impairments due to silver migration or other mechanisms.

Radiofrequency heating at 9.4T: in vivo temperature measurement results in swine.

In vivo temperatures were correlated to the whole head average specific absorption rate (SAR(avg)) at 9.4T using 12 anesthetized swine (mean animal weight = 52 kg, standard deviation = 6.7 kg). Correlating the temperatures and SAR(avg) is necessary to ensure safe levels of human heating during ultra-high field MR exams. The temperatures were measured at three depths inside the brain, in the rectum, and at the head-skin of swine. A 400 MHz, continuous wave RF power was deposited to the head using a volume coil. The SAR(avg) values were varied between 2.7-5.8 W/kg. The RF power exposure durations were varied between 1.4-3.7 hr. To differentiate the temperature response caused by the RF from that of the anesthesia, the temperatures were recorded in four unheated swine. To study the effect of the spatial distribution of the RF and tissue properties, the temperature probes were placed at two brain locations (n = 4 swine for each location). Results showed that the in vivo brain temperatures correlated to the SAR(avg) in a geometry-dependent manner. Additionally, 1) the skin temperature change was not the maximum temperature change; 2) the RF heating caused an inhomogeneous brain temperature distribution; and 3) the maximum temperature occurred inside the brain.

Digital oxide deposition of SiO2 layers for III-nitride metal-oxide-semiconductor heterostructure field-effect transistors

We present a digital-oxide-deposition (DOD) technique to deposit high quality SiO2 dielectric layers by plasma-enhanced chemical vapor deposition using alternate pulses of silicon and oxygen precursors. The DOD procedure allows for a precise thickness control and results in extremely smooth insulating SiO2 layers. An insulating gate AlGaN∕GaN heterostructure field-effect transistor (HFET) with 8nm thick DOD SiO2 dielectric layer had a threshold voltage of −6V (only 1V higher than that of regular HFET), very low threshold voltage dispersion, and output continuous wave rf power of 15W∕mm at 55V drain bias.

Synthesis and characterization of plasma-polymerized tert-butylacrylate films

Plasma polymerized tert-butylacrylate (pp- t-BA) film was prepared using tert-butylacrylate monomer under 100 Pa of vapor pressure with varying RF power of 10–250 W and continuous wave RF power of 13.56 MHz. The deposition rates of pp- t-BA films were determined using quartz crystal microbalance (QCM) method. The chemical structure of pp- t-BA films was characterized using FT-IR, contact angle and XPS techniques. The gas sorption properties of pp- t-BA were measured using a QCM sensor array. Results showed that deposition rates of pp- t-BA film were proportional to the polymerization time at 100 Pa of monomer pressure under the same RF power. The deposition amount of pp- t-BA films increased gradually with increasing RF power of 30–150 W. Increasing the RF power on plasma polymerization decreased the amount of ester group in pp- t-BA films. Sensitivity of gas sorption on pp- t-BA films is related to the RF power of polymerization.

Extraordinary Lattice Displacement by Pulse Thickness Extension Mode Resonance Oscillation of Acoustic Wave and Its Effect on the Activity for Ethanol Oxidation of a Thin Pd Film Catalyst

Pulse radio frequency (rf) electric power was applied to a ferroelectric single crystal of z-cut LiNbO3 to generate pulse thickness extension mode resonance oscillation (p-TERO) of bulk acoustic wave, and the effects of p-TERO on the catalytic activity for ethanol oxidation of a Pd catalyst deposited on the crystal were studied. The employment of the pulse method made it possible to introduce several times larger rf power to the catalyst, compared to conventional continuous rf power introduction. The p-TERO remarkably increased catalytic activity for acetaldehyde production, and its effects were particularly great in a high rf power region. A laser Doppler method was developed to monitor lattice displacement caused by p-TERO. The patterns of lattice displacement became nonuniform in a high rf power region by the appearance of extraordinary lattice displacement. Very large activity increases in the region were associated with the lattice displacement. The mechanism of catalyst activation by p-TERO is discu...

Application of high performance inverter circuit to plasma generation and heating in fusion devices

We have constructed a rf power source using static induction transistor (SIT) inverters to generate high heat flux plasmas in the diverter plasma simulator, NAGDIS-II. Since the SIT can enlarge the operating frequency range of the high power inverter above 1 MHz with output power of several tens of kW, SIT inverters can be used to generate rf plasmas and to heat plasmas by exciting low frequency plasma waves, such as Alfven waves and ion cyclotron waves in plasma devices. The rf conversion efficiency of a SIT inverter with matched load is &amp;gt;85% for frequencies up to 1.2 MHz. The third harmonic antenna current is found to be less than −30 dB. Using four sets of inverter power supplies, we can generate more than 56 kW in continuous rf power or 80 kW in 1 s pulses to produce high heat flux plasmas in the diverter plasma simulator.

Advances in High-Power RF Amplifiers

Several powerful accelerators and storage rings are being considered that will require tens or even hundreds of megawatts of continuous rf power. The economics of such large machines can be dictated by the cost and efficiency of the rf amplifiers. The overall design and performance of such narrow-band amplifiers, operating in the 50- to 1500-MHz region, are being theoretically studied as a function of frequency to determine the optimum rf amplifier output power, gain, efficiency, and dc power requirements. The state of the art for three types of amplifiers (gridded tubes, klystrons, and gyrocons) is considered and the development work necessary to improve each is discussed. The gyrocon is a new device, hence its various embodiments are discussed in detail. The Soviet designs are reviewed and the gyrocon's strengths and weaknesses are compared to other types of microwave amplifiers. The primary advantages of the gyrocon are the very large amount of power available from a single device and the excellent efficiency and stable operation. The klystron however, has much greater gain and is simpler mechanically. At very low frequencies, the small size of the gridded tube makes it the optimum choice for all but the most powerful systems.

RF System for the PEP Storage Ring

The main parameters of the rf system for the PEP storage ring are given. A total of 6 MW will be provided by twelve klystrons, each capable of delivering 500 kW of continuous rf power. Each klystron feeds two accelerating cavity sections, each 2.1 meters in length. The general layout is shown. The klystrons are housed in shelters above ground. The wave guides run through vertical penetrations to connect the klystrons to the accelerating cavities in the tunnel. The shelters are placed at three symmetrical regions around the ring. At each of regions 4, 8, and 12 there will be four klystrons and their eight cavities with the eight cavities all being grouped together in the long straight section on one side of an interaction region. The klystron power supplies are on pads outside the shelters. They are unregulated but their voltage may be slowly varied over a 40 percent range by means of variable voltage transformers in the 12 kV input line to each power supply. The output of each power supply passes into the shelter to a high voltage cabinet which contains filter capacitors, crowbars, interlock circuits, and control and metering circuits.