High Power RF Sources Research Articles

Pulse Compression Photoconductive Switching Using Negative Differential Mobility

This work demonstrates a novel optoelectronic device with the potential for use as a high-frequency, high-power RF source or amplifier. The device is a gallium–arsenide coplanar waveguide with a small gap in the signal trace for optical illumination. A confined charge cloud is generated by illumination through an aperture in an opaque mask over this gap. An electric field above the threshold for negative differential mobility (NDM) enables pulse compression, which prevents the charge cloud from spreading temporally during the drift process. Due to the NDM phenomenon, the output electrical pulse is temporally compressed compared to the input optical pulse. This phenomenon is demonstrated using three different experiments with varied laser pulsewidth (28–700 ps) and device geometry (50- and 100- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> -length gaps). A 66% reduction in the full-width at half-maximum of the electrical pulse relative to the input optical pulse was demonstrated. This novel coupled optoelectronic device opens avenues for high-frequency, high-power, compact devices that could enable next-generation satellite communication systems with faster data rates and longer ranges.

Spin-Wave Linewidth Measurement of Microwave Gyromagnetic Materials in a Low RF Power

In this paper, a novel test system for spin-wave linewidth has been proposed. As the traditional test system requires a high-power RF source, a high Q cylindrical resonator with efficiency compression of microwave magnetic field is designed by a partially filled dielectric in this paper. Experimental results show that this method can achieve spin-wave linewidth measurement with lower RF power compare to the original rectangular cavity. Simultaneous, replacing the traditional tuned resonant cavity with the frequency adjustable signal source can reduce the system interference to cavity field distribution and lays the foundation for fully automated testing. Finally, the wide linewidth spin-wave linewidth test is realized in a low RF power environment, and the test uncertainty for the system is estimated within ±10 %.

6 MeV novel hybrid (standing wave - traveling wave) photo-cathode electron gun for a THz superradiant FEL

A novel 6 MeV hybrid photo-injector has been designed and constructed, based on a smaller-scale prototype previously built in UCLA’s Particle Beam Physics Laboratory. It has been commissioned at Ariel University in Israel as an on-going collaboration between the two universities. This unique, new generation design provides a radically simpler approach to RF feeding of a gun/buncher system, leading to a much shorter beam via velocity bunching owed to an attached traveling wave section of the photo-injector. This design offers better performance in beam parameters, providing a high quality electron beam, with energy of 6 MeV. The hybrid photo-injector was designed to provide emittance of approximately 3 μm, and pulse duration of 150 fs at up to 1 nC per pulse. The hybrid gun is driven by a SLAC XK5 klystron as the high power RF source, and third harmonic of a ∼35 fs IR Laser amplifier (266 nm) to extract electrons from the photo-cathode. The unique electron gun will produce a bunched electron pulse to drive a THz FEL, which will operate at the super-radiant regime, and therefore requires ambitious beam properties. This paper describes the gun and presents experimental results from the gun and its sub-systems, including energy and charge measurements, compared with the design simulations.

Numerical modeling of a high power triode-based self-excited oscillator: a path forward more efficient designs and a better frequency stability of low cost high power RF sources

Triode-based circuits are still the less expensive and more reliable high power sources for industrial applications between 13.56 and 40.68 Mhz. Among them, self-excited oscillators feature the lower cost but with a frequency stability preventing them to address all ISM frequencies and applications. This article demonstrates the possibility to model numerically self-excited oscillators with power triodes using a class-C practical circuit. Advantages over existing analytical calculations and graphical derivations are shown. A method is proposed to evaluate the frequency stability in presence of internal imperfections and external perturbations. These tools offer the possibility to design and analyze more efficiently this family of oscillators especially for demanding applications regarding frequency stability.

Compact and Highly Efficient Kilowatt Lumped Push–Pull Power Amplifier for Cyclotron in Radioisotopes Production

This article presents a very compact highly efficient solid-state power amplifier (SSPA) module at kilowatt level in a push-pull architecture. This SSPA is intended to be used as a modular RF source for a 20 kW cyclotron that operates in the low VHF band for radioisotopes production. For a compact design, a lumped LC balun is adopted instead of a traditional ferrite transformer for the 180° phase shift requirement of the push-pull architecture. Lumped harmonic traps are employed as harmonic tuning to achieve higher efficiency. This implementation enables an ultracompact design realized on a 90 mm × 70 mm circuit board. Improved efficiency at reduced output power is achieved through a static supply voltage variation, which gives more than 90% efficiency over a 5 dB output power range. This approach enables considerable energy saving when applied to the whole cyclotron system. The achieved results show that the proposed SSPA with drain supply modulation is a very promising architecture for very high-power RF sources in cyclotrons for radioisotope production.

Experimental demonstration of externally driven millimeter-wave particle accelerator structure

We report the experimental demonstration of a mm-wave electron accelerating structure powered by a high-power rf source. We demonstrate reliable coupling of an unprecedented rf power—up to 575 kW into the mm-wave accelerator structure using a quasi-optical setup. This standing wave accelerating structure consists of a single-cell copper cavity and a Gaussian to TM01 mode converter. The accelerator structure is powered by 110 GHz, 10-ns long rf pulses. These pulses are chopped from 3 ms pulses from a gyrotron oscillator using a laser-driven silicon switch. We show an unprecedented high gradient up to 230 MV/m that corresponds to a peak surface electric field of more than 520 MV/m. We have achieved these results after conditioning the cavity with more than 105 pulses. We also report preliminary measurements of rf breakdown rates, which are important for understanding rf breakdown physics in the millimeter-wave regime. These results open up many frontiers for applications not only limited to the next generation particle accelerators but also x-ray generation, probing material dynamics, and nonlinear light-matter interactions at mm-wave frequency.

On the limits of multipactor in rectangular waveguides

Multipactor in WR-284-like geometries is measured utilizing local and global detection techniques. To emulate conditions one may find in a waveguide filter structure while maintaining the fundamental microwave mode, a standard rectangular waveguide geometry with the reduced waveguide height set to 2.1 or 5.5 mm was adopted. Two high power RF sources were used to investigate a large range of input power (few kWs to MWs): a solid state source using GaN HEMTs allowing for larger pulse widths than standard magnetrons (100 μs as opposed to ∼4 μs) and a MW level S-band coaxial magnetron for the high power end. Particular interest was taken in capturing the lower and upper limits of multipactor threshold. Lower multipactor thresholds for finite pulse duration are governed by the appearance of one or more electrons in the multipactor gap during the applied pulse as well as a minimum power (electric field) level that affects a secondary electron emission yield above unity. As shown, such initial electrons(s) may easily be seeded via an external UV source illuminating the gap. However, wall collisions of excited metastable molecules may be another source of electrons, an observation based on the experiment and prior research. A multipactor upper threshold was non-existent in the experiment, even at powers over 200 kW within a 2.1 mm test gap, which numerically yielded a gap transit time significantly shorter than one half-period of the GHz wave. This is attributed to the electric field distribution within the waveguide structure, which results in the multipactor's spatial position moving to more favorable locations within the test gap.

Combining high-power heterogeneous RF sources for accelerator applications

Facilities with a synchrotron light source at present utilize both vacuum-tube and solid-state RF sources to accelerate an electron beam, thanks to the rapid development of semiconductor technology. In NSRRC, a retired 500-MHz 100-kW klystron-based RF transmitter and a newly developed 80-kW solid-state RF transmitter became available for experimental study. We proposed the usage of a 3-dB waveguide hybrid coupler to combine these two heterogeneous RF sources to provide twice the RF-power of each alone. The intrinsic difference in RF properties of these two RF-power sources seen by the 3-dB hybrid coupler is compensated with an analogue low-level feedback system that locks the klystron side to the solid-state RF source side for a quasi-equal power level and a quadrature phase offset to obtain the greatest efficiency of the combination. In this manner, we demonstrated an efficiency up to 98.3% for combined RF-power 140 kW measured at the target port of a 3-dB waveguide hybrid coupler; the dummy load after the non-target port received negligible power within the full power range. The robustness to a power imbalance between these two RF-power sources was studied theoretically and experimentally; the efficiency of the RF-power combination was maintained greater than 90% for an available power ratio of these two RF sources up to 3 or phase error within ±45°. The results demonstrate an economical power-combination scheme for heterogeneous RF-power sources with high tolerance of imbalance for a symmetric power combiner. The usage of an asymmetric power combiner is foreseen to extend the imbalance tolerance.

Gyromagnetic source of high power wideband pulses

In this paper we present the results on simulation and experimental research of RF oscillations excitement in gyromagnetic nonlinear transmission line. The oscillations were observed in the frequency range 1.5-1.7 GHz. The choice of the appropriate ferrite type and irradiating helix antenna was followed by the development of high power RF source. The irradiated RF pulses were characterized by 80 kV effective potential and close to circular polarization. The gyromagnetic RF source has shown stable operation at 400 Hz repetition rate.

Corrosion Mitigation Coatings for RF Sources and Components

RF sources are used in many applications, including high-power radar, communications, cancer therapy and diagnosis, electronic countermeasures, and high-energy accelerators. Many of these sources are located in well controlled environments where high-quality coolant fluids are readily available. Others, however, are located on platforms where harsh environmental conditions exist, and high-quality coolants are not easily provided. This includes ships at sea and remote transmitters in the second-world and third-world countries. Corrosive ethylene glycol mixtures may be required where extreme temperatures are encountered. Essentially, all high-power RF sources are fabricated from brazed copper structures, which are particularly susceptible to corrosion from corrosive coolants. This paper demonstrates using atomic layer deposition (ALD) to deposit nanometer-scale ceramic coatings inside coolant passages to separate the coolant from the underlying metal surface. This effectively reduces or eliminates corrosion and erosion, thereby increasing lifetime and reliability of RF sources. We describe ALD equipment and process specifically focused on RF sources and related components. The results of numerous experiments are reported, confirming the efficacy of the technology.

Femto-second synchronisation with a waveguide interferometer

CERN’s compact linear collider CLIC requires crab cavities on opposing linacs to rotate bunches of particles into alignment at the interaction point (IP). These cavities are located approximately 25 metres either side of the IP. The luminosity target requires synchronisation of their RF phases to better than 5 fs r.m.s. This is to be achieved by powering both cavities from one high power RF source, splitting the power and delivering it along two waveguide paths that are controlled to be identical in length to within a micrometre. The waveguide will be operated as an interferometer. A high power phase shifter for adjusting path lengths has been successfully developed and operated in an interferometer. The synchronisation target has been achieved in a low power prototype system.

Four channel high power rf source with beam steering based on gyromagnetic nonlinear transmission lines.

The synchronized operation of four gyromagnetic nonlinear transmission lines (NLTLs) was tested with a pulse repetition frequency up to 1 kHz during 1 s bursts. High voltage pulses with a duration of ∼5 ns from the solid state driver S-500 were split into four 48 Ω channels reaching about -200 kV in each channel with ∼10% variation in the amplitude. The maximum peak voltage at the NLTL output was within 220-235 kV with the maximum modulation depth of decaying oscillations up to 90% at the center frequency near 2.1 GHz. The relative delay between channels reached the half-period of the center frequency of oscillations. The associated beam steering by four element array of conical helical antennas was demonstrated in a horizontal plane at 17°. The effective potential of radiation reached 360 kV at the radiation axis. The effect of ferrite temperature on the shock wave velocity in gyromagnetic NLTL is observed.

Characteristics of a high-power RF source of negative hydrogen ions for neutral beam injection into controlled fusion devices

An injector of hydrogen atoms with an energy of 0.5–1 MeV and equivalent current of up to 1.5 A for purposes of controlled fusion research is currently under design at the Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences. Within this project, a multiple-aperture RF surface-plasma source of negative hydrogen ions is designed. The source design and results of experiments on the generation of a negative ion beam with a current of >1 A in the long-pulse mode are presented.

Waveguide-Mode Terahertz Free Electron Lasers Driven by Magnetron-Based Microtrons

We have developed small-sized terahertz free-electron lasers by using low-cost and compact microtrons combining with magnetrons as high-power RF sources. We could stabilize the bunch repetition rate by optimizing a modulator for the magnetron and by coupling the magnetron with an accelerating cavity in the microtron. By developing high-performance undulators and low-loss waveguide-mode resonators having small cross-sectional areas, we could strengthen the interaction between the electron beam and the THz wave inside the FEL resonators to achieve lasing even with low-current electron beams from the microtron. We used a parallel-plate waveguide in a planar electromagnet undulator for our first THz FEL. We try to reduce the size of the FEL resonator by combining a dielectric-coated circular waveguide and a variable-period helical undulator to realize a table-top THz FEL for applying it to the security inspection on airports.

SRF LINAC FOR FUTURE EXTENSION OF THE PEFP

A study on the superconducting RF linac is underway in order to increase the beam energy up to 1 GeV by extending the Proton Engineering Frontier Project (PEFP) 100-MeV linac. The operating frequency of the PEFP superconducting linac (SCL) is 700 MHz, which is determined by the fact that the frequency of the existing normal conducting linac is 350 MHz. A preliminary study on the beam dynamics showed that two types of cavities with geometrical betas of 0.50 and 0.74 could cover the entire energy range from 100 MeV to 1 GeV. An inductive output tube (IOT) based RF system is under consideration as a high-power RF source for the SCL due to its low operating voltage and high efficiency. As a prototyping activity for a reduced beta cavity, a five-cell cavity with a geometrical beta of 0.42 was designed and fabricated. A vertical test of the prototype cavity at low temperatures was performed to check the performance of the cavity. The design study and the prototyping activity for the PEFP SCL will be presented in this paper.

Balmer Hα, Hβ and Hγ Spectral Lines Intensities in High-Power RF Hydrogen Plasmas

Hα (Balmer-alpha), Hβ (Balmer-beta) and Hγ (Balmer-gamma) spectral line intensities in atomic hydrogen plasma are investigated by using a high-power RF source. The intensities of the Hα, Hβ and Hγ spectral lines are detected by increasing the input power (0–6 kW) of ICPs (inductively coupled plasmas). With the increase of net input power, the intensity of Hα improves rapidly (0–2 kW), and then reaches its dynamic equilibrium; the intensities of Hβ can be divided into three processes: obvious increase (0–2 kW), rapid increase (2–4 kW), almost constant (4–6 kW); while the intensities of Hγ increase very slowly. The energy levels of the excited hydrogen atoms and the splitting energy levels produced by an obvious Stark effect play an important role in the results.

ARIEL e-linac

The design and implementation of a 1/2 MW beam power electron linear accelerator (e-linac) for the production of rare isotope beams (RIB) via photo-fission in the context of the Advanced Rare IsotopE Laboratory, ARIEL (Koscielniak et al. 2008; Merminga et al. 2011; Dilling et al., Hyperfine Interact, 2013), is described. The 100 % duty factor e-linac is based on super-conducting radiofrequency (SRF) technology at 1.3 GHz and has a nominal energy of 50 MeV. This paper provides an overview of the accelerator major components including the gun, cryomodules and cryoplant, high power RF sources, and machine layout including beam lines. Design features to facilitate operation of the linac as a Recirculating Linear Accelerator (RLA) for various applications, including Free Electron Lasers, are also noted.

Performance and modelling of 70kVdc power supply with solid-state crowbar

The experimental activities of tokamak research involve development of high power RF and microwave sources for fusion related heating and current drive applications. High power RF and microwave tubes like Klystron, Gyrotron and Tetrode are in general operated with high voltage DC power supplies. These HVDC power supplies of the order of 70kVdc, must be equipped with necessary arc fault protection in addition to general over current and over voltage protection.The arc fault protection must act within few microseconds to prevent permanent damage to the RF tube, window, etc. When an arc fault is detected, output voltage of the DC power supply is short circuited using a crowbar device (generally Ignitron, Thyratron, thyristor, rail-gap, etc.) that operates in few microseconds. This diverts the fault current from the load to crowbar device, thereby protecting the load. This is necessary as conventional protection in the power supply input takes ∼100ms to switch-off. The crowbar device must be able to take the fault current till the circuit breaker placed at power supply input is switched off.The arc fault protection is tested for its effectiveness by “wire-burn” test. Full power short circuit of ∼1.5MW DC power supply puts enormous stress on the power supply, utility and the crowbar, therefore frequent wire-burn testing is to be avoided.This report presents simulation of wire-burn test using PSIM software. Optimization of the component values without conducting actual wire-burn test could be achieved.

High-power microwave attenuator employing slow wave structure

Using present pulsed microwave amplifier, we can obtain RF peak power beyond one hundred MW. However, it is not easy to test such a high-power RF. To overcome this difficulty we developed a high-power microwave attenuator employing a slow wave structure.For example, the output power of RF pulse compressor for present electron linear accelerator reaches a few hundreds MW RF power, but the existing dummy loads can absorb only a few tens MW of RF power.The attenuator we developed has a kind of periodic structure and is made of metal only. We operated this attenuator using a high-power RF source, and found that it could be operated fewer than 50pps RF output at 40MW, 2.5μs or 100MW, 0.5μs.

Optimizing the laser absorption technique for quantification of caesium densities in negative hydrogen ion sources

The performance of negative hydrogen ion sources, which rely on the formation of negative hydrogen ions on a surface with low work function, depends strongly on the caesium dynamics in the source. A quantitative measurement of the amount of caesium in the source during plasma-on and plasma-off (vacuum phase) is highly desirable. The laser absorption technique is optimized for the diagnostics of neutral caesium densities close to the extraction surface on which the negative hydrogen ions are generated. The setup is simplified as much as possible utilizing also an automatic data evaluation for online measurements at high power rf sources. The setup is tested and calibrated in a small scale laboratory experiment. The system and the analysis of the D2 caesium line at 852.1 nm are described in detail, including effects of line saturation and density depletion. The system is sensitive in the density range 1013–1017 m−3 (path length of about 15 cm), allowing also for a temporal resolution of 40 ms. First very promising results from the negative hydrogen ion source are presented, such as the increase in the caesium density due to the caesium evaporation and time traces before, during, and after the discharge indicating a strong caesium redistribution.