High Power Test Research Articles

High RF power tests of the first 1.3\xa0GHz fundamental power coupler prototypes for the SHINE project

The Shanghai High Repetition Rate XFEL and Extreme Light Facility (SHINE) project will use 600 1.3 GHz fundamental power couplers, which are modified based on TTF-III power couplers, for continuous-wave operation with input power up to approximately 7 kW. The first batch of 20 sets of 1.3 GHz coupler prototypes was fabricated from three domestic manufacturers for the SHINE project. To better characterize the radio frequency conditioning phenomena for validating the performance of power couplers, a room temperature test stand was designed, constructed, and commissioned for the SHINE 1.3 GHz power couplers. In addition, a horizontal test cryostat was built to test the 1.3 GHz superconducting cavities, fundamental power couplers, tuners, and other components as a set. The results of these tests indicate that the 1.3 GHz couplers are capable of handling up to 14 kW continuous waves. Herein, the main aspects of the radio frequency design and construction of the test stand, along with the test results of the high-power conditioning of the 1.3 GHz couplers, are described.

Design and Implementation of High-Power Fiber Laser Cham-ber Test Under Different Aerosols Conditions

Design and Implementation of High-Power Fiber Laser Cham-ber Test Under Different Aerosols Conditions

High Power RF Discharge Detection Technique Based on the In-Phase and Quadrature Signals

High power radio frequency (RF) breakdown testing is a subject of great relevance in the space industry, due to the increasing need of higher transmission power and smaller devices. This work presents a novel RF breakdown detection system, which monitors the same parameters as the microwave nulling system but with several advantages. Where microwave nulling--a de facto standard in RF breakdown testing--is narrowband and requires continuous tuning to keep its sensitivity, the proposed technique is broadband and maintains its performance for any RF signal. On top of that, defining the detection threshold is cumbersome due to the lack of an international standardized criterion. Small responses may appear in the detection system during the test and, sometimes, it is not possible to determine if these are an actual RF breakdown or random noise. This new detection system uses a larger analysis bandwidth, thus reducing the cases in which a small response is difficult to be classified. The proposed detection method represents a major step forward in high power testing as it runs without human intervention, warning the operator or decreasing the RF power automatically much faster than any human operator.

Selective mode filtering for gyrotron traveling-wave tube systems

In this paper, a method to filter the parasitic modes for gyrotron traveling-wave tube (gyro-TWT) system is proposed. The difference of surface current between the operating TE01 mode and the parasitic modes is analyzed to achieve the mode discrimination. Based on the analysis, a slot array with high mode-selectivity is introduced, and a Ka-band oversized waveguide mode filter is designed to attenuate the parasitic modes. A prototype is fabricated and measured for validation purpose. The measurement of S-parameter demonstrates that parasitic modes can be effectively suppressed by the mode filter, while the TE01 mode suffers little attenuation after propagating through the proposed device. The high-power capacity of the mode filter is verified by a high-power test based on a gyro-TWT. The proposed mode filter features high performance in purifying TE01 mode as well as a high-power capacity, which makes it suitable for gyro-TWT systems.

Analysis and design of parallel-coupled high-gradient structure for ultrashort input power pulses

Shortening the pulse length of input power is a possible approach to raise the accelerating gradient limit of normal conducting structures. We present a novel design of a parallel-coupled structure that operates at ultrashort input power pulses. The principle of the design is to shorten the filling time of the whole structure by feeding the cells individually so that the duration of power transmission between cells can be saved. An X-band (11.994 GHz) parallel-coupled structure design with a 10 ns pulse length is presented in this work. Previous high-power tests of X-band structure prototypes show that a gradient of $120\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ could be achieved for a 200 ns pulse length. Based on the law requiring that ${E}^{30}{t}^{5}=\text{constant}$, this 10-ns-structure design should be able to reach a gradient of $200\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. A detailed circuit model and real-time electromagnetic field simulation methods for designing the parallel-coupling structure are also presented and discussed in this paper.

Development of a compact X-band variable-ratio RF power splitter

A novel X-band (11.424 GHz) radio frequency (RF) power splitter has been designed, fabricated and tested at Tsinghua University. With E-bend waveguide structures and a modified concave bottom polarizer introduced in this work, a compact geometry is achieved. The RF power division ratio of two output ports can be adjusted arbitrarily by changing the position of a short circuit piston, controlled by a servo motor precisely. The input port maintains a good match at any output ratio. The parameters are optimized to acquire high RF power transmission capacity and large bandwidth. The cold test indicated that the max input reflection is −33.1 dB and the max output transmissions are −0.07 dB and −0.12 dB respectively, which agrees with the simulation results well. The high-power test was operated on Tsinghua High Power Test Stand. After days of conditioning, the RF power splitter can be operated steadily at 53 MW 250 ns input power with 20 Hz repetition frequency.

High-gradient rf tests of welded X -band accelerating cavities

Linacs for high-energy physics, as well as for industry and medicine, require accelerating structures which are compact, robust, and cost-effective. Small foot-print linacs require high-accelerating gradients. Currently, stable-operating gradients, exceeding $100\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, have been demonstrated at SLAC National Accelerator Laboratory, CERN, and KEK at X-band frequencies. Recent experiments show that accelerating cavities made out of hard copper alloys achieve better high-gradient performance as compared with soft copper cavities. In the scope of a decade-long collaboration between SLAC, INFN-Frascati, and KEK on the development of innovative high-gradient structures, this particular study focuses on the technological developments directed to show the viability of novel welding techniques. Two novel X-band accelerating structures, made out of hard copper, were fabricated at INFN-Frascati by means of clamping and welding. One cavity was welded with the electron beam and the other one with the tungsten inert gas welding process. In the technological development of the construction methods of high-gradient accelerating structures, high-power testing is a critical step for the verification of their viability. Here, we present the outcome of this step---the results of the high-power rf tests of these two structures. These tests include the measurements of the breakdown rate probability used to characterize the behavior of vacuum rf breakdowns, one of the major factors limiting the operating accelerating gradients. The electron beam welded structure demonstrated accelerating gradients of $90\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ at a breakdown rate of ${10}^{\ensuremath{-}3}/(\mathrm{pulse}\text{ }\mathrm{meter})$ using a shaped pulse with a 150 ns flat part. Nevertheless, it did not achieve its ultimate performance because of arcing in the mode launcher power coupler. On the other hand, the tungsten inert gas welded structure reached its ultimate performance and operated at about a $150\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ gradient at a breakdown rate of ${10}^{\ensuremath{-}3}/(\mathrm{pulse}\text{ }\mathrm{meter})$ using a shaped pulse with a 150 ns flat part. The results of both experiments show that welding, a robust, and low-cost alternative to brazing or diffusion bonding, is viable for high-gradient operation. This approach enables the construction of multicell standing and traveling-wave accelerating structures.

High Power Tests of Brazeless Accelerating Structures

High Power Tests of Brazeless Accelerating Structures

High Power Test of the Antenna Adjustable Power Coupler for 325 MHz Superconducting Cavities

High Power Test of the Antenna Adjustable Power Coupler for 325 MHz Superconducting Cavities

High-Power Test of a Highly Over-Coupled X-Band RF Gun Driven by Short RF Pulses

High-Power Test of a Highly Over-Coupled X-Band RF Gun Driven by Short RF Pulses

Ultrawideband, high-power, microstripline test setup for experimental study and characterization of multipactor.

This paper discusses different components of a high-power test setup for replicating multipactor in a laboratory environment. We developed a broadband test cell for parallel-plate multipactor discharges that can operate from DC to 1.2GHz. The proposed test cell design features a multi-step transition from a coaxial line to a microstripline with negligible insertion loss suitable for high-power breakdown experiments. The multipactor section is adjustable and replaceable, offering flexibility in conducting various multipactor tests, such as different gap distances and local surface treatments. We incorporated two local multipactor detection methods, an electron multiplier tube and a biased standalone probe to rapidly and reliably detect the growth of secondary electrons in the multipactor vicinity. The driving circuits of these detection methods have been designed to filter out RF coupling while preserving the detection signal due to multipactor current. To demonstrate the accuracy of the proposed test setup, we validated the multipactor thresholds determined in simulation using the 3D particle-in-cell module of CST Microwave Studio. We obtained very good agreement between simulation and experimental results over the broadband frequency range. The topics discussed in this paper further inform how to address the design obstacles encountered in developing a bench-top multipactor test setup.

The development of the 499.8 MHz superconducting cavity system for BEPCII

In order to accommodate the luminosity improvement strategy, two superconducting 499.8 MHz radio frequency (rf) cavities have been adopted for the Upgrade project of Beijing Electron Positron Collider (BEPCII), a double-ring electron–positron collider as well as a synchrotron radiation (SR) light source. The in-house development of the 499.8 MHz superconducting cavity system was initiated at IHEP since 2008. One prototype niobium cavity with all the parts except for the flanges and the pickup port formed by spinning was fabricated and then vertical-tested successfully in July 2011. Four prototype input couplers were manufactured and high-power tested with one reaching a maximum rf power of continuous wave (cw) 420 kW. One prototype HOM damper with a satisfactory absorbing efficiency at the desired frequency band was also constructed and high power tested up to cw 4.4 kW. In the meanwhile, a frequency tuner consisting of a stepping motor and a piezoelectric oscillator was developed. Finally, the bare cavity was dressed with one input coupler, two HOM dampers and one frequency tuner, mounted in a domestic-developed cryostat and then high-power tested successfully in Oct. 2011. The achieved accelerating voltage is 2.2 MV with corresponding Q0 measured to be 5.8 × 108, beyond the designed goal. This is the first in-house developed 499.8 MHz HOM-damped superconducting cavity system in China. The design, fabrication, post processing and tests of the niobium bare cavity and its auxiliaries, as well as the horizontal high-power test of the cavity system are presented.

Development of the antenna adjustable power coupler for 325 MHz superconducting cavities

In this paper, we present the multi-physics design and high-power tests of a fundamental power coupler for superconducting cavities. The fabricated power coupler was a coaxial capacitive-type coupler based on a conventional 318-inch Electronic Industries Alliance 50-Ω coaxial transmission line with a titanium nitride (TiN)-coated single ceramic window. The coupler had three diagnostic ports for vacuum, arc, and electron pick-ups to monitor radio frequency (RF) breakdowns. To reduce the thermal load required to reach the cryogenic temperature, the thermal intercepts were located at 4.5 K and 40 K for liquid helium use and at 4.5 K and 77 K for liquid helium and nitrogen use. A transition box for adjusting the antenna penetration depth and matching impedance was designed. A high-power test was performed with a 20 kW and 325 MHz solid-state power amplifier. The power coupler was connected to a rectangular test cavity with high vacuum and various measuring equipment, such as an arc detector, a power meter, and an electron pick-up probe. The interlock system under vacuum and arc instrumentations prevented the RF window from breaking the power coupler window during the high-power test. We conducted high-power tests for more than 12 h at 12 kW in a 325 MHz continuous wave mode to verify the performance of the designed power coupler.

Development of an automatic control and analysis system for the conditioning of high-power couplers at IHEP

A new radio frequency (rf) conditioning system for power couplers has been developed for the High Energy Photon Source (HEPS) project. The system adopted a function-driven modular design, and the control software development was based on Experimental Physics and Industrial Control System (EPICS) platform to be compatible with the HEPS control system. Dedicated data acquisition system and in-house developed interlock system, as well as universal instruments and devices, constitute the main hardware infrastructure, while distributed EPICS input/output controllers, tailor-made operator interfaces, and data archiving comprise the control software pillars. Automatic control logics were developed to adjust the conditioning parameters according to vacuum pressure readings, thus largely spares human misoperations and considerably reduces the operator’s workload. Limited human supervision is required and the conditioning efficiency may be improved. Besides, data analysis and automatic report generation programs were developed and integrated into the conditioning system. Performances and behaviors of the couplers being conditioned can be easily obtained to help identify potential issues. The system has been used in the conditioning of power couplers and has also been customized to accommodate high-power tests of several other rf components thanks to its modular nature. The performance of both the hardware and the control software was excellent during the tests. The design, realization, and operation of the new rf conditioning system are presented.

The performance of RF power couplers with capacitive coupling for SRF accelerators

Different from common coaxial couplers used for superconducting radio frequency (SRF) accelerators, power couplers with capacitive coupling in which the inner conductor is separated into two unconnected parts on both sides of the cold ceramic window transmit RF power by capacitive coupling. Capacitive coupling type power couplers have the advantages of easy fabrication of ceramic window, reduced thermal conduction from room temperature side to the cryogenic system, and convenient assembling to cryomodule. To further investigate the performance of this type of RF power couplers, an upgraded power coupler with capacitive coupling has been designed and fabricated at Peking University. The high-power test demonstrated that RF power up to 15 kW in continuous wave (CW) mode can be transmitted steadily. In this paper, the structure, simulation, and high-power test results of this power coupler are presented in detail.

High-power laser testing of calcium-phosphate-based bioresorbable optical fibers

Silica optical fibers are employed in endoscopy and related minimally invasive medical methods thanks to their good transparency and flexibility. Although silicon oxide is a biocompatible material, its use involves a serious health risk due to its fragility and the fact that potential fiber fragments can freely move inside the body without the possibility of being detected by conventional methods such as X-ray imaging. A possible solution to this issue can be the development of optical fibers based on bioresorbable (i.e., biodegradable and biocompatible) materials, which exhibit the important benefit of not having to be explanted after their functionality has expired. The optical power transmission tests of recently developed single-mode (SM) and multi-mode (MM) bioresorbable optical fibers based on calcium-phosphate glasses (CPGs) are here reported. A continuous-wave (CW) fiber laser at 1080 nm with output power up to 13 W and picosecond laser sources at 515 and 1030 nm with MW pulse peak power were used to test the transmission capabilities of the CPG fibers. No degradation of the CPG fibers transmission under long-term illumination by CW laser was observed. A laser-induced damage threshold (LIDT) at a fluence higher than 0.17 J/cm2 was assessed with the picosecond laser sources.

Development of medium-frequency cavity loaded with multi-ring magnetic alloy cores cooled by chemically inert liquid

Magnetic alloys (MAs) are commonly applied in the radio-frequency (RF) cavities of proton synchrotrons. The high saturation permeability and Curie temperature of MAs facilitate the development of RF cavities with large accelerating gradients that could not be achieved with ferrite. The conventional MA core is a disk-shaped toroid made by winding MA ribbons. Here, we propose a multi-ring core structure that consists of three concentrically arranged toroidal ring cores of different radial sizes. Neither epoxy resin impregnation nor a waterproof coating is applied to the cores, and thus the thermal stress during high-power operation is expected to be relaxed. A set of multi-ring cores is sandwiched between two glass-epoxy plates with flow channels. This modular structure increases the velocity of the coolant flow, making the flow turbulent to obtain the desired cooling efficiency. Coolant-induced corrosion of the Fe-based MA is prevented by adopting a low-viscosity, chemically inert, and electrically insulating liquid. The RF and thermal designs of a cavity loaded with multi-ring cores are presented and two types of test cavity are developed. A series of high-power tests demonstrate stable operation with the designed performance. The results of experimental performance tests for two cavities loaded with multi-ring MA cores cooled by a chemically inert liquid are reported.

Compact S -band linear accelerator system for ultrafast, ultrahigh dose-rate radiotherapy

Radiation therapy is currently the most utilized technique for the treatment of tumors by means of ionizing radiation, such as electrons, protons and x/gamma rays, depending on the type, size and depth of the cancer mass. Radiation therapy has in general fulfilled the main requirement of targeting thus damaging the malignant cells and sparing the healthy tissues as best as possible. In this scenario, electron linear accelerators have been operated as viable tools for the delivery of both high-energetic electrons and x-ray beams, which are obtained via the bremsstrahlung process of the electrons hitting on a high-Z material. Recently, it has been experimentally demonstrated that ultrahigh dose-rate bursts of electrons and x-ray beams increase the differential response between healthy and tumor tissues. This beneficial response is referred to as the FLASH effect. For this purpose, we have developed the first dedicated compact $S$-band linear accelerator for FLASH radiotherapy. This linac is optimized for a nominal energy of 7 MeV and a pulsed electron beam current of 100 mA and above. The accelerator is mounted on a remote-controlled system for preclinical research studies in the FLASH regime. We will show the rf and beam dynamics design of the $S$-band linac as well as the commissioning and high-power rf tests. Furthermore, the results of the dosimetric measurements will be illustrated.

Test and operation of high-power RF system for KSTAR helicon current drive system

The helicon current drive system for the efficient off-axis current drive of KSTAR is currently being upgraded with 476 MHz, 1.2 MW klystrons and a new high-voltage power supply (HVPS). Four klystrons, which were used in PEP-II, were transferred to NFRI from the Stanford Linear Accelerator Center. Each klystron can generate 1.2 MW of CW RF power at 476 MHz with an 84 kV 24 A beam. Two klystrons will be powered in parallel using a single pulse step modulator HVPS with a rating of 92 kV and 64 A. Herein, a detailed description of the klystron test setup and results obtained during its test is presented. A high-power test of the klystrons is performed, wherein an RF power of 620 kW is generated by a 70 kV and 18.8 A beam.

High-power tests and solution to overheating at cavity–coupler interface of the 166.6-MHz beta = 1 superconducting quarter-wave resonator for HEPS

166.6-MHz superconducting cavities have been chosen for the High Energy Photon Source (HEPS) as the main accelerating structures to provide 900 kW of beam power and 5.4 MV of accelerating voltage. A proof-of-principle cavity adopting the quarter-wave beta = 1 geometry was previously developed. Excellent performance was achieved in vertical tests at cryogenic temperatures. The cavity was later welded with a helium jacket, dressed with a power coupler and other ancillaries, and high-power tested in a test cryomodule. Performance degradation was observed and analyzed. Evidence from temperature sensor readout and heat loss measurement results suggested an overheating in the cavity–coupler interface region causing a “thermal runaway” and eventually quenching the cavity at its design voltage. Electromagnetic-fluid-thermal coupled simulation has thus been conducted, and the hypothesis was nicely validated. Finally, solutions were proposed including an elongated niobium extension tube at the coupler port and an optimized helium gas cooling of the power coupler’s outer conductor. These modifications have been subsequently applied on the 166.6-MHz higher-order-mode damped superconducting cavities for the HEPS. Heat loss at 4.2 K contributed by the power coupler can be largely reduced with a modest gas cooling scheme. Similar design approaches can also be applied to other non-elliptical superconducting structures with on-cavity high-power coupler mountings.