RF Power Coupler Research Articles

Electromagnetic properties of superconductive low-beta 325 MHz half-wave resonators at low RF field

Abstract A niobium prototype and two test unjacketed 325 MHz coaxial half-wave cavities with beta=0.21 were designed, fabricated, and tested at low RF fields. Electromagnetic properties were investigated using a vector network analyzer in the frequency domain and an RF generator, directional couplers, and power detectors in the time domain (both continuous wave and pulse mode).
We analyzed and compared the response of the low-beta cavity in both normal and superconducting states.
We studied the relationship between the resonator parameters and coupling devices to optimize the design of the RF power coupler and field probe antenna, aiming to achieve accurate cavity characterization.
We proposed a tunable power coupler design that allows for varying the coupling coefficient when operating in a vacuum at liquid helium temperature.
The typical responses of the cavity in case of under-, critical-, and over-coupling conditions obtained within a single cool-down are presented and analyzed.
We introduced a simplified method based on decay measurements with cavity port switching for effective cavity characterization at low RF amplitudes.
This method aligns well with standard Q-factor measurement protocols and can expedite experimental investigations with small RF signals.
The Q0(Eacc) curve for half-wave resonators at ultra-low amplitudes is analyzed and discussed.
Experiments with coaxial half-wave cavities showed an almost constant Q0 at RF fields ranging from 10-4MV/m to 10-1MV/m.
Additionally, a significant increase in the quality factor was observed after 120○C baking.
The presented results are discussed in the context of potential low-field applications of superconductive cavities. 

RF power coupling of RAON RFQ

A radio frequency quadrupole (RFQ) has been constructed as a front accelerator for a rare isotope accelerator facility. The RFQ has two power couplers that are operated simultaneously for high power operation to accelerate from proton to uranium. Coaxial transmission lines have been adopted to connect each power coupler to the 80 kW solid state power amplifier(SSPA) which is consisted with combiners, circulators and 5 kW unit amplifiers. In this study, the coupling to the cavity with two power couplers was investigated for the failure of RF power units. Moreover, the operation scenario of the RFQ linac was also presented in considering the availability.

A method to measure the electric parameters of the driver in a fusion-relevant RF ion source

RF ion sources for the neutral beam injection application contain a part called the driver where the plasma is generated. Measurement of the driver's external electrical parameters, such as coil current Icoil and driver equivalent impedance ZD, is useful for assessing the RF power coupling process and estimating the internal plasma parameters. The direct measuring method using RF I-V sensors at the driver's input terminal is inconvenient since it requires the installation of I-V sensors with very high phase (difference) accuracy near the driver. Thus, an indirect measuring method is developed for the RF ion sources equipped with the transformer-isolated Γ-type matching network in this paper. The method uses the electric parameters measured before the matching network and an impedance matching model of the real-world matching network. In the matching model, the matching transformer is treated as a mutual inductance circuit, and the parasitic impedance of the matching network is considered. Then the method is applied to an RF ion source, HINT. With the real-time impedance and power measured before the matching network as inputs, the driver's electric parameters and the RF power transfer efficiency η of HINT are monitored. Without plasma discharge, Icoil and ZD measured indirectly are consistent with those measured directly. With discharges (at transmitted power of 5∼12 kW and filling pressure of 0.45∼0.7 Pa), the deviations of indirect measurements from direct measurements are below 10% in Icoil, and then below 0.06 (absolute) in η. The comparisons show that the indirect measuring method can be used as an alternative to evaluate the driver's electric parameters and η in HINT.

Predictive fluid model for self-consistent description of inductive RF coupling in powerful negative hydrogen ion sources

RF-driven negative hydrogen ion sources are typically operated at low frequencies around 1 MHz, gas pressures around or below 1 Pa and large power densities up to 10 Wcm-3. Owing to these conditions as well as the current discharge geometries and antenna layouts, the RF power coupling is far from optimized, i.e. only a fraction η of the power delivered by the generator is absorbed by the plasma. This considerably limits the performance and reliability of RF-driven ion sources. To study the bidirectional RF power coupling a self-consistent fluid model is introduced. Taking into account the interplay between the nonlinear RF Lorentz force and the electron viscosity (usually neglected in state-of-the-art fluid models) a steady state solution is obtained, where the trends reflect the experimental data. Solutions calculated in hydrogen but with increased ion masses indicate that the latter are responsible for the systematically increased η, which is observed experimentally when deuterium instead of hydrogen is used as feed gas.

A 2D semi-analytical electromagnetic model for cylindrical ICP sources with Faraday shield

Faraday shield(FS) is a metallic shield with slits through which electromagnetic energy is coupled into cylindrical ICP sources. Thus, the effect of FS on the RF field and power coupling is important. A 2D semi-analytical electromagnetic model considering both FS and plasma is proposed. The results compared to a 2D numerical model shows that the semi-analytical model is available in electromagnetic field calculations. It is found that both FS and plasma have no influence on the magnetic field outside them. FS strengthens the electric field around the slits and weakens the electric field around the middle of the stripes, and this effect is due to the gradient field generated by the electric charge accumulated on the FS surfaces. From the point of view of the pure 2D electromagnetic field, FS affects the electric field distribution in a small region, and it seems to not affect the total power coupling into plasma.

Design studies of a continuous-wave normal conducting buncher for European X-ray Free Electron Laser

A three-cell 1.3 GHz, Normal Conducting (NC) buncher is designed for a possible future upgrade of the European X-ray Free Electron Laser (Eu-XFEL) to operate in a continuous-wave (CW)/ long pulse (LP) mode. The RF geometry of the buncher is optimized for high shunt impedance, large mode separation as well as multipacting free in the operation range. The nominal buncher voltage is 400 kV with an RF power dissipation of 14 kW, and its shunt impedance is ∼50% higher than existing CW bunchers. The RF power coupler cell is optimized for field symmetry to minimize coupler kick effect on beam dynamics. The thermal load due to RF power dissipation is analyzed using Multiphysics simulations in CSTMWS and a cooling scheme is designed.

Influence of external parameters on RF inductive discharge plasma characteristics

Systematic experimental studies of the electron density and temperature, the efficiency of RF power coupling to the RF inductive discharge plasma have been carried out in the pressure range of helium, neon, argon, and krypton 0.1–133 Pa, at an RF generator power of 100–500 W and frequencies of 2, 4 and 13.56 MHz. It is shown that the electron density reaches a maximum, and the temperature reaches a minimum in the pressure range 1.33–13.3 Pa. Taking into account the presence of a parasitic capacitive coupling between the inductor and the plasma, which forms the capacitive channel of RF power input, makes it possible to conclude that the maximum values of the electron density were observed at the pressure at which the power input through the inductive channel is maximal. At pressures of the order of 0.133 Pa and below, an increase in the electron temperature is observed in the peripheral part of the discharge. Numerical modeling by the PIC method shows that one of the reasons is the formation of a directed azimuthal motion of electrons in the region of the skin layer. As the pressure increases, a transition occurs from the nonlocal to the local electron kinetics, which is reflected in the ratio between the electron temperature in the peripheral and central parts of the discharge.

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.

Magnetic nozzle radiofrequency plasma thruster approaching twenty percent thruster efficiency

Development of a magnetic nozzle radiofrequency (rf) plasma thruster has been one of challenging topics in space electric propulsion technologies. The thruster typically consists of an rf plasma source and a magnetic nozzle, where the plasma produced inside the source is transported along the magnetic field and expands in the magnetic nozzle. An imparted thrust is significantly affected by the rf power coupling for the plasma production, the plasma transport, the plasma loss to the wall, and the plasma acceleration process in the magnetic nozzle. The rf power transfer efficiency and the imparted thrust are assessed for two types of rf antennas exciting azimuthal mode number of m=+1 and m=0, where propellant argon gas is introduced from the upstream of the thruster source tube. The rf power transfer efficiency and the density measured at the radial center for the m=+1 mode antenna are higher than those for the m=0 mode antenna, while a larger thrust is obtained for the m=0 mode antenna. Two-dimensional plume characterization suggests that the lowered performance for the m=+1 mode case is due to the plasma production at the radial center, where contribution on a thrust exerted to the magnetic nozzle is weak due to the absence of the radial magnetic field. Subsequently, the configuration is modified so as to introduce the propellant gas near the thruster exit for the m=0 mode configuration and the thruster efficiency approaching twenty percent is successfully obtained, being highest to date in the kW-class magnetic nozzle rf plasma thrusters.

Commissioning and initial operation of the W7-X neutral beam injection heating system

Abstract The first, of two planned, neutral beam injectors for the stellarator Wendelstein 7-X (W7-X) was commissioned for and participated in the experimental campaign (OP1.2b) from July to October 2018. The injector was equipped with two RF driven ion sources from which 90A of positive hydrogen ions were extracted at 55 kV. After neutralization, the two sources provided >3 MW of neutral beam heating power to the stellarator plasma. During the experimental campaign >300 shots were successfully performed for plasma heating or to allow for measurement of the ion temperature profile by charge exchange recombination spectroscopy (CXRS). The initial operation of the NBI system on W7-X was very successful, demonstrating both increased central plasma density and stored plasma energy, or allowing for the collection of the time resolved ion temperature over the bulk of the plasma. This paper presents, briefly, source conditioning and performance, focusing on some of the initial problems encountered. Described in more detail is the most significant challenge overcome during commissioning: failure to couple RF power into one of the sources.

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.

Measurement of RF characteristics for Superconducting Quarter-wave Resonator

RF characteristics of unloaded quality factor (Q0) and accelerating electric field (Eacc) were measured for superconducting quarter-wave resonator (QWR) in order to verify the performance. For the accurate measurement of RF characteristics, the measurement method of Q0 based RF power and RF coupling coefficient was used, and the measurement uncertainty was calculated with statistics analysis of systematic error in RF measurement system. The superconducting QWR was designed with a resonant frequency of 101.28 MHz and an accelerating electric field (Eacc) of 6 MV/m at a cavity dissipation power of 10 W in the high-intensity and energy isotope separator online device (HIE-ISOLDE) project at CERN. Q-slopes and RF coupling coefficients were measured for three QWRs at the resonant frequency 101.28 MHz. Q0 values of QWRs were measured 4.55 × 108, 3.78 × 108 and 3.17 × 108 at the 6 MV/m, respectively, and the measurement uncertainty of Q0 and Eacc were calculated 2.92% and 3.32%. Performances of superconducting QWRs were acceptable to operate the beam acceleration with consideration of cryomodule capacity in HIEISOLDE project.

Time-Resolved Electron Density Measurement Characterization of E–H-Modes for Inductively Coupled Plasma Instabilities

Inductively coupled plasma sources driven by RF power at low-pressure regimes are well adopted for high-volume manufacturing of semiconductor devices. One vexing challenge to the utility of these plasma processing reactors is the existence of the E–H-mode transition. Industry notably avoids the process region associated with this transition, where plasma instabilities and bimodal power coupling prohibit reliable RF power delivery. One plasma instability detailed in this paper is associated with a hysteresis in coupled RF power (current) varying for the E-mode, or weakly capacitive coupling to the plasma, in comparison to the stronger current coupling in the H-mode, where inductive coupling is preferentially dominant. As a result, approximately two orders of magnitude of electron density is relinquished in this transition region from serving industrial manufacturing processes. We characterize the plasma parameter variation through the E-mode to H-mode with a time-resolved measurement of the electron density. Electronegative chemistries are incorporated into our experimental setup. The experimental scheme serves to evaluate RF power delivery and ameliorate its coupling through the transition region. We seek to extend this paper to adopt more efficient power coupling for toroidal plasma sources.

New compact ion source design and implementation for low current applications

Abstract A new compact electron cyclotron resonance (ECR) ion source for low current applications, designed and built in-house at the University of the Basque Country (UPV/EHU) is presented. The source was designed to use as many commercially available components as possible for key tasks such as electromagnetic resonators, magnetic structure, RF power couplers and beam transport, to lower the cost and lessen reliance on specially made parts for spares. This leads to a cost-effective yet compact and fully functional design. The main design decisions are described and experiments on plasma generation and the proton beam extracted using the source are shown and discussed.

Field tuning and rf measurements of the four-vane radio frequency quadrupole with ramped inter-vane voltage

This paper describes the physical design, rf measurement, and tuning result of a 325 MHz four-vane radio frequency quadrupole (RFQ) with a ramped inter-vane voltage. The 3-meter-long RFQ will accelerate the negative hydrogen ion beam from 50 keV to 3 MeV with a peak current of 5 mA, a pulse width of $10--40\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$, and a maximum repetition rate of 0.5 Hz. During tuning we reveal that the coupling loop of the coaxial coupler adopted for this RFQ enhances the interference between the dipole field distribution and the coupling coefficient. Therefore, an iteration method is adopted to simultaneously achieve the desired field distribution and coupling coefficient. After tuning, the relative error between the measured field distribution and the designed curve for the operating quadrupole mode is within $\ifmmode\pm\else\textpm\fi{}2.7%$, and the dipole components are about $\ifmmode\pm\else\textpm\fi{}1.9%$ of the quadrupole mode. The measured coupling coefficient of the rf power coupler equals 1.03, with the desired value of 1.04.

Electromagnetic simulations, manufacturing and low power measurements of a 352.21 MHz RF power coupler for the SANAEM RFQ project

Radio frequency quadrupole (RFQ) linear accelerators occupy an important place among modern accelerators since the days it was conceived and constructed because of its ability to focus, bunch and accelerate the low energy beams at high efficiency. In Turkey, recently a 352.21 MHz RFQ accelerator cavity is designed and manufactured within a proton beamline located at the Turkish Atomic Energy Authority (TAEK), Sarayköy Nuclear Research and Training Center (SANAEM). The SANAEM RFQ cavity, operating in pulsed mode needs an overall RF power of 120 kW to accelerate the proton beam up to a final beam energy of 1.3 MeV . A new prototype RF power coupler, considering multipacting effect is designed, developed and manufactured to be used to deliver the required power for the RFQ. At the beginning of this study, design and 3D EM simulations of a coaxial power coupler considered for the SANAEM RFQ accelerator is presented. The RF properties of the coupler such as external quality factor, power loss on the loop, coupling coefficient and scattering parameters are evaluated and the geometrical parameters are optimized with the CST Microwave Studio. Also, various dielectric window materials to be used in the coupler are discussed. Moreover, multipacting simulations for the coupler are carried out using the CST Particle Studio and RF power levels at which the multipacting effect occurs are obtained. Finally, the manufacturing of a prototype coupler, low power measurements and discussions are presented in the present article.

An algorithm for the design and tuning of RF accelerating structures with variable cell lengths

An algorithm is proposed for the design of a π mode standing wave buncher structure with variable cell lengths. It employs a two-parameter, multi-step approach for the design of the structure with desired resonant frequency and field flatness. The algorithm, along with analytical scaling laws for the design of the RF power coupling slot, makes it possible to accurately design the structure employing a freely available electromagnetic code like SUPERFISH. To compensate for machining errors, a tuning method has been devised to achieve desired RF parameters for the structure, which has been qualified by the successful tuning of a 7-cell buncher to π mode frequency of 2856 MHz with field flatness <3% and RF coupling coefficient close to unity.The proposed design algorithm and tuning method have demonstrated the feasibility of developing an S-band accelerating structure for desired RF parameters with a relatively relaxed machining tolerance of ∼25μm. This paper discusses the algorithm for the design and tuning of an RF accelerating structure with variable cell lengths.

Room temperature quarter wave resonator re-buncher development for a high power heavy-ion linear accelerator

In the medium energy beam transport (MEBT) line system of the RAON which consists of several quadrupole magnets, three normal-conducting re-bunchers, and several diagnostic devices, a quarter wave resonator type re-buncher was chosen for minimizing longitudinal emittance growth and manipulating a longitudinal phase ellipse into the longitudinal acceptance of the low energy linac. The re-buncher has a resonant frequency of 81.25 MHz, geometrical beta (βg) of 0.049, and physical length of 24 cm. Based on the result of numerical calculations of electromagnetic field using CST-MWS and mechanical analysis of the heat distribution and deformation, an internal structure of the re-buncher was optimized to increase the effective voltage and to reduce power losses in the wall. The criteria of the multipacting effect was estimated and it was confirmed by the experiment. The position and specification of cooling channels are designed to recover a heat load up to 15 kW with reasonable margin of 25%. The coaxial and loop type RF power coupler are positioned on the high magnetic field region and two slug tuners are installed perpendicularly to the beam axis. The frequency sensitivity as a function of the tuner depth and cooling water temperature is measured and the frequency shift is in all cases within the provided tuner range. The test with a high power of 10 kW and the continuous wave is performed and the reflection power is observed less than 450 W.

The ESS Spoke cryomodule and its test valve box

The European Spallation Source project aims at being the most powerful neutron source feeding multidisplinary researches. Based on superconducting radiofrequency technologies, the linear accelerator will operate for the first time a 56 meter long section of niobium double Spoke cavities. Paired in 13 cryomodules, each cavity will generate an accelerating pulsed field of 9MV/m. A prototype cryomodule housing two superconducting Spoke cavities and their RF power couplers is now being fabricated and assembled. It provides the cryogenic environment for their operation in a 2K saturated superfluid helium bath: a 50 K thermal shield, a cold magnetic shield, and integrates all the interfaces necessary to be operational within the linac machine. This prototype will be tested in 2016 at IPNO site and then at Uppsala university FREIA facilities. A valve box was designed to take into account the specific features of this prototype cryomodule and of both test sites. This valve box is also considered as a prototype of the cryogenic distribution of the linac Spoke section. This element will then be used for the tests of the series cryomodules.

Effect of the radial plasma nonuniformity on the propagation of guided m=+1 and m=−1 modes in helicon discharges

Theoretical as well as numerical analyses of the full set of Maxwell's equations is carried out to study non-axisymmetric (m≠ 0) guided modes in radially nonuniform helicon (HE) discharges. Unlike the axisymmetric (m = 0) modes, these modes reveal a non-reciprocal behavior with respect to the azimuthal direction. We develop the conditions for propagation and non-propagation of the various modes in the helicon parameter range, thereby focussing on the important role of the radial density gradient. Three types of modes occurring in different parameter ranges are described, i.e., the helicon (HE) mode, the electrostatic (ES) or Trivelpiece-Gould mode, and the locally coupled (LC) mode that is characterized by mode coupling (MC) in a certain region of the plasma density profile. In contrast to m=+1 modes, the parameter range of m=−1 modes is much more restricted as rather high densities are needed for the propagation of the helicon and LC modes. An important issue of the investigations is the rf power coupling and absorption via the various modes. Computations based on a simple antenna-plasma model show that the axial wavenumber of the antenna determines decisively which type of mode is excited. In case of LC mode excitation, the dominant role of the MC layer for the absorption is demonstrated. Finally, the rf power coupling to helicon modes is studied. The density limit for m=−1 helicon mode propagation and the narrow magnetic field profiles of these modes are the main reasons why the rf power absorption in helicon discharges occurs via m=+1 helicon modes.