RF Sources for High Gradient Accelerators
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high power klystrons, multiple beam klystrons, MBK↗
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Alternative modeling methods for plasma-based Rf ion sources
Rf-driven ion sources for accelerators and many industrial applications benefit from detailed numerical modeling and simulation of plasma characteristics. For instance, modeling of the Spallation Neutron Source (SNS) internal antenna H{sup −} source has indicated that a large plasma velocity is induced near bends in the antenna where structural failures are often observed. This could lead to improved designs and ion source performance based on simulation and modeling. However, there are significant separations of time and spatial scales inherent to Rf-driven plasma ion sources, which makes it difficult to model ion sources with explicit, kinetic Particle-In-Cell (PIC) simulation codes. In particular, if both electron and ion motions are to be explicitly modeled, then the simulation time step must be very small, and total simulation times must be large enough to capture the evolution of the plasma ions, as well as extending over many Rf periods. Additional physics processes such as plasma chemistry and surface effects such as secondary electron emission increase the computational requirements in such a way that even fully parallel explicit PIC models cannot be used. One alternative method is to develop fluid-based codes coupled with electromagnetics in order to model ion sources. Time-domain fluid models can simulate plasma evolution, plasma chemistry, and surface physics models with reasonable computational resources by not explicitly resolving electron motions, which thereby leads to an increase in the time step. This is achieved by solving fluid motions coupled with electromagnetics using reduced-physics models, such as single-temperature magnetohydrodynamics (MHD), extended, gas dynamic, and Hall MHD, and two-fluid MHD models. We show recent results on modeling the internal antenna H{sup −} ion source for the SNS at Oak Ridge National Laboratory using the fluid plasma modeling code USim. We compare demonstrate plasma temperature equilibration in two-temperature MHD models for the SNS source and present simulation results demonstrating plasma evolution over many Rf periods for different plasma temperatures. We perform the calculations in parallel, on unstructured meshes, using finite-volume solvers in order to obtain results in reasonable time.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Alternative RF coupling configurations for H{sup −} ion sources
RF heated sources for negative hydrogen ions both for fusion and accelerators require very high RF powers in order to achieve the required H{sup −} current what poses high demands on the RF generators and the RF circuit. Therefore it is highly desirable to improve the RF efficiency of the sources. This could be achieved by applying different RF coupling concepts than the currently used inductive coupling via a helical antenna, namely Helicon coupling or coupling via a planar ICP antenna enhanced with ferrites. In order to investigate the feasibility of these concepts, two small laboratory experiments have been set up. The PlanICE experiment, where the enhanced inductive coupling is going to be investigated, is currently under assembly. At the CHARLIE experiment systematic measurements concerning Helicon coupling in hydrogen and deuterium are carried out. The investigations show that a prominent feature of Helicon discharges occurs: the so-called low-field peak. This is a local improvement of the coupling efficiency at a magnetic field strength of a few mT which results in an increased electron density and dissociation degree. The full Helicon mode has not been achieved yet due to the limited available RF power and magnetic field strength but it might be sufficient for the application of the coupling concept to ion sources to operate the discharge in the low-field-peak region.
07 ISOTOPE AND RADIATION SOURCES↗
Overview of High Power Vacuum Dry RF Load Designs
A specific feature of RF linacs based on the pulsed traveling wave (TW) mode of operation is that only a portion of the RF energy is used for the beam acceleration. The residual RF energy has to be terminated into an RF load. Higher accelerating gradients require higher RF sources and RF loads, which can stably terminate the residual RF power. RF feeders (from the RF source though the accelerating section to the load) are vacuumed to transmit multi-megawatt high power RF. This overview will outline vacuumed RF loads only. A common method to terminate multi-MW RF power is to use circulated water (or other liquid) as an absorbing medium. A solid dielectric interface (a high quality ceramic) is required to separate vacuum and liquid RF absorber mediums. Using such RF load approaches in TW linacs is troubling because there is a fragile ceramic window barrier and a failure could become catastrophic for linac vacuum and RF systems. Traditional loads comprising of a ceramic disk have limited peak and average power handling capability and are therefore not suitable for high gradient TW linacs. This overview will focus on ''vacuum dry'' or ''all-metal'' loads that do not employ any dielectric interface between vacuum and absorber. The first prototype is an original design of RF loads for the Stanford Two-Mile Accelerator.
43 PARTICLE ACCELERATORS↗
RF Conditioning of the Photo-Cathode RF Gun at the Advanced Photon Source - NWA RF Measurements
A new S-band Photo-cathode (PC) gun was recently installed and RF conditioned at the Advanced Photon Source (APS) Injector Test-stand (ITS) at Argonne National Lab (ANL). The APS PC gun is a LCLS type gun fabricated at SLAC [1]. The PC gun was delivered to the APS in October 2013 and installed in the APS ITS in December 2013. At ANL, we developed a new method of fast detection and mitigation of the guns internal arcs during the RF conditioning process to protect the gun from arc damage and to RF condition more efficiently. Here, we report the results of RF measurements for the PC gun and an Auto-Restart method for high power RF conditioning.
Smith, T. L.↗
Simplified method for measuring the impedance of RF power sources - A concept
Bolometer detector and bridge circuit measure the RF power. A varied bridge reference resistor achieves maximum power transfer allowing the output impedance of the RF source to be determined from the known circuit parameters.
Oakley, E. C.↗
Ion extraction from a saddle antenna RF surface plasma source
Existing RF Surface Plasma Sources (SPS) for accelerators have specific efficiencies for H{sup +} and H{sup −} ion generation around 3 to 5 mA/cm{sup 2} per kW, where about 50 kW of RF power is typically needed for 50 mA beam current production. The Saddle Antenna (SA) SPS described here was developed to improve H{sup −} ion production efficiency and SPS reliability and availability. At low RF power, the efficiency of positive ion generation in the plasma has been improved to 200 mA/cm{sup 2} per kW of RF power at 13.56 MHz. Initial cesiation of the SPS was performed by heating cesium chromate cartridges by discharge as was done in the very first versions of the SPS. A small oven to decompose cesium compounds and alloys was developed and tested. After cesiation, the current of negative ions to the collector was increased from 1 mA to 10 mA with RF power ∼1.5 kW in the plasma (6 mm diameter emission aperture) and up to 30 mA with ∼4 kW RF power in the plasma and 250 Gauss longitudinal magnetic field. The ratio of electron current to negative ion current was improved from 30 to 2. Stable generation of H{sup −} beam without intensity degradation was demonstrated in the AlN discharge chamber for a long time at high discharge power in an RF SPS with an external antenna. Continuous wave (CW) operation of the SA SPS has been tested on the small test stand. The general design of the CW SA SPS is based on the pulsed version. Some modifications were made to improve the cooling and cesiation stability. The extracted collector current can be increased significantly by optimizing the longitudinal magnetic field in the discharge chamber. CW operation with negative ion extraction was tested with RF power up to 1.8 kW from the generator (∼1.2 kW in the plasma) with production up to Ic=7 mA. Long term operation was tested with 1.2 kW from the RF generator (∼0.8 kW in the plasma) with production of Ic=5 mA, Iex ∼15 mA (Uex=8 kV, Uc=14 kV)
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
A Novel Technique for Pulsed Operation of Magnetrons without Modulation of Cathode Voltage
Modern pulsed superconducting accelerators of megawatt beams require efficient RF sources controllable in phase and power. For each Superconducting RF (SRF) cavity is desirable a separate RF source with power up to hundreds of kW with pulse duration in the millisecond range. The efficiency of the traditional RF sources (klystrons, IOTs, solid-state amplifiers) is lower than that of the magnetrons, while the cost of a unit of RF power is much higher. Therefore the magnetron-based RF sources would significantly reduce the capital and operation costs in comparison with the traditional RF sources. A recently developed an innovative technique makes possible the pulsed generation of magnetrons powered below the self-excitation threshold voltage. This technique does not require pulse modulators to form RF pulses. The magnetron operation in this regime is stable, low noise, controllable in phase and power, and provides higher efficiency than other types of RF power sources. It allows operation in pulsed modes with large duty factor. The developed technique and its experimental verification are considered and discussed.
43 PARTICLE ACCELERATORS↗
A 12 GHz RF Power Source for the CLIC Study
The CLIC RF frequency has been changed in 2008 from the initial 30 GHz to the European X-band 11.9942 GHz permitting beam independent power production using klystrons for CLIC accelerating structure testing. A design and fabrication contract for five klystrons at that frequency has been signed by different parties with SLAC. France (IRFU, CEA Saclay) is contributing a solid state modulator purchased in industry and specific 12 GHz RF network components to the CLIC study. RF pulses over 120 MW peak at 230 ns length will be obtained by using a novel SLED-I type pulse compression scheme designed and fabricated by IAP, Nizhny Novgorod, Russia. The X-band power test stand is being installed in the CLIC Test Facility CTF3 for independent structure and component testing in a bunker, but allowing, in a later stage, for powering RF components in the CTF3 beam lines. The design of the facility, results from commissioning of the RF power source and the expected performance of the Test Facility are reported.
43 PARTICLE ACCELERATORS↗
Simulations and Experimental Plans for a High-Repetition-Rate Field-Enhanced Conduction-Cooled Superconducting RF Electron Source
We present a novel RF design for a field enhanced electron source driven by field emission cathodes. The proposed electron source relies on the enhanced high electric field gradients at the cathode to simultaneously extract and accelerate electrons. The system will be tested in a conduction-cooled superconducting radio-frequency cavity recently demonstrated at Fermilab. In this paper, we present electromagnetic and thermal simulations of the setup that support the feasibility of the design.
43 PARTICLE ACCELERATORS↗
Simulations for Field-Enhanced Conduction-Cooled Superconducting RF Electron Source
We present a novel RF design for a field enhanced electron source driven by field emission cathodes. The proposed electron source relies on the enhanced high electric field gradients at the cathode to simultaneously extract and accelerate electrons. The system will be tested in a conduction-cooled superconducting radiofrequency cavity recently demonstrated at Fermilab. In this work, we present electromagnetic and thermal simulations of the setup that support the feasibility of the design.
43 PARTICLE ACCELERATORS↗
Physics-electrical hybrid model for real time impedance matching and remote plasma characterization in RF plasma sources
Plasma characterization and impedance matching are an integral part of any radio frequency (RF) based plasma source. In long pulse operation, particularly in high power operation where plasma load may vary due to different reasons (e.g. pressure and power), online tuning of impedance matching circuit and remote plasma density estimation are very useful. In some cases, due to remote interfaces, radio activation and, due to maintenance issues, power probes are not allowed to be incorporated in the ion source design for plasma characterization. Therefore, for characterization and impedance matching, more remote schemes are envisaged. Two such schemes by the same authors are suggested in these regards, which are based on air core transformer model of inductive coupled plasma (ICP) [M. Bandyopadhyay et al., Nucl. Fusion 55, 033017 (2015); D. Sudhir et al., Rev. Sci. Instrum. 85, 013510 (2014)]. However, the influence of the RF field interaction with the plasma to determine its impedance, a physics code HELIC [D. Arnush, Phys. Plasmas 7, 3042 (2000)] is coupled with the transformer model. This model can be useful for both types of RF sources, i.e., ICP and helicon sources.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Note: A versatile radio-frequency source for cold atom experiments
A radio-frequency (RF) source designed for cold atom experiments is presented. The source uses AD9858, a direct digital synthesizer, to generate the sine wave directly, up to 400 MHz, with sub-Hz resolution. An amplitude control circuit consisting of wideband variable gain amplifier and high speed digital to analog converter is integrated into the source, capable of 70 dB off isolation and 4 ns on-off keying. A field programmable gate array is used to implement a versatile frequency and amplitude co-sweep logic. Owing to modular design, the RF sources have been used on many cold atom experiments to generate various complicated RF sequences, enriching the operation schemes of cold atoms, which cannot be done by standard RF source instruments.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Muons Inc Master CRADA with Annex A for Magnetron Development
Modern CW or pulsed superconducting accelerators of megawatts beams require efficient RF sources controllable in phase and power. It is desirable to have an individual RF power source with power up to hundreds of kW for each Superconductive RF (SRF) cavity. For pulsed accelerators the pulse duration in millisecond range is required. The efficiency of the traditional RF sources (klystrons, IOTs, solid-state amplifiers) in comparison to magnetrons is lower and the cost of unit of RF power is significantly higher. Typically, the cost of RF sources and their operation is a significant part of the total project cost and operation. The magnetron-based RF sources with a cost of power unit of 1-3 dollars per Watt would significantly reduce the capital and operation costs in comparison with the traditional RF sources. This arouses interest in magnetron RF sources for use in modern accelerators. A recently developed kinetic model describing the principle of magnetron operation and subsequent experiments resulted in an innovative technique producing the "stimulated" generation of magnetrons powered below the self-excitation threshold voltage. The magnetron operation in this regime is stable, low noise, controllable in phase and power, and provides higher efficiency than other types of RF power sources. It allows operation in CW and pulse modes (at large duty factor). For pulsed operation this technique does not require pulse modulators to form RF pulses. It also looks as a promising opportunity to extend magnetron life time.
43 PARTICLE ACCELERATORS↗
Stimulated Generation of Magnetrons powered below the Self-Excitation Threshold Voltage
Modern CW or pulsed superconducting accelerators of megawatts beams require efficient RF sources controllable in phase and power. It is desirable to have an individual RF power source with power up to hundreds of kW for each Superconductive RF (SRF) cavity. For pulsed accelerators the pulse duration in millisecond range is required. The efficiency of the traditional RF sources (klystrons, IOTs, solid-state amplifiers) in comparison to magnetrons is lower and the cost of unit of RF power is significantly higher. Typically, the cost of RF sources and their operation is a significant part of the total project cost and operation. The magnetron-based RF sources with a cost of power unit of 1-3 dollars per Watt would significantly reduce the capital and operation costs in comparison with the traditional RF sources. This arouses interest in magnetron RF sources for use in modern accelerators. A recently developed kinetic model describing the principle of magnetron operation and subsequent experiments resulted in an innovative technique producing the "stimulated" generation of magnetrons powered below the self-excitation threshold voltage. The magnetron operation in this regime is stable, low noise, controllable in phase and power, and provides higher efficiency than other types of RF power sources. It allows operation in CW and pulse modes (at large duty factor). For pulsed operation this technique does not require pulse modulators to form RF pulses. It also looks as a promising opportunity to extend magnetron life time. The developed technique, its experimental verification and a brief explanation of the kinetic model substantiating the technique are presented and discussed in this article.
43 PARTICLE ACCELERATORS↗
Improved Magnetron Stability and Reduced Noise in Efficient Transmitters for Superconducting Accelerators
State of the art high-current superconducting accelerators require efficient RF sources with a fast dynamic phase and power control. This allows for compensation of the phase and amplitude deviations of the accelerating voltage in the Superconducting RF (SRF) cavities caused by microphonics, etc. Efficient magnetron transmitters with fast phase and power control are attractive RF sources for this application. They are more cost effective than traditional RF sources such as klystrons, IOTs and solid-state amplifiers used with large scale accelerator projects. However, unlike traditional RF sources, controlled magnetrons operate as forced oscillators. Study of the impact of the controlling signal on magnetron stability, noise and efficiency is therefore important. This paper discusses experiments with 2.45 GHz, 1 kW tubes and verifies our analytical model which is based on the charge drift approximation.
Kazakevich, G.↗
High-Power Magnetron Transmitter for the Electron Collider Ring of the MEIC Facility
Operation of the 3-12 GeV electron collider 8-shape ring of the MEIC facility causes a Synchrotron Radiation (SR) of electrons in arcs with energy loss of ~20 kW/m at beam current of ~3 A. The total SR loss up to 2 MW per a revolution is presumed to compensate by Superconducting RF (SRF) accelerating cavities. To minimize the beam emittance, each individual SRF cavity is proposed to feed by an individual and independent RF source allowing a wide-band control in phase and power. Most efficient and less expensive in capital and maintenance costs the high-power transmitters based on magnetrons, injection-locked by phase-modulated signals, controlled in wide-band are proposed as the RF sources. The magnetron RF sources utilizing 2-cascade magnetrons allowing a wide-band phase and power control by the injection-locking phase-modulated signals were experimentally modelled by 2.45 GHz, CW, 1 kW magnetrons. Results of the modelling and adequacy of the transmitters for the SRF cavities are discussed in the presented article.
Kazakevich, Grigory↗
Study of synchronous RF pulsing in dual frequency capacitively coupled plasma
Low-pressure multi-frequency capacitively coupled plasmas (CCPs) are used for numerous etch and deposition applications in the semiconductor industry. Pulsing of the radio-frequency (RF) sources enables control of neutral and charged species in the plasma on a millisecond timescale. The synchronous (i.e. simultaneous, in-phase) pulsing of both power sources in a dual frequency CCP is examined in this article. Due to the low gas pressure, modeling has been done using the electrostatic particle-in-cell/Monte Carlo collision method. Here, the objective of this work is to investigate the sensitivity of the plasma properties to small changes in timing during synchronous pulsing of the two RF sources. It is demonstrated that small deviations in the on and off times of the two RF sources can lead to major changes in the plasma characteristics. This high sensitivity is of concern for process repeatability but can be utilized to enable better control of the dynamics of plasma-surface interaction. In the simulations, the pulsing parameters (on and off times and ramp rates) are varied and the temporal evolution of plasma characteristics such as electron density (n e ), species current at the electrode, and electron temperature are examined. It is demonstrated that if the low-frequency (LF) source is turned off a few μs before (or after) the high-frequency source, n e during the off-state is significantly higher (or lower) due to the frequency coupling effect. Similarly, turning on the LF source with a small delay results in a sharp increase in the plasma density when the RF sources are turned on.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗