Superconducting RF Accelerator Research Articles

Utilization of the CW magnetrons as coherent RF sources for superconducting RF accelerators

CW magnetrons designed and optimized for industrial heaters, driven by an injection-locking signal, were suggested to power Superconducting RF (SRF) cavities. However, the CW magnetrons, which are regenerative generators, are intended to operate in self-excitation mode, so, for reliable starting they use a large regenerative gain, causing a large regenerative instability in operation. Thus, power supply ripple, even shot noise, etc., lead to multiple sidebands forming a quasi-continuous noise spectrum at the carrier frequency. Therefore, in the traditionally used mode of operation and control, such magnetrons are inapplicable for powering high Q-factor SRF cavities. A new approach to operation and control of CW magnetrons considering non-stationary processes during start-up and operation makes it possible to find a mode of almost coherent RF generation of tubes with a significant reduction in regenerative instability and noise and increased efficiency. In our work is presented substantiation for the newly developed mode with an analysis of the new approach based on experimental results.

Method for in situ and in operando cavity loaded Q extraction in superconducting rf accelerators

A novel method has been developed that allows extracting the loaded quality factor of particle accelerating radio-frequency cavities at any time during beam operation from acquired control system process parameters without the need of additional measurement equipment. Concurrently it assesses the attenuation in a cavities' power transmission line for any rf generator to rf cavity mismatch condition, and thus can correct calibration inaccuracies of measured forward and reflected power levels. The mathematical formalism is described. It is specifically derived for cryogenically cooled superconducting cavities for which the loaded quality factor can be equated with the external quality factor of the input power coupler. The described method has been applied for 418 superconducting cavities in operation at the Continuous Electron Beam Accelerator Facility at the Thomas Jefferson National Accelerator Facility. The validation of the method as well as its beneficial use as a diagnostic tool is demonstrated for a finite number of examples utilizing process variables archived during a period in the past when the linac beam current was ramped up multiple times. The method is in the process of being implemented in the machines' control system for an in situ and in operando evaluation of the power coupler's external quality factor.

Radiation Generation with an Existing Demonstrator of an Energy-Recovery Continuous-Wave Superconducting RF Accelerator

Over the past decades, many accelerator laboratories have put much effort into the development of compact energy-recovery linac (ERL) demonstrators to verify various physical and technical aspects of the generation, acceleration, transport and energy recovery of high brightness and high average current electron beams in a superconducting radio-frequency (SRF) linear accelerator. Beyond these goals, the ERL demonstrator also offers unique opportunities to study novel schemes for THz and X-ray radiation generation. In this paper, we discuss feasible options for schemes generating THz and X-ray radiation at low-energy continuous-wave (CW) SRF ERL demonstrators such as the bERLinPro accelerator.

Record high-gradient SRF beam acceleration at Fermilab

Many modern and future particle accelerators employ high gradient superconducting RF (SRF) to generate beams of high energy, high intensity and high brightness for research in high energy and nuclear physics, basic energy sciences, etc. In this paper we report the record performance large-scale SRF system with average beam accelerating gradient matching the International Linear Collider (ILC) specification of 31.5 MV m−1. Design of the eight cavity 1.3 GHz SRF cryomodule, its performance without the beam and results of the system commissioning with high intensity electron beam at Fermilab Accelerator Science and Technology (FAST) facility are presented. We also briefly discuss opportunities for further beam studies and tests at FAST including those on even higher gradient and more efficient SRF acceleration, as well as exploration of the system performance with full ILC-type beam specifications.

Resonant interaction of the electron beam with a synchronous wave in controlled magnetrons for high-current superconducting accelerators

A simplified analytical model of the resonant interaction of the beam of Larmor electrons drifting in the crossed constant fields of a magnetron with a synchronous wave providing a phase grouping of the drifting charge was developed to optimize the parameters of an rf resonant injected signal driving the magnetrons for management of phase and power of rf sources with a rate required for superconducting high-current accelerators. The model, which considers the impact of the rf resonant signal injected into the magnetron on the operation of the injection-locked tube, substantiates the recently developed method of fast power control of magnetrons in the range up to 10 dB at the highest generation efficiency, with low noise, precise stability of the carrier frequency, and the possibility of wideband phase control. Experiments with continuous wave 2.45 GHz, 1 kW microwave oven magnetrons have verified the correspondence of the behavior of these tubes to the analytical model. A proof of the principle of the novel method of power control in magnetrons, based on the developed model, was demonstrated in the experiments. The method is attractive for high-current superconducting rf accelerators. This paper also discusses vector methods of power control with the rates required for superconducting accelerators, the impact of the rf resonant signal injected into the magnetron on the rate of phase control of the injection-locked tubes, and a conceptual scheme of the magnetron transmitter with highest efficiency for high-current accelerators.

SUPERCONDUCTING RF ACCELERATOR CAVITY TECHNIQUES AS A BASIS FOR FUTURE ACCELERATORS --- Learn from the development and beam operation for Compact ERL ---

SUPERCONDUCTING RF ACCELERATOR CAVITY TECHNIQUES AS A BASIS FOR FUTURE ACCELERATORS --- Learn from the development and beam operation for Compact ERL ---

Studies on the Mass-Production Fabrication of ILC Superconducting RF Accelerator Cavity

Studies on the Mass-Production Fabrication of ILC Superconducting RF Accelerator Cavity

Erratum: "Industrialization of Superconducting RF Accelerator Technology"

Reviews of Accelerator Science and TechnologyVol. 06, pp. 311 (2013) Free AccessErratum: "Industrialization of Superconducting RF Accelerator Technology"is erratum ofIndustrialization of Superconducting RF Accelerator TechnologyMichael Peiniger, Michael Pekeler and Hanspeter VogelMichael PeinigerRI Research Instruments GmbH, Friedrich-Ebert-Straße 1, 51429 Bergisch Gladbach, Germany, Michael PekelerRI Research Instruments GmbH, Friedrich-Ebert-Straße 1, 51429 Bergisch Gladbach, Germany and Hanspeter VogelRI Research Instruments GmbH, Friedrich-Ebert-Straße 1, 51429 Bergisch Gladbach, Germanyhttps://doi.org/10.1142/S179362681392001XCited by:0 Previous AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsRecommend to Library ShareShare onFacebookTwitterLinked InRedditEmail [Reviews of Accelerator Science and Technology, Vol. 5 (2012)]. FiguresReferencesRelatedDetailsRelated articlesIndustrialization of Superconducting RF Accelerator Technology27 Jan 2013Reviews of Accelerator Science and Technology Recommended Vol. 06 Metrics History PDF download

Industrialization of Superconducting RF Accelerator Technology

Superconducting RF (SRF) accelerator technology has basically existed for 50 years. It took about 20 years to conduct basic R&D and prototyping at universities and international institutes before the first superconducting accelerators were built, with industry supplying complete accelerator cavities. In parallel, the design of large scale accelerators using SRF was done worldwide. In order to build those accelerators, industry has been involved for 30 years in building the required cavities and/or accelerator modules in time and budget. To enable industry to supply these high tech components, technology transfer was made from the laboratories in the following three regions: the Americas, Asia and Europe. As will be shown, the manufacture of the SRF cavities is normally accomplished in industry whereas the cavity testing and module assembly are not performed in industry in most cases, yet. The story of industrialization is so far a story of customized projects. Therefore a real SRF accelerator product is not yet available in this market. License agreements and technology transfer between leading SRF laboratories and industry is a powerful tool for enabling industry to manufacture SRF components or turnkey superconducting accelerator modules for other laboratories and users with few or no capabilities in SRF technology. Despite all this, the SRF accelerator market today is still a small market. The manufacture and preparation of the components require a range of specialized knowledge, as well as complex and expensive manufacturing installations like for high precision machining, electron beam welding, chemical surface preparation and class ISO4 clean room assembly. Today, the involved industry in the US and Europe comprises medium-sized companies. In Japan, some big enterprises are involved. So far, roughly 2500 SRF cavities have been built by or ordered from industry worldwide. Another substantial step might come from the International Linear Collider (ILC) project currently being designed by the international collaboration GDE ('global design effort'). If the ILC will be built, about 18,000 SRF cavities need to be manufactured worldwide within about five years. The industrialization of SRF accelerator technology is analyzed and reviewed in this article in view of the main accelerator projects of the last two to three decades.

Surface characterization of Nb samples electropolished with real superconducting rf accelerator cavities

We report the results of surface characterizations of niobium (Nb) samples electropolished together with a single cell superconducting radio-frequency accelerator cavity. These witness samples were located in three regions of the cavity, namely at the equator, the iris and the beam-pipe. Auger electron spectroscopy (AES) was utilized to probe the chemical composition of the topmost four atomic layers. Scanning electron microscopy with energy dispersive X-ray for elemental analysis (SEM/EDX) was used to observe the surface topography and chemical composition at the micrometer scale. A few atomic layers of sulfur (S) were found covering the samples non-uniformly. Niobium oxide granules with a sharp geometry were observed on every sample. Some Nb-O granules appeared to also contain sulfur.

A new measurement tool for characterization of superconducting rf accelerator cavitiesusing high-performance LTS SQUIDs

This paper presents a new system to measure very low currents in an acceleratorenvironment, using a cryogenic current comparator (CCC). In principle a CCC is aconventional current transformer using the high-performance SQUID technology to sensethe magnetic fields caused by the beam current. Since the system is sensitive on a pA level,it is an optimum device to detect dark currents of superconducting cavities. The systempresented here is designed for the test facilities of the superconducting accelerator modulesfor the European XFEL at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg.Measurements in a quiet environment showed that an intrinsic noise level of the CCC of40 pA Hz−1/2 could be achieved.

International Linear Collider damping ring lattice design with two wiggler sections

The International Linear Collider (ILC), which is based on super-conducting RF acceleration technology, requires the damping rings to provide beams with extremely small equilibrium emittance, and large acceptance to ensure good injection efficiency for high-emittance, and high-energy spread beam from the positron source. In order to reduce the cost for ILC damping rings, an alternative lattice which is different from the baseline configuration design has been designed with modified FODO arc cells, and the total quadrupole number has been reduced by half. At the same time, to decrease the total cost involved in constructing access shafts needed to supply power, cryogenics etc. for the wigglers and other systems, the number of wiggler sections is decreased from 8 to 4,and further to 2.This new lattice has been optimized to have a good dynamic aperture. This alternative ILC damping ring lattice design will reduce the cost largely as compared with the baseline design.

Designing Focusing Solenoids for Superconducting RF Accelerators

The design of a focusing solenoid for use in a superconducting RF linac requires resolving a range of problems with conflicting requirements. Providing the required focusing strength contradicts the goal of minimizing the stray field on the surfaces of adjacent superconducting RF cavities. The requirement of a compact solenoid, able to fit into a gap between cavities, contradicts the need of mechanical support necessary to restrain electromagnetic forces that can result in coil motion and subsequent quenching. In this report we will attempt to address these and other issues arising during the development of focusing solenoids. Some relevant test data will also be presented.

Ponderomotive instabilities and microphonics—a tutorial

Phase and amplitude stabilization of the fields in superconducting cavities in the presence of ponderomotive effects and microphonics was one of the major challenges that had to be surmounted in order to make superconducting rf accelerators practical. This was of particular concern in low-velocity proton and ion accelerators since the beam loading was often negligible, but was usually not relevant in electron accelerators since the beam loading was often high and the gradients low. More recent or future applications of electron linacs—for example JLab upgrade, energy recovering linacs (ERLs)—will operate at increasingly higher gradients with little beam loading, and the issues associated with microphonics and ponderomotive instabilities will again become relevant areas of research. This paper will describe the ponderomotive instabilities and the conditions under which they can occur, and review the methods by which they, and microphonics, can be overcome.

DC photoemission electron guns as ERL sources

Very-high-voltage DC electron guns, delivering moderate duration bunches from photoemission cathodes, and followed by conventional drift bunching and acceleration, offer a practical solution for an ERL injector. In a variant of this scheme, a DC gun is placed in close proximity to a superconducting RF accelerator cavity, with few or no active elements between the gun and cavity. The principal technical challenge with such electron guns arises from field emission from the cathode electrode and its support structure. Field emission may result in voltage breakdown across the cathode–anode gap, or a punch-through failure of the insulator holding off the cathode potential, as well as lesser though still serious problems. Various means to mitigate these problems are described. The operational lifetime of high quantum efficiency photocathodes in these guns is determined by the vacuum conditions, through phenomena such as chemical poisoning and ion back-bombardment. Minimization of the field strength on electrode structures pushes high-voltage DC guns toward large dimensions and, correspondingly, large outgassing loads, but it is also true that these guns offer many opportunities for achieving excellent vacuum conditions. Good solutions to vacuum problems that had previously limited cathode lifetime have been demonstrated in recent years. Designs for DC guns presently in use and planned for the near future will be described. The parameters necessary for a 100 mA average current, very-high-voltage DC gun with a photocathode operational lifetime greater than 100 h appear to be within reach, but have yet to be demonstrated. A 1 A average current source with good cathode operational lifetime will require developments beyond the present state-of-the-art.

Parameter study of the VUV FEL at the TESLA Test Facility

Abstract We present a short description of the theoretical and numerical activity for the design of the VUV FEL proposed at DESY. The FEL will be a 6 nm SASE device driven by a 1 GeV beam from the superconducting rf accelerator which is under development by the international TESLA collaboration.

Oxygen desorption during niobium sputtering for superconducting RF accelerators

Superconducting niobium RF resonators are an integral part of the various moderm accelerators — from low energy LINACs for heavy ions to electron-positron colliders with energy up to hundreds of GeV. Traditionally, the resonators have been made from high purity sheet niobium, but recently this design started to give way to copper substructures onto which a thin niobium film is deposited. Resonators with sputtered niobium are less expensive and higher acceleration fields can be achieved because the high thermoconductivity of copper eliminates thermal quenches. Unfortunately, that advantage is counterbalanced by degradation of the quality factor, Q, with increasing accelerating field. It is generally believed that Q degradation is caused by non-metallic impurities embedded into the niobium film during discharge sputtering. However, neither the amount and composition nor sources of the impurities are known with any certainty. This paper examines the hypothesis that the mechanism of film contamination is the decomposition of the surface oxides on the copper substrate and the niobium cathode when they are exposed to the sputtering discharge. Quantitative connections between the impurity concentrations, the pressure of active gases during sputtering and the residual resistivity ratio of niobium are established and applied to the experimental data published by CERN. Calculations show that, with ignition of the sputtering discharge, the residual pressure of oxygen-containing gases increases by two orders of magnitude. To reduce the contamination of niobium films during sputtering, outgassing of the sputtering system in helium discharge is proposed.

Oxygen desorption during niobium sputtering for superconducting RF accelerators

Oxygen desorption during niobium sputtering for superconducting RF accelerators

A path towards a compact, CW FEL using superconducting rf accelerator technology

Recently, several compact FELs have been built based on pulsed linacs and RF photocathode guns. There is now an exciting opportunity to extend the capability of the technique to CW operation by incorporating the recent development in superconducting electron accelerators. The higher average power possible with CW operation will enhance the utility and the versatility of the compact FELs for fundamental research as well as for industrial application. Our vision of an eventual compact high power FEL is one that is targeted to have a foot print of 1 × 6m2. It would consist of a CW RF superconducting photocathode gun closely coupled to a superconducting accelerator cavity, all in a single cryostat, producing a 6–15 MeV beam. The helium refrigerator will be built into the compact dewar as is currently being done for the JAERI FEL in Japan. The FEL output will consist of 10–100 μJ pulses at 10–70 MHz repetition rate with average power greater than 1 kW. The FEL performance will be optimized in the wavelength range between 10 and 50 μm, which is a difficult region for conventional laser techniques. Longer wavelengths can be generated via Cherenkov FEL, and X-rays can be produced via Thomson scattering.

Status of cryogenics for the LEP200 energy upgrade project at CERN

The cryogenics for the LEP200 project covers the liquid helium bath cooling requirements of about 200 superconducting RF acceleration cavities to be installed in the ring tunnel of CERN's electron-positron collider LEP. It includes two cryoplants of 6 kW refrigeration capacity, four of 12 kW, about 2.6 km of transfer lines with actively cooled, flexible screens, and a helium inventory of about 80'000 Nm 3. At present all equipment is ordered; about one quarter is installed and operational. Special design features needed to cope with space problems in the existing pit/tunnel system are presented and first operational experience from running the two 6 kW plants with 550 m of transfer lines and a number of cavities is reported.