Cryogenic superconducting niobium resonators are key to accelerators for free-electron lasers and experiments in particle and nuclear physics. A resonator's performance is often degraded by a magnetic field that is trapped in it during a quench event. Without a clear understanding of this field's origin, the problem has been addressed by thermal cycling above the cavity's critical temperature. Now the authors pinpoint the mechanism responsible for the trapped flux, and show how to fully recover the quality factor while staying $b\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}w$ the critical temperature, which is of great practical interest.

https://link.aps.org/pdf/10.1103/PhysRevApplied.5.044019

Quench-Induced Degradation of the Quality Factor in Superconducting Resonators

Project-X, a multi-MW proton source, is now under development at Fermilab. In this paper we present study of high order modes (HOM) excited in continues-wave (CW) superconducting linac of Project-X. We investigate effects of cryogenic losses caused by HOMs and influence of HOMs on beam dynamics. We find that these effects are small. We conclude that HOM couplers/dampers are not needed in the Project-X SC RF cavities.

High order modes in Project-X linac

The European Spallation Source (ESS) is a multi-disciplinary research centre under design and construction in Lund, Sweden. This new facility is funded by a collaboration of 17 European countries and is expected to be up to 30 times brighter than today’s leading facilities and neutron sources. The ESS will enable new opportunities for researchers in the fields of life sciences, energy, environmental technology, cultural heritage and fundamental physics. A 5 MW long pulse proton accelerator is used to reach this goal. The pulsed length is 2.86 ms, the repetition frequency is 14 Hz (4 % duty cycle), and the beam current is 62.5 mA. It is composed of one string of spoke cavity cryomodule and two strings of elliptical cavity cryomodules. This paper introduces the thermo-mechanical design and expected operation of the ESS spoke cavity cryomodules. These cryomodules contain two double spoke bulk Niobium cavities operating at 2 K and at a frequency of 352.21 MHz. The superconducting section of the Spoke Linac accelerates the beam from 90 MeV to 220 MeV. A Spoke Cavity Cryomodule Technology Demonstrator will be built and tested in order to validate the ESS series production.

/pdf/the-ess-spoke-cavity-cryomodules-1p55z7ngk2.pdf

The ESS spoke cavity cryomodules

The Lorentz force can dynamically detune pulsed Superconducting RF cavities. Considerable additional RF power can be required to maintain the accelerating gradient if no effort is made to compensate for this detuning. Compensation systems using piezo actuators have been used successfully at DESY and elsewhere to control Lorentz Force Detuning (LFD). Recently, Fermilab has developed an adaptive compensation system for cavities in the Horizontal Test Stand, in the SRF Accelerator Test Facility, and for the proposed Project X.

/pdf/resonance-control-in-srf-cavities-at-fnal-8nbjnm23u9.pdf

Resonance control in SRF cavities at FNAL

The Lorentz force can dynamically detune pulsed Superconducting RF cavities. Considerable additional RF power can be required to maintain the accelerating gradient if no effort is made to compensate for this detuning. An adaptive feed-forward Lorentz Force Detuning (LFD) compensation algorithm developed at Fermilab is described. Systems based on this approach have been used to successfully reduce LFD from several hundred Hz to several 10s of Hz or better.

/pdf/adaptive-compensation-of-lorentz-force-detuning-in-4sbj0x2p05.pdf

Adaptive compensation of Lorentz force detuning in superconducting RF cavities

In the recently commissioned superconducting RF cavity test facility at Fermilab (SCTF), a 325 MHz, {\beta}=0.22 superconducting single-spoke resonator (SSR1) has been tested for the first time with its input power coupler. Previously, this cavity had been tested CW with a low power, high Qext test coupler; first as a bare cavity in the Fermilab Vertical Test Stand and then fully dressed in the SCTF. For the tests described here, the design input coupler with Qext ~ 106 was used. Pulsed power was provided by a Toshiba E3740A 2.5 MW klystron.

First high power pulsed tests of a dressed 325 MHz superconducting single spoke resonator at Fermilab

In the recently commissioned superconducting RF cavity test facility at Fermilab (SCTF), a 325 MHz, {beta} = 0.22 superconducting single-spoke resonator (SSR1) has been tested for the first time with its input power coupler. Previously, this cavity had been tested CW with a low power, high Q{sub ext} test coupler; first as a bare cavity in the Fermilab Vertical Test Stand and then fully dressed in the SCTF. For the tests described here, the design input coupler with Q{sub ext} {approx} 10{sup 6} was used. Pulsed power was provided by a Toshiba E3740A 2.5 MW klystron.

/pdf/first-high-power-pulsed-tests-of-a-dressed-325-mhz-379wc6q8hp.pdf

Performance degradation of superconducting RF (SRF) cavities induced by magnetic field trapped in its walls is fairly well understood phenomenon; nevertheless, criteria for setting requirements for acceptable level of magnetic field generated inside cryomodules after cooling down have not been well established. Superconducting walls of cavities in cryomodules protect the cavities from this component of the magnetic field unless thermal breakdown (quench) happens; magnetic flux penetrates inside the cavity through normally conducting opening generated during the quench and becomes trapped in the walls after it cools down again. The amount of the trapped magnetic flux depends on the size of normally conducting zone developed in walls of the cavity during quenching; it can be evaluated by modelling quench propagation (QP). In this study, we compared performance degradation of several cavities of different shapes and frequencies predicted by the QP modelling with the data obtained by direct RF measurements; the magnetic field was generated by superconducting coils mounted in the vicinity of quenching cavities. A criterion is suggested for establishing a requirement for the maximum magnetic field generated inside cryomodules; it can be used for any RF structure and magnetic system.

/pdf/performance-degradation-of-a-superconducting-cavity-3q72hzdm2j.pdf

Performance degradation of a superconducting cavity quenching in magnetic field

Concept of the 650 MHz cavities for the Project X is presented. Choice of the basic parameters, i.e., number of cells, geometrical {beta}, apertures, coupling coefficients, etc., is discussed. The cavity optimization criteria are formulated. Results of the RF design are presented for the cavities of both the low-energy and high-energy sections.

/pdf/concept-em-design-of-the-650-mhz-cavities-for-the-project-x-5bkx333am0.pdf

Concept em design of the 650 mhz cavities for the project-x*

A coaxial blade tuner has been selected for the 1.3GHz SRF cavities of the Fermilab SRF Accelerator Test Facility. Results from tuner cold tests in the Fermilab Horizontal Test Stand are presented. Fermilab is constructing the SRF Accelerator Test Facility, a facility for accelerator physics research and development. This facility will contain a total of six cryomodules, each containing eight 1.3 GHz nine-cell elliptical cavities. Each cavity will be equipped with a Slim Blade Tuner designed by INFN Milan. The blade tuner incorporates both a stepper motor and piezo actuators to allow for both slow and fast cavity tuning. The stepper motor allows the cavity frequency to be statically tuned over a range of 500 kHz with an accuracy of several Hz. The piezos provide up to 2 kHz of dynamic tuning for compensation of Lorentz force detuning and variations in the He bath pressure. The first eight blade tuners were built at INFN Milan, but the remainder are being manufactured commercially following the INFN design. To date, more than 40 of the commercial tuners have been delivered.

/pdf/test-of-a-coaxial-blade-tuner-at-hts-fnal-3dl8xjoxam.pdf

T. Khabiboulline

Papers

First high power pulsed tests of a dressed 325 MHz superconducting single spoke resonator at Fermilab

First high power pulsed tests of a dressed 325 MHz superconducting single spoke resonator at Fermilab

Performance degradation of a superconducting cavity quenching in magnetic field

Concept em design of the 650 mhz cavities for the project-x*

Test of a coaxial blade tuner at HTS FNAL