A high-current, compact, conduction cooled superconducting radio-frequency cryomodule for particle accelerators. The cryomodule will accelerate an electron beam of average current up to 1 ampere in continuous wave (CW) mode or at high duty factor. The cryomodule consists of a single-cell superconducting radio-frequency cavity made of high-purity niobium, with an inner coating of Nb3Sn and an outer coating of pure copper. Conduction cooling is achieved by using multiple closed-cycle refrigerators. Power is fed into the cavity by two coaxial couplers. Damping of the high-order modes is achieved by a warm beam-pipe ferrite damper.

FIELD OF THE INVENTION

The present invention relates to superconducting radio-frequency (SRF) cryomodules used in particle accelerators, and in particular to a compact, conduction-cooled SRF cryomodule suitable to accelerate a high-current beam.

BACKGROUND OF THE INVENTION

Superconducting Radio-Frequency (SRF) accelerators are important tools for scientific research due to the small RF losses and the higher continuous-wave (CW) accelerating fields than normal conducting cavities. These devices are predominantly used in nuclear and high-energy physics research, as well as light sources for experiments in material and biological sciences. In conventional SRF accelerators, the superconducting state is achieved by cooling niobium SRF cavities, the accelerating structures inside the cryomodule, to below the transition temperature of 9.2K, typically to 4.3 K or lower, by means of immersing them in a liquid helium (He) bath.

Cryogenic plants required to supply the liquid helium to SRF cryomodules are complex, of substantial size, constitute a major fraction of the capital and operating cost of SRF accelerators, and are one of the main obstacles towards a more widespread use of SRF technology. Although SRF technology is applicable to many industrial applications, such as environmental remediation, the high cost of producing and operating the cryogenic plant substantially limits the application of SRF technology.

Accordingly, what is needed is a compact, low-cost SRF accelerator for cost-effective use in industrial applications such as environmental remediation, which includes the treatment of waste-water and flue-gases. An SRF electron accelerator required for those applications should be capable of operating at high-current (˜1 ampere) and low energy (1-10 MeV).

OBJECT OF THE INVENTION

An object of this invention is to provide a compact, conduction cooled, high-current SRF cryomodule for use in particle accelerators for industrial applications.

A further object is to provide an SRF cryomodule that greatly reduces the capital cost, operating cost, and operational complexity of a cryomodule for use in a particle accelerator.

A further object is to provide an SRF cryomodule that eliminates the need for a helium liquefier, a pressure vessel, and a cold tuner.

Another object is to significantly lower investment and operating costs of an SRF accelerator.

A further object is to provide an SRF cryomodule that is free of liquid cryogen hazards.

Another object of the invention is to provide an SRF cryomodule in which the conventional cryogenic plant is replaced by a closed-cycle refrigerator at much lower cost.

A still further object of the invention is to provide a compact, conduction-cooled SRF cryomodule capable of accelerating a high-current beam operating at a current of 1 ampere or greater and at an energy of 1-10 MeV.

A still further object of the invention is to provide a high current SRF cryomodule that can be used for cleaning flue gases, such as converting nitrous oxides in the flue gases, or for treating wastewater streams, such as hospital or municipal waste streams, to remove biological materials, or to modify the sludge in waste treatment plants.

These and other objects and advantages of the present invention will be better understood by reading the following description along with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

The present invention is a compact, conduction-cooled, high-current SRF cryomodule for particle accelerators. The cryomodule includes a multi-layer SRF cavity, dual coaxial input couplers, high-order modes (HOM) dampers, thermal shield, magnetic shields, support structure, a vacuum vessel and multiple cryocoolers. In such a cryomodule, the cryogenic plant is replaced by commercial Gifford-McMahon (GM) closed-cycle refrigerators at much lower cost. The SRF cryomodule will allow the development of low-cost SRF accelerators for industrial applications, particularly for environmental remediation.

DETAILED DESCRIPTION OF THE INVENTION

With reference toFIG. 2, the invention is a compact, conduction cooled SRF cryomodule10for accelerating a high current beam. The meaning of “high current beam” as used herein refers to a beam that includes a current of up to or greater than 1 ampere. The meaning of “compact” as used herein refers to a conduction cooled SRF cryomodule that has an overall size of 1.5 m by 1.5 m or less. The conduction cooled SRF cryomodule10includes an SRF cavity12located inside a vacuum vessel14.FIG. 2depicts a single-cell cavity although other arrangements such as multiple-cell cavities are within the scope of the invention.

The SRF cavity12is preferably of elliptical shape and geometric β tailored to the energy of the incoming beam. The SRF cavity12is preferably fabricated from high-purity niobium (Nb) having a residual resistivity ratio of greater than 300 and includes a thickness of 3-5 millimeters. Alternatively, metals with thermal conductivity greater than 500 W/(m K) at 4 K, such as tungsten or copper, could also be used.

As shown inFIG. 3, the cavity inner surface16is coated with a thin (1-1.5 μm thick) superconducting inner layer18preferably formed by thermal diffusion of Sn vapor in a vacuum furnace at 1000-1200° C. The inner layer18is preferably constructed of Nb3Sn, Nb3Ge, NbN, or NbTiN, and is most preferably constructed of Nb3Sn. The thin film coating is a superconductor having a critical temperature greater than 15 K. The use of Nb3Sn as the inner layer18of the cavity results in an SRF cavity with substantially lower RF losses as compared to an uncoated cavity constructed of bulk Nb at 4.3 K.

The SRF cavity12outer surface20is coated with a layer22preferably of copper or tungsten, and most preferably of pure copper having a purity of greater than 99.98%. The method of applying the outer layer22is preferably by electroplating, vacuum plasma spraying, or by a combination of vacuum plasma-spraying and electroplating. The outer coating is not required if the cavity is fabricated from a metal other than Nb.

Referring toFIG. 1, two symmetrically located coaxial power couplers24are used to feed RF power into the SRF cavity12. Each power coupler24is capable of sustaining a minimum of 500 kW of RF power into the SRF cavity12. As shown inFIG. 5, a section of the inner surface of the outer conductor of the power coupler is preferably coated with a thin layer25(1-1.5 μm thick) of a high-temperature superconductor to minimize the static and dynamic heat load from the coupler. Preferably, the thin layer25of high-temperature superconductor material is YBCO (yttrium barium copper oxide) having a critical temperature greater than 90 K. The high-temperature superconductor is preferably applied to the inner surface of the outer conductor by methods including physical-chemical vapor deposition, pulsed laser deposition, or a combination of physical-chemical vapor deposition and pulsed laser deposition.

With reference toFIG. 2, cooling of the SRF cavity to below 15 K, preferably to less than or equal to 4.3 K, is provided by one or more cryocoolers26. The cryocoolers26each include a first stage cold head28and a second stage cold head30. The second stage cold head30of each cryocooler is connected to the SRF cavity12by means of a mechanical contact joint32with a malleable indium interlayer34and a high thermal conductivity strain relief section36. The outer copper layer20(seeFIG. 3) of the SRF cavity12will provide a high thermal conduction path from the SRF cavity surfaces to the cryocooler second stage cold heads30. The first stage cold head28of the cryocooler is preferably at a temperature of 50-80 K and the second stage cold head30of the cryocooler is preferably at a temperature of 4.3-9 K A preferred cryocooler such as described herein is the Gifford-McMahon (GM) type cryocooler, available from Sumitomo (SHI) Cryogenics of America, in Allentown, Pa. Most preferably, the cryocooler26would have a second stage capacity greater than or equal to 1.5 watts W at 4.2 K. A preferred strain relief section is preferably constructed of copper or tungsten and most preferably consists of copper thermal straps such as those available from Technology Applications, Inc., in Boulder, Colo.

With reference toFIG. 2, the conduction cooled SRF cryomodule10preferably includes a thermal shield38with a structure core40, wherein said structure core is connected to the cryocooler first stage cold heads28by means of a mechanical contact joint with a malleable indium interlayer. High thermal conductivity strain relief sections are located along the shield structure core40. Thermal shield38, preferably constructed of oxygen-free electronic copper, takes infrared heat away from the SRF cavity. Multi-layer insulation blankets are wrapped around the thermal shield to further reduce radiative heat transfer.

Magnetic fields are preferably minimized in the SRF cavity12through the use of an inner magnetic shield42and an outer magnetic shield44. With reference toFIG. 2, the magnetic shields are preferably constructed of a material with the ability to support the absorption of a magnetic field within itself. The magnetic shields are constructed of a shielding alloy that will attract magnetic flux lines of the interfering fields to itself and divert the unwanted field away from sensitive areas or components. The magnetic shields are preferably constructed of a high permeability metal having high magnetic shielding properties. The magnetic shields are most preferably constructed of MuMETAL®, a metal alloy available from Magnetic Shield Corporation of Bensenville, Ill., CRYOPERM® 10 or Amumetal 4K, both available from Amuneal Manufacturing Corp., in Philadelphia, Pa. Most preferably, multi-layer insulation blankets are wrapped around the inner magnetic shield.

With reference toFIG. 2, the conduction cooled SRF cryomodule10according to the present invention preferably includes an entrance beam tube46and an exit beam tube48connected to the SRF cavity12. Most preferably, damping of the high-order modes of the accelerated particles is achieved by enlarging the exit beam tube48of the SRF cavity. As shown inFIG. 2, the diameter of the exit beam tube48is larger than the diameter of the entrance beam tube46. Preferably, the SRF cryomodule includes a water-cooled beam pipe higher-order mode ferrite damper50for damping of higher-order modes and allowing their propagation to a room-temperature. A conduction cooled SRF cryomodule10with 1 MW RF power fed into the SRF cavity by the power couplers24is capable of generating a 1 ampere beam (high current SRF beam) at 1 MW RF power.

The volume within the cavity is isolated from the outside environment by means of two vacuum valves52outside the vacuum vessel, which are preferably all-metal gate valves. A vacuum valve52is included on the entrance46and on the exit beam tube48.