RF cavity using liquid dielectric for tuning and cooling

A system for accelerating particles includes an RF cavity that contains a ferrite core and a liquid dielectric. Characteristics of the ferrite core and the liquid dielectric, among other factors, determine the resonant frequency of the RF cavity. The liquid dielectric is circulated to cool the ferrite core during the operation of the system.

TECHNICAL FIELD

This document relates generally to particle accelerators and particularly to a radio-frequency (RF) cavity for use in a synchrotron or beam line where the velocity of particles changes.

BACKGROUND

An RF cavity is used in a particle accelerator to accelerate a particle beam using an RF electromagnetic field. Development of fixed-field alternating gradient (FFAG) synchrotron technology has sparked interest in its use in various commercial machines. Upgrades of existing synchrotrons and improvements to other particle accelerator designs are also in need of RF cavities that fit into physically limited spaces of a practical machine and rapidly change frequency over a wide range. Thus, there is a need for RF cavities that are suitable for use in an accelerator whose acceptance depends on its functional capability as well as physical size.

SUMMARY

A system for accelerating particles includes an RF cavity that contains a ferrite core and a liquid dielectric. Characteristics of the ferrite core and the liquid dielectric, among other factors, determine the resonant frequency of the RF cavity. The liquid dielectric is circulated to cool the ferrite core during the operation of the system.

In one embodiment, a system for accelerating a particle beam in a particle accelerator includes a vacuum beam pipe, an RF cavity, and a coolant circulation system. The vacuum beam pipe allows passage of the particle beam. The RF cavity surrounds the beam pipe and includes a wall forming a chamber that contains a liquid dielectric and a ferrite core. The ferrite core surrounds the vacuum beam pipe. A liquid inlet and a liquid outlet on the wall allow the liquid dielectric to be circulated through the chamber to cool the ferrite core. The coolant circulation system is coupled to the RF cavity to circulate the liquid dielectric through the chamber at a circulation speed that is controlled using a temperature of the liquid dielectric.

In one embodiment, a method for accelerating a particle beam in a particle accelerator is provided. The particle beam is passed in a vacuum beam pipe through an RF cavity including a ferrite core surrounding the vacuum beam pipe. A liquid dielectric is circulated through the RF cavity to cool the ferrite core. The circulation speed at which the liquid dielectric is circulated through the RF cavity is adjusted using the sensed temperature of the liquid dielectric.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description provides examples, and the scope of the present invention is defined by the appended claims and their legal equivalents.

It should be noted that references to “an”, “one”, or “various” embodiments in this disclosure are not necessarily to the same embodiment, and such references contemplate more than one embodiment.

This document discusses, among other things, a compact, tunable radio-frequency (RF) cavity for use in a synchrotron or beam line where the velocity of particles changes, such as a fixed-field alternating gradient (FFAG) synchrotron. New developments in the design of FFAG synchrotrons have sparked interest in their use as rapid-cycling, high intensity accelerators for particles such as ions, protons, muons, and electrons. Potential FFAG applications include medical accelerators of protons and light ions for cancer therapy, proton drivers for neutron or muon production, rapid muon accelerators, and electron accelerators for synchrotron light sources. In various embodiments, the present RF cavity establishes or enhances feasibility of the FFAG synchrotron for such applications. In various embodiments, the compact size of the RF cavity allows for expansion of capabilities of an existing machine with limited space for additional components.

In one embodiment, the RF cavity includes an orthogonally biased ferrite core that allows for rapid tuning over various frequency ranges. A liquid dielectric is used for adjusting the frequency range and cooling the ferrite core.

One example of an application of the RF cavity is to improve the performance of the 8 GeV Fermilab (Fermi National Accelerator Laboratory, Batavia, Ill.) Booster synchrotron. The rapid tunability of the RF cavity can be exploited to improve synchrotron performance, and the compactness of the cavity permits it to be located in tight spaces. For example, a single second-harmonic cavity for the Fermilab Booster will improve proton capture from a linear accelerator, also referred to as a linac.

Another example of an application of the RF cavity is its potential use in the Main Injector synchrotron at Fermilab. When a new high-intensity superconducting 8 GeV linac eventually replaces the Booster synchrotron, additional RF cavities will be needed in the Main Injector. Again, the rapid tunability and the compactness will provide higher performance in terms of beam intensity.

FIG. 1is an illustration of an embodiment of an RF cavity system100for accelerating particles. RF cavity system100is part of a particle accelerator and includes a vacuum beam pipe102B, an RF cavity104surrounding vacuum beam pipe102B, and various devices and systems supporting the operation of RF cavity104. Vacuum beam pipe102B is a portion of a vacuum beam pipe102, which is part of a vacuum system of the particle accelerator and allows passage of a particle beam that is being accelerated. The portions of vacuum beam pipe102outside RF cavity system100are referred to as a vacuum beam pipe102A. In various embodiments, vacuum beam pipe102B includes a ceramic beam pipe.

RF cavity104is a type of pillbox cavity. In the illustrated embodiment, RF cavity104includes a wall106forming a chamber110. Wall106includes an exterior surface107and an interior surface108. Interior surface108faces chamber110. In various embodiments, wall106is made of a non-magnetic metallic material, such as copper or aluminum. Chamber110is configured to contain a liquid dielectric112. A liquid dielectric is also referred to as a dielectric liquid or a dielectric fluid. A liquid inlet114on wall106allows liquid dielectric112to flow into chamber110. A liquid outlet116on wall106allows liquid dielectric112to flow out of chamber110.

A ferrite core118is placed in chamber110and surrounds a portion of vacuum beam pipe102B. During operation of the particle accelerator, ferrite core118occupies substantially the region of high magnetic field, and space between ferrite core118and vacuum beam pipe102B, which is filled by liquid dielectric112, occupies substantially the region of high electric field. The dielectric constant of liquid dielectric112determines the frequency range of RF cavity104. In various embodiments, the frequency range is to be chosen according to the requirements of each particular application. Liquid dielectric112also provides for cooling of ferrite core118during the operation of the particle accelerator. Liquid inlet114and liquid outlet116on wall106allow liquid dielectric112to be circulated through chamber110to remove heat from ferrite core118. A coolant circulation system130coupled to RF cavity104pumps liquid dielectric112into liquid inlet114and out of liquid outlet116. In the illustrated embodiment, ferrite core118includes a plurality of ring-shaped ferrite cores118A-J. Spacers120are used to keep open cooling channels between ring-shaped ferrite cores118A-J to ensure effective cooling of ferrite core118using liquid dielectric112circulating around and between the ring-shaped ferrite cores. A vacuum safety system132coupled to RF cavity104isolates vacuum beam pipe102B using valves136A-B in case of a leak of liquid dielectric112into vacuum beam pipe102B. As illustrated inFIG. 1, valves136A-B separates vacuum beam pipe102B from vacuum beam pipe102A. In other words, vacuum beam pipe102B refers to the portion of vacuum beam pipe102between valves136A-B, and vacuum beam pipe102A refers to the remaining portions of vacuum beam pipe102. When being closed, valves136A-B retain the leaked liquid dielectric112within portion102B, thereby preventing liquid dielectric112from leaking into vacuum beam pipe102A. Coolant circulating system130and vacuum safety system132are further discussed below with reference toFIGS. 2 and 8, respectively.

A solenoidal biasing coil122surrounds RF cavity104over exterior surface107of wall106. Solenoidal biasing coil122produces a biasing magnetic field that is orthogonal to the RF magnetic field in RF cavity104. When compared to other possible field directions, the orthogonal biasing provides for faster frequency tuning with less RF heating loss. Solenoid power source138delivers electric current though solenoidal biasing coil122to generate the biasing magnetic field. An RF powering system134coupled to RF cavity104provides for the RF magnetic field in RF cavity104. RF powering system134is further discussed below with reference toFIG. 7.

An iron yoke124surrounds solenoidal biasing coil122and substantially encloses RF cavity104. Iron yoke124shunts the biasing magnetic field and reduces its effect on the particle beam in vacuum beam pipes102A-B. In the illustrated embodiment, vacuum beam pipe102B, chamber110, ferrite core118, solenoidal biasing coil122, and iron yoke124are approximately concentric. In one embodiment, one or more solid-state or permanent magnets are incorporated onto, or included in, one or more portions of iron yoke124to strengthen the biasing magnetic field. This may lower the power requirement for operating RF cavity system100by reducing the current applied to solenoidal biasing coil122. In a specific embodiment, ring-shaped permanent magnets placed co-axially with ferrite core118replace the portions of iron yoke124at the end of ferrite core118(outside wall106).

Ferrite core118and liquid dielectric112allow for tuning of the resonant frequency of RF cavity104. Changing the magnetic field imposed on ferrite core118allows for rapid change of the resonant frequency, such as within a small fraction of a second. In one embodiment, the frequency tuning range of RF cavity104is changed by suspending the operation of RF cavity104for replacing the currently used liquid dielectric112with a liquid dielectric of a different type. In various embodiments, the tunable range of the resonant frequency of RF cavity104is determined by the range of magnetic field imposed on ferrite core118and types of liquid dielectric available for use as liquid dielectric112.

Ferrite core118is made of a ferrite material. As used in this document, a “ferrite material” includes a material having a permeability (μ) being a function of a magnetic field imposed on that material. This permeability is relatively constant when the intensity of the magnetic field is above a characteristic field, Bsat. Lower characteristic field requirements allow for faster tuning times for an RF cavity with variable resonant frequency. Another desirable characteristic of the ferrite material is low microwave loss. Lower microwave loss means less heat needs to be removed from the RF cavity for its proper operation.

Ferrite materials that have been tested in a model for RF cavity104(discussed below) include (1) a Ni—Zn ferrite having low microwave loss but high magnetic field intensity requirement, and (2) a yttrium iron garnet (YIG) having microwave loss but low magnetic field intensity requirement. Examples of potentially suitable ferrite materials are listed in Table 1.

Liquid dielectric112has a dielectric constant and a thermal conductivity. A liquid dielectric with a relatively high dielectric constant allows for a relatively small size of the RF cavity. A liquid dielectric with a relatively high thermal conductivity allows for a relatively more efficient removal of the heat generated in the ferrite core.

Liquid dielectric tested in the model for RF cavity104includes silicone oil. Examples of potentially suitable liquid dielectrics are listed in Table 2.

FIG. 2is a block diagram illustrating an embodiment of a coolant circulation system230coupled to RF cavity104via liquid inlet114and liquid outlet116.

Properties of the ferrite material vary with temperature. Therefore, liquid dielectric112is used to maintain suitable operating properties of ferrite core118by controlling its temperature.

Coolant circulation system230is an embodiment of coolant circulation system130and includes a primary pump240, a primary pump controller242, a heat exchanger244, a secondary pump246, a secondary pump controller248, coolant pipes252, and a temperature sensor254. Primary pump240pumps liquid dielectric112through chamber110of RF cavity104. Primary pump controller242controls the pumping speed of primary pump240. Heat exchanger244cools liquid dielectric112using a secondary coolant250. In one embodiment, secondary coolant250is water.

Secondary pump246pumps secondary coolant250through heat exchanger244. Secondary pump controller248controls the pumping speed of secondary pump246. Coolant pipes252connect between RF cavity104, primary pump240, and heat exchanger244and allow for circulation of liquid dielectric112through RF cavity104, primary pump240, and heat exchanger244. In the illustrated embodiment, primary pump240pumps liquid dielectric112out of RF cavity104through liquid outlet116and into heat exchanger244, such that liquid dielectric112circulates through RF cavity104and heat exchanger244.

In various embodiments, primary pump controller242controls a primary circulation speed at which liquid dielectric112circulates through RF cavity104and heat exchanger244, and secondary pump controller248controls a secondary circulation speed at which the secondary coolant circulates through heat exchanger244. Temperature sensor254senses a temperature of liquid dielectric112. In one embodiment, temperature sensor254is located in or near RF cavity104such that the sensed temperature approximates the temperature of liquid dielectric112in chamber110of RF cavity104. In one embodiment, temperature sensor254represents a plurality of temperature sensors in RF cavity system100. In one embodiment, one or both of primary pump controller242and the secondary pump controller248are used to stabilize the temperature of liquid dielectric112in chamber110of RF cavity104by adjusting one or both of the primary circulation speed and the secondary circulation speed using the temperature sensed by temperature sensor254. A substantially stable temperature of liquid dielectric112ensures that the properties of ferrite core118are substantially stable.

A design analysis for RF cavity104was performed using computer modeling with Poisson SuperFish provided by Los Alamos Accelerator Code Group (LAACG, Los Alamos National Laboratory, New Mexico, U.S.A.) and ANSYS Multiphysics software (ANSYS, Inc., Canonsburg, Pa., U.S.A.). The analysis was performed using parameters approximately appropriate for the Fermilab Booster. The RF cavity model represents a simple pillbox cavity with a reentrant beam pipe where the accelerating gap is sealed with a ceramic pipe. The fluid inside the RF cavity has a dielectric constant of 4.5 and is used for adjusting the frequency range of the RF cavity and to cool the ferrite core. The biasing coil is a simple water-cooled solenoid. An iron yoke is used to return the biasing field flux. The biasing field is parallel to the axis of the particle beam and orthogonally biases the ferrite core. The RF cavity has a radius of about 30 cm and a length of about 50 cm long.

Table 3 shows the SuperFish calculations of cavity parameters for cases of unbiased ferrite core and biased ferrite core. The calculated parameters are associated with the mesh geometry only, using standard room-temperature copper. The ceramic permittivity is not included in the SuperFish analysis but is included in the ANSYS analysis. This is an ideal calculation without loss factors, so the shunt impedance needs to be corrected when losses are considered in practice.

In the design and fabrication of an RF cavity that can be operated at full power, it is important that the resonant frequency of the completed RF cavity is predictable with high confidence. Therefore, a physical model for RF cavity104, with solenoidal biasing coil122and iron yoke124, has been constructed and tested to verify mathematical predictions for frequency and quality factor. An aluminum body, which was designed to be easily reconfigured to hold different ring-shaped ferrite cores and/or different liquid dielectrics, was built around a ceramic beam pipe, with rubber O-rings such that the gap between the irises is adjustable. The solenoidal biasing coil winding is water cooled and can provide a magnetic field up to 0.15 T.

FIG. 3is a graph presenting frequency and quality factor measurements performed with the physical model. The graph is generated using only data from experimental measurements, without using data from calculations and simulations. The effectiveness of using the biasing current to control and change the ferrite permeability and, consequently, to change the resonant frequency of the RF cavity is investigated. The curves shown inFIG. 3include (i) resonant frequency of the RF cavity with the liquid dielectric (F WITH LIQ), (ii) quality factor of the RF cavity with the liquid dielectric (Q WITH LIQ), (iii) resonant frequency of the RF cavity without the liquid dielectric (F WITHOUT LIQ), (iv) quality factor of the RF cavity without the liquid dielectric (Q WITHOUT LIQ), each as a function of the biasing current. These curves indicate that the quality factor of the RF cavity is a non-linear function of the biasing current. Also, it appears that the power losses in the RF cavity take place mainly in the ferrite core, and not in the liquid dielectric, because the two quality factor curves are similar in shape and displaced horizontally from one another. Presence of the liquid dielectric changes the frequency tuning range of the RF cavity. The ferrite permeability, as determined by the biasing current, controls the frequency of the RF cavity within the tuning range.

Results of measurements with the physical model show excellent frequency agreement with the numerical simulations carried out with SuperFish and ANSYS based on the measured parameters of the ferrite cores (Ni—Zn ferrite and YIG). The measurements with a liquid dielectric (the Dow Corning 561 Silicone Oil) are also in good agreement with the simulations. These results demonstrate that the properties of the ferrite core and liquid dielectric were accurately measured and that the operation of the models for the RF cavity is well understood. Therefore, operating parameters of the RF cavity is accurately predictable by numerical simulations.

FIG. 4is a block diagram illustrating an embodiment of a powering system434for RF cavity104. Powering system434is an embodiment of RF cavity powering system134and includes a microwave power source460, an RF input coupler462, and a dielectric barrier464that is in contact with RF cavity104.

RF cavity104is powered by electromagnetic energy generated from microwave power source460and transmitted via RF input coupler462and dielectric barrier464. RF input coupler462is configured to match the broad frequency bandwidth of RF cavity104for approximately maximum power transmission efficiency, with an approximately minimum amount of power reflected back to microwave power source460. Because RF cavity104is filled with liquid dielectric112, the electromagnetic energy travels through dielectric barrier464, which is coupled between RF input coupler462and RF cavity104. Dielectric barrier464is an air-to-cavity dielectric barrier, or window, that is configured to transmit the electromagnetic energy into RF cavity104. In one embodiment, RF input coupler462is an coaxial cable or a waveguide, and dielectric barrier464is a rugged ceramic to metal brazed assembly configured to minimize any thermally induced stress that occurs therein.

FIG. 5is a block diagram illustrating an embodiment of a vacuum safety system532. Vacuum safety system532is an embodiment of vacuum safety system132and includes fast-acting vacuum valves536A-B, a sensor568, and a vacuum safety controller566.

RF cavity104, which contains liquid dielectric112, is separated from the vacuum system of the particle accelerator by the wall of vacuum beam pipe102B. A leak of liquid dielectric112into the vacuum system will potentially prevent the particle beam from circulating and damage the components of the particle accelerator. Therefore, vacuum safety system532is provided to prevent potentially serious damages in case liquid dielectric112leaks into vacuum beam pipe102A from RF cavity104.

Sensor568represents one or more sensors each sensing a signal indicative of a leak of liquid dielectric112into vacuum beam pipe102B. In one embodiment, sensor568includes one or more pressure sensors each detect a pressure signal indicative of the leak. Fast-acting vacuum valves536A-B seal off a portion of vacuum beam pipe102near RF cavity104when being closed. Vacuum safety controller566detects the leak using the one or more signals sensed by sensor568. In response to a detection of the leak, vacuum safety controller566closes the fast-acting vacuum valves536A-B, thereby isolating the portion of vacuum beam pipe102B from vacuum beam pipe102A and the rest of the vacuum system of the particle accelerator.

FIG. 6is a block diagram illustrating an embodiment of a circular particle accelerator670, which includes RF cavity system100. In the illustrated embodiment, circular particle accelerator670includes RF cavity system100, bend magnets672A-D, focus magnets674A-D, and an injection system676, connected together through an approximately circular vacuum system602. Vacuum system602includes vacuum chambers and pipes constructed of materials such as metal or ceramic.

Circular particle accelerator670accelerates particles, such as electrons, protons, and heavy ions, to high energy levels. The particles are collected in a series of particle bunches. Bend magnets672A-D include electromagnets configured to bend the particle beam around in a trajectory that is approximately circular. Focus magnets674A-D include electromagnets configured to focus the particle beam to a small transverse size. Injection system676injects a low-energy particle beam into vacuum system602. In one embodiment, injection system676includes a linear accelerator, also referred to as linac. RF cavity system100accelerates the particle beam by providing an electric field at the time that the particle bunches pass through RF cavity104.

In one embodiment, bend magnets672A-D and/or focus magnets674A-D include one or more electromagnets each formed by two skewed solenoid coils that are energized to produce a dipole field, while the fields produced by the two solenoid coils cancel each other on the longitudinal direction (along the axes of the solenoid coils). An example of such an electromagnet is discussed in D. I. Meyer and R. Flasck, “A New Configuration for a Dipole Magnet for Use in High Energy Physics Applications,”Nuclear Instruments and Methods,80 (1970): 339-341.

FIG. 7is a flow chart illustrating an embodiment of a method700for operating an RF cavity for accelerating particles in a particle accelerator. In one embodiment, method700is performed by RF cavity system100, including its various embodiments as discussed in this document.

At710, a liquid dielectric is circulated through the RF cavity and a heat exchanger. The RF cavity surrounds a portion of a vacuum beam pipe and includes a ferrite core that is in direct contact with the circulating liquid dielectric. The liquid dielectric cools the ferrite core during operation. An example of the liquid dielectric is silicone oil. The dielectric constant of the liquid dielectric determines the resonant frequency of the RE cavity. In one embodiment, the frequency range of the RF cavity is adjusted by adjusting the dielectric constant of the liquid dielectric. For example, the frequency range of the RF cavity is substantially changed by replacing the liquid dielectric with another liquid dielectric having substantially different dielectric constant. In one embodiment, microwave electromagnetic power is transmitted to the RF cavity from a power source through a dielectric barrier providing for air-to-cavity power transmission. In one embodiment, a pressure signal indicative of a leak of the liquid dielectric into the beam pipe is sensed to provide for detection of the leak. In response to a detection of the leak, valves are closed to confine the leaked liquid dielectric to a portion of the beam pipe near the RF cavity. At720, a secondary coolant is circulated through the heat exchanger to cool the liquid dielectric. An example of the secondary coolant is water.

At730, a temperature of the liquid dielectric is sensed. In one embodiment, the temperature is sensed in or near the RF cavity such that it indicates the temperature of the liquid dielectric within the RF cavity. At740, the speed at which the liquid dielectric is circulated through the RF cavity and the heat exchanger is adjusted using the sensed temperature. At750, the speed at which the secondary coolant is circulated through the heat exchanger is adjusted using the sensed temperature. In one embodiment, the speed adjustments at740and/or750allow for stabilization of the temperature of the liquid dielectric in the RF cavity, which in turn stabilizes the properties of the ferrite core during the operation of the particle accelerator.

At760, the particle beam is passed in the vacuum beam pipe through the RF cavity. In one embodiment, the particle beam is passed when the temperature of the liquid dielectric in the RF cavity is stable. Examples of the particles in the particle beam include ions, protons, muons, and electrons.

It is to be understood that the above detailed description is intended to be illustrative, and not restrictive. For example, structures as shown in various figures, including but not limited to shape and relative size of each system component, arrangement of the components, and number of components (such as the number of the ring-shaped ferrite cores, the number of turns of the solenoidal biasing coil, the number of pipes and magnets, the number of pumps, and the number of RF cavities), is for illustrative purposes only. While specific examples of ferrite materials and liquid dielectric are presented, other suitable materials are also usable as recognizable by those skilled in the art. Other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.