High power adjustable RF coupling loop

This non-provisional application is a continuation of a previously filed provisional application with application No. 60/253,545 and the filing date Nov. 28, 2000.

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The present invention relates generally to resonance acceleration of charged particles and specifically to excitation of resonant cavities of a resonance accelerator with a radio-frequency electromagnetic wave. The invented device is a high power coupling loop which is used to interface a resonant cavity to a transmission line.

In its broadest classification, there are two types of charged particle accelerators; electrostatic, and resonance. In an electrostatic accelerator charged particles gain energy as they move between two regions that are held at two different electric potentials, for example two electrodes. Associated with the electric potential is the electric field E. The integral of the electric field along the path traversed by the particle that connects the two regions is equal to the potential difference between the two region. The accelerating force is equal to qE, where q is the electric charge of the particle. In an electrostatic accelerator, as the name “electrostatic” suggests, the electric potential is independent of time. Accelerating charged particles to megavolt energies requires a very large electric filed. The convenient unit in this context is megavolt/cm. Because both isolating materials and practical vacuum break down under strong electric fields the limit of the electrostatic acceleration is from several to around 10 MeV. The resonance accelerator, however, does not have this limitation.

Resonance accelerators like all electromagnetic devices owes their existence to the genius of J. C. Maxwell who added the displacement current to the Ampere's law and implied new physical phenomena which has been substantiated in all details by experiment. Accordingly, a time varying magnetic fields give rise to electric fields and vice-versa. One of the device which is directly related to the present invention is the resonant cavity. In its simplest form a resonant cavity is a hollow volume enclosed by metallic walls. The hollow volume, as predicted by the Maxwell equations, supports electromagnetic oscillations in which the energy in the cavity oscillates between the electric and magnetic fields. In a resonance accelerator the particles are accelerated by electric field of the cavity or an array of cavities. Since both magnitude and direction of the electric field of a resonant cavity changes with time there must be an exact correlation between the movement of accelerating particles and the frequency of the resonant cavity: any time that the particles reaches the cavity field the electric field of the cavity should be in a direction to accelerate the particles. (The alternative is deceleration of charged particles which results in amplification of the cavity fields.) The term “resonance” in resonance accelerator refer to this requirement.

The resonant frequency of almost all cavities that are used for particle acceleration fall in the radio-frequency (rf for short) range. To resonate a cavity and keep it in the excited state the rf power from an rf amplifier must be continuously fed into the cavity. The transfer of power from the rf amplifier to the cavity is by a transmission line which connects the rf amplifier to the cavity. The end of the transmission line on the cavity side is connected to a coupling device which is housed inside the cavity. The coupling device interfaces the transmission line to the resonant cavity and plays a vital role in both operation of the accelerator and protection of the rf circuit elements.

From practical point of view the coupling device must possess some key features. First, the reflected rf power, the power that reflects back to the rf amplifier, should not exceed more than a few percent of the rf forward power. Here, the primary issue is not efficient use of the power but protection of the downstream components—the power amplifiers. A large amount of reflected power can easily ruin circuit elements on its pass. Second, the physical size of the coupling device should be much smaller that the physical size of the cavity. This condition warrants that the effect of the coupling device on the cavity is not more than a small perturbation. This requirement comes from the fact that in accelerator applications a resonant cavity with a large Q is desired. (The Q of the cavity is defined by

Q=ω o Stored energy/Power loss

where ω o is the angular frequency of the cavity.) A coupling device, however, reduces Q of the cavity. This reduction in Q gets worse as the physical size of the coupling device becomes larger.

With regard to the features of the present invention which will be discussed shortly, all existing high power coupling devices are nonadjustable and water cooled. The fact that they are nonadjustable means that many of them with different physical size and shape must be built and tried until one of them can provide a tolerable reflected power. The fact that they are water cooled means they are prone to leak water in the vacuum where they operate and the water cooling adds additional cost and maintenance. From these considerations it is highly desirable to come up with an adjustable coupling device that also does not use water cooling. Finally, a coupling device should be purely metallic. Any nonmetal part, such as ceramic or teflon, which is sometimes used for electrical isolation of a coupling device components will melt under high power.

The device to be described in the next section is the first adjustable high power rf coupling loop. Because it is adjustable it provides perfect impedance matching and subsequently renders almost zero reflected power. Moreover, it fulfills all other requirements discussed in this section; it is purely metallic, relatively small, and does not use water cooling. This coupling loop has been installed in an accelerator and has shown excellent performance.

A new coupling loop at radio frequency (rf) is presented which is used for interfacing a transmission line to a resonant cavity. All parts of the loop are metallic and subsequently the loop is ideal for high power rf applications. Its key parts comprises of two parallel metallic rods and a sliding clamps. One of the rod is connected to the center line of the transmission line and the other is hard soldered to the return; the ground. The two rods are shorted by the sliding clamp. The impedance matching is achieved by simply adjusting the position of the clamp. This is the first adjustable coupling loop and also the first loop that does not use water cooling. Since the loop is adjustable it can be adjusted to produce practically zero reflected power which is a highly desirable feature in resonance accelerators.

FIG. 1 is a schematic view of the coupling loop.

FIG. 2 is a schematic view of the adjustable clamp.

FIG. 3 is a sectional view of the coupling loop and resonant cavities of an accelerator in which the coupling loop has been installed.

FIG. 1 shows a cross section view of the coupling loop. The main components of the loop are the two parallel quarter-inch solid metallic rods 11 land 12 , clamp 13 , and metallic structure 14 . We refer to this metallic structure as the holder. Rod 12 is part of the commercially available high voltage feed through. This rod is electrically isolated from holder 14 by structure 15 and also vacuum sealed by structure 15 . One end of rod 12 is connected to the center conductor of 50-ohm elbow 16 . A transmission line, not shown in FIG. 1, connects elbow 16 to an rf amplifier. Rod 11 is hard soldered to holder 14 . Both rod 11 and holder 14 are the return part of the circuit interfacing the transmission line and are electrically connected to the outer shield of the transmission line through elbow 16 .

Clamp 13 electrically shorts rod 11 and 12 . As shown in FIG. 2, excluding mounting screws 13 c , clamp 13 comprises of two top and bottom symmetrical pieces, 13 a and 13 b , respectively. The diameters of half-circle grooves in 13 a and 13 b are chosen to snugly house parallel rods 11 , and 12 . The screw holes in 13 b are tapped and the screw holes 13 a are straight, untapped. The position of clamp 13 , please see FIG. 1, is adjusted by loosening the three mounting screws. Upon loosening 13 c , the clamp can slide back and fourth along the two rods. As will be described shortly, the location of clamp 13 is the major determining factor for impedance matching.

Holder 14 also functions as a heat sink for rods 11 , 12 , and clamp 13 . It should be noted that the amount of heat generated in the coupling loop is not significant. All previous designs of coupling loops is based on the assumption that without a coolant the loop will melt. This is not the case, however. There is no reason to support that the coupling loop should get any hotter than, for example, any part of the transmission line. Yet the transmission line stands the heat without direct cooling. This assessment of the situation that the loop should not get hot is substantiated by the present design of the loop. As a result, using the holder as a heat sink is more than adequate for keeping the loop temperature down.

From the functional point of view the coupling loop should provide perfect impedance matching. Specifically, the loop should function such that the combined impedance of the resonant cavity and the loop be equal to the impedance of the transmission line connected to the loop. When this happens the reflected rf power is zero and all forward power will be absorbed by the resonant cavity. For the sake of discussion, we assume that the impedance of the transmission line connecting the rf amplifier to the coupling loop is 50 ohm. This is generally true, since all commercially available high power transmission lines are 50 ohm. With this assumption the reflected power vanishes if the impedance of the combination of the coupling loop and the resonant cavity as seen by the transmission line is 50 ohm. In that case the incident wave in the transmission line does not see any discontinuity and subsequently is not reflected back

Denoting the voltage across the coupling loop by V 1 and the current through the coupling loop by I 1 and the combined impedance of the coupling loop and resonant cavity as seen by the transmission line by Z 1 we have the relation Z 1 ≡V 1 /I 1 . This expression is the definition of Z 1 . Both V 1 and I 1 depend on the geometry of the coupling loop, geometry of the cavity, location and orientation of the coupling loop with respect to the cavity. In general, Z 1 has a reactive and a resistive component. As noted above we require that the reactive component be equal to zero and the resistive component be equal to 50 ohm. There are two parameters that can be adjusted to fulfill these two requirements. They are the position of clamp 13 , please see FIG. 1, and the orientation of the coupling loop with respect to the cavity, please see FIG. 2 . The latter parameter is varied by rotating the coupling loop in its mounting location. When the above two requirements are met the reflected power vanishes.

As noted above, fulfilling the above two requirements are equivalent to elimination of the reflected power which in turn depends on the exact location of clamp 13 and if necessary small rotation of the loop. A simple device called voltage standing wave ratio analyzer, or VSWR analyzer for short, which is a standard tool in rf technology can determine the orientation of the loop and location of the clamp. The VSWR analyzer is a low power rf generator which has a dial to show the reflected power in terms of VSWR which is defined by VSWR = 1 + ρ v 1 - ρ v ,

where ρ v is the magnitude of the reflection coefficient. When ρ v is zero the reflected power vanishes and VSWR converges to 1. The reflected power is measured by connecting the VSWR analyzer to elbow 16 of FIG. 3 . As noted above, the value of VSWR=1 indicates zero reflected power. Therefore, the object is to vary the location of the clamp very slightly and systematically until VSWR converges to 1. In general, the orientation of the loop should be chosen such that the rectangle defined by rod 11 , 12 and clamp 13 intercept the maximum flux of the cavity mode.

The present invention has been installed in a cyclotron called RDS-11 which is marketed by SIEMENS. The schematic of this cyclotron is shown in FIG. 3 . The cyclotron operates with a forward rf power of around 10 kWatts and the operating frequency of the resonant cavity is around 73 MHz. By adjusting the position of clamp 13 the reflected power of the present invention can be set as low as 20 to Watts. This gives a reflected power of (20/10000)×100=0.2%, which is very small.

Finally, the actual dimensions of the coupling loop is as follows. Rod 11 is about 4.5 inches long and the nominal dimensions of the other parts can be determined from FIG. 1 based on the dimension of rod 11 .