Elekta Ab

Elekta Ab (publ)

Terry Arthur Large

John Allen

Terence Bates

Leonard Knowles Brundle

Kevin J Brown

Wei Yu

The present invention provides a linear accelerator in which a rotatable conductive vane is employed to vary the electromagnetic coupling between adjacent accelerating cells. The vane is sealed off from the rest of the linear accelerator by an insulating partition, so the pressure around the vane can be higher than in the rest of the accelerator. This greatly simplifies the mechanisms which may be used to control the rotation of the vane, allowing a higher bakeout temperature in manufacture and a higher rate of rotation in use.

Linear accelerator

A linear accelerator comprises a series of accelerating cavities, adjacent pairs of which are coupled via coupling cavities, in which at least one coupling cavity comprises a rotationally asymmetric element that is rotateable thereby to vary the coupling offered by that cavity. A control means for the accelerator is also provided, adapted to control operation of the accelerator and rotation of the asymmetric element, arranged to operate the accelerator in a pulsed manner and to rotate the asymmetric element between pulses to control the energy of successive pulses. A beneficial way of doing so is to rotate the asymmetric element continuously during operation of the linear accelerator. Then, the control means need only adjust the phase of successive pulses so that during the brief period of the pulse, the asymmetric element is 'seen' to be at the required position. The asymmetric element can disposed within an evacuated part of the accelerator and rotated by way of an electromagnetic interaction with parts outside the evacuated part. No parts associated with the drive need therefore pass through the vacuum seal. This could be achieved by providing at least one magnetically polarized member on the asymmetric element and at least one electrical coil outside the evacuated part.

An accelerator comprises a plurality of accelerating cells arranged to convey a beam, adjacent cells being linked by a coupling cell, the coupling cells being arranged to dictate the ratio of electric field in the respective adjacent accelerating cells, at least one coupling cell being switchable between a positive ratio and a negative ratio. Such an accelerator in effect inserts a phase change into the E field by imposing a negative ratio, meaning that the beam will meet a reversed electric field in subsequent cells and will in fact be decelerated. As a result, the beam can be developed and bunched in early cells while accelerating to and/or at relativistic energies, and then bled of energy in later cells to bring the beam energy down to (say) between 100 and 300 KeV. Energies of this magnitude are comparable to diagnostic X-rays, where much higher contrast of bony structures exists. Hence the accelerator can be used to take kilovoltage portal images.

A standing wave linear accelerator has a plurality of resonant cavities located along a particle beam axis. One or more pairs of resonant cavities are electromagnetically coupled via a coupling cavity. A rotationally asymmetric element within the coupling cavity is adapted to rotate about an axis that is substantially parallel to the axis of the coupling cavity. The coupling cavity is imperfectly symmetric about its axis due to a relative excess of material disposed within the cavity in the portion opposed to the apertures. Rotation of the polarization of a TE mode inside the cylindrical cavity provided a simple single mechanical control of coupling value, that has negligible effect on the phase shift across the device. A slight frequency dependence on the angle of rotation is correctable by a relative excess of material located opposite the apertures between the coupling cavity and the accelerating cavities.

This device allows the variation of the coupling between two points in an RF circuit in a very simple way while maintaining the RF phase relationship and varying the relative magnitude of the RF fields. The device is characterized by a simple mechanical control of coupling value, that has negligible effect on the phase shift across the device. This is achieved by the simple rotation of the polarisation of a TE mode inside a cylindrical cavity. Such a device does not contain resistive elements, and the sliding mechanical surfaces are free from high RF currents. This device finds an application in standing wave linear accelerators, where it is desirable to vary the relative RF field in one set of cavities with respect to another, in order that the accelerator can operate successfully over a wide range of energies.

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Lindfield, United Kingdom

Terry Arthur Large