Particle accelerator with vacuum pump connected to the low voltage side

Abstract: The disclosure relates to a particle accelerator which comprises two accelerating tubes as well as a charge-exchange channel, whereby the vacuum pump for evacuating from said charge-exchange channel gas particles injected therein is not connected to the high-voltage side but, via a vacuum tube, to the low-voltage side of the particle accelerator. This means a substantial simplification of the maintenance of the vacuum pump. In addition to that a motor and a generator are no longer required. By providing the vacuum pump with equipotential plates, which comprise pump holes arranged eccentrically round the centre of the vacuum tube, the gas particles are in this configuration are accelerated while retaining the high breakdown strength.

Description: The present invention relates to a tandem particle accelerator comprising at least two accelerating tubes having a high-voltage side and a low-voltage side, with a charge-exchange channel comprising provisions for the injection of a gas therein, which charge-exchange channel is provided between the high-voltage sides of said accelerating tubes, and at least one high-vacuum pump which is in open connection with said charge-exchange channel. A tandem particle accelerator of this type is generally known. In particular a particle accelerator is known wherein a gas is injected into the charge-exchange channel (=CEC), which gas is pumped from the CEC by the vacuum pump after having interacted with a beam consisting of one or more types of charged or neutral particles. Said gas must be prevented from finding its way into the accelerating tubes as much as possible, because this leads to undesirable charge-exchange processes in said accelerating tubes, resulting in a reduced output and quality of the particle accelerator. The vacuum pump is supplied with the necessary power by a generator, for example, which is coupled with a motor via a driving mechanism, which for example comprises an electrically insulating driving shaft or driving belt. Said insulating driving mechanism electrically separates the vacuum pump and the generator present in the terminal, which are on high voltage, from a motor on a much lower voltage, namely on earth potential. The drawback of the known tandem particle accelerator is that the high-vacuum pump present in the terminal lacks in accessibility. Frequently the encasing housing the terminal and the accelerating tubes, which is usually made of metal, must be opened in order to carry out maintenance work, and the necessary electric screening facilities must be dismounted, and be mounted again after said maintenance work on the vacuum pump has been completed. This makes the required regular maintenance work on the high-vacuum pump(s) time-consuming and costly and, moreover, reduces the effective output of the known particle accelerator. Another drawback of the known tandem particle accelerator is the fact that a generator and a motor as well as an insulated driving mechanism mounted therebetween are required to provide the necessary electric power for the high-vacuum pump. Said parts require maintenance at an awkward place, and they are vulnerable to high-voltage breakdowns, which inevitably occur in particle accelerators of this type. The object of the present invention is to provide a particle accelerator which is reliable and which is simple to maintain. In order to accomplish that objective the tandem particle accelerator according to the invention is characterized in that said particle accelerator includes a vacuum-tube which is in open connection with said charge-exchange channel, and which vacuum-tube is connected between said high-voltage side and said low-voltage side, and in that said high-vacuum pump is connected to said vacuum tube at the low-voltage side. The advantage of the particle tandem accelerator according to the invention is that the high-vacuum pump is on earth potential, thus making maintenance simpler, quicker and less costly. In addition to that the driving mechanism consisting of insulating material and the generator and the motor are no longer required. Furthermore the vulnerability due to high-voltage breakdowns of the high-vacuum pump, which was previously on high-voltage, but which is on earth potential now, has been reduced, resulting in an enhanced reliability of the particle accelerator according to the invention. Another advantage is moreover the fact that a greater freedom of choice is obtained with regard to the dimensions and the type of high-vacuum pump to be used, because usually more space is available for such an externally mounted pump. One embodiment of the particle accelerator is according to the invention characterized in that it comprises a valve connected between the low-voltage side of said vacuum tube and said high-vacuum pump. The advantage of this embodiment of the tandem-particle accelerator according to the invention is the fact that the valve, which is on earth potential, is closed prior to maintenance work being carried out on the high-vacuum pump, as a result of which it is not necessary to open the accelerating tube and the vacuum tube, which leads to a further saving on maintenance time with regard to the high-vacuum pump. Another embodiment of the tandem particle accelerator is according to the invention characterized in that equipotential plates are provided in the vacuum tube, which plates are provided with pump holes, which pump holes are arranged spiral-wise (eccentrically) round the centre of the vacuum tube. The advantage of this further embodiment of the particle accelerator according to the invention is that charged particles, which will inevitably be present in such an accelerating tube, and which are accelerated through the electric field perpendicularly to the equipotential surfaces, strike against an equipotential surface before reaching a high velocity. This results in a high breakdown strength of the vacuum tube, and the undesirable yet inevitable production of radiation is reduced. The present invention and its concomitant further advantages will be discussed in more detail hereafter with reference to the drawing, wherein like parts are numbered alike in the various Figures. In the drawing: Figure 1 shows an embodiment of a known particle accelerator; Figure 2 shows another embodiment of a known particle accelerator; Figure 3 shows an embodiment of the particle accelerator according to the invention. Figures 1, 2 and 3 show a relevant part of a particle accelerator 1, in which electrically charged or neutral particles are accelerated/transported in at least one accelerating tube 2 by an emission source (not shown). To that end accelerating tube 2 is with its high-voltage side connected to terminal 3 and with its low-voltage side to the, usually metal, encasing 4, which is on earth potential. In a similar manner second accelerating tube 15 is connected between terminal 3 and encasing 4. Accelerating tubes 2 and 15 are provided with electrodes (not shown) known per se, which are separated from each other by insulators, and which are surrounded by corona rings. Furthermore an accelerating tube of this type comprises spark apertures, equipotential sections, if desired, and a resistor network in order to realize a substantially uniform voltage grading over accelerating tubes 2 and 15. Particle accelerator 1 may be a tandem accelerator, for example, in which the high voltage difference is used for accelerating charged particles coming from an emission source in the direction of or from the terminal being on high voltage in accelerating tubes 2 and 15. In accelerating tube portions 2 and 15, in which a vacuum is generated by means of one or more vacuum or high-vacuum diffusion pumps, absorption pumps or cryogenic pumps, charge exchange processes occur in a CEC 6, which is positioned in terminal 3, which charge exchange processes are necessary in order to ensure the desired exit velocity. Furthermore particle accelerator 1 may be of a type in which neutral particles are transported from the earth potential to terminal 3 and wherein said particles are ionized in CEC 6, followed by an acceleration of said ions in the second accelerating tube 15, in order to give them the desired exit velocity. In order to maintain this charge exchange process, which is desirable in CEC 6, a gas is introduced into CEC 6 from a gas cylinder 7, via a supply pipe 8, which gas interacts with the beam of electrically charged or neutral particles moving through CEC 6. This region of increased gas pressure within CEC 6 must be maintained locally in CEC 6 as best as possible, because gas flowing into accelerating tubes 2 and 15 would lead to undesirable charge exchange processes. The gas introduced into CEC 16 flows to a vacuum pump 11 present in terminal 3, which is on high voltage, via the two ends 9 and 10. The evacuated gas can be recirculated via a return pipe 12, if desired. Said recirculating is optional, however. The electric power required for vacuum pump 11 is generated by generator 14, which is in turn driven by a motor 16 being on earth potential. The required mechanical coupling between generator 14 and motor 16 may be a driving shaft or a driving belt, for example, which is electrically insulating in order to maintain the voltage difference between motor 16 and generator 14. Figure 2 shows another embodiment of a known particle accelerator, which does not comprise the vacuum pump 11, the generator 14 or the motor 16. The drawback of this embodiment is the fact that the gas injected into charge-exchange channel 6 must be completely evacuated through accelerating tubes 2 and 15, resulting in a stronger increase of the pressure in said tubes, as a result of which more charge-exchange processes take place in said tubes. Figure 3 shows an embodiment wherein a vacuum tube 13 is connected between terminal 3 and encasing 4, which is generally made of metal, whereby the vacuum pump on encasing 4 is connected to vacuum tube 13. Such a vacuum tube is provided with means similar to the means described above with reference to accelerating tubes 2 and 15. In this embodiment the vacuum pump is on earth potential, however, as a result of which maintenance of said vacuum pump is much simpler, because it is directly accessible. By mounting a valve (not shown in Figure 3) in vacuum tube 13, the maintenance work on the part of particle accelerator 1 positioned before said valve may even take place without the vacuum in accelerating tubes 2 and 15 and vacuum tube 13 being broken. Positioned within vacuum tube 13 are equipotential plates 5, which are provided with a plurality of pump holes (not shown), which function to enable the discharge of the gas to vacuum pump 11. In practice it has appeared to be advantageous to provide the pump holes off the centre of the vacuum tube. As a result of this charged particles, which are inevitably present within such an accelerating tube and which are accelerated through the electric field in a direction perpendicularly to the equipotential surfaces, strike against an equipotential surface before reaching a high velocity. This results in a high breakdown strength of the tube, and the undesirable yet inevitable production of radiation is reduced.

Claims: A tandem particle accelerator (1) comprising at least two accelerating tubes (2,15) having a high-voltage side (3) and a low-voltage side (4), with a charge-exchange channel (6) comprising provisions for the injection of a gas therein, which charge-exchange channel (6) is provided between the high-voltage sides (3) of said accelerating tubes (2,15), and at least one high-vacuum pump (11) which is in open connection with said charge-exchange channel (6), characterized in that said particle accelerator (1) includes a vacuum tube (13) which is in open connection with said charge-exchange channel (6), and which vacuum-tube (13) is connected between said high-voltage side (3) and said low-voltage side (4), and in that said high-vacuum pump (11) is connected to said vacuum tube (13) at the low-voltage side (4). A tandem particle accelerator (1) according to claim 1, wherein it comprises a valve connected between the low-voltage side (4) of said vacuum tube (13) and said high-vacuum pump (11). A tandem particle accelerator (1) according to claim 1 or 2, wherein said vacuum tube (13) is provided with means for achieving a more uniform potential gradient from said high-voltage side (3) to the high vacuum-pump (11) connected to the low-voltage side (4). A tandem particle accelerator (1) according to claim 3, wherein said means for achieving the uniform potential gradient comprise a resistor network. A tandem particle accelerator (1) according to any one of the claims 1 - 4, wherein said vacuum tube (13) is provided with corona rings. A tandem particle accelerator (1) according to claim 5, wherein said corona rings and/or said vacuum tube (13) are provided with insulators and/or spark apertures. A tandem particle accelerator (1) according to any one of the claims 1 - 6, wherein said vacuum tube (13) is provided with equipotential plates. A tandem particle accelerator (1) according to claim 7, wherein said equipotential plates are provided with pump holes. A tandem particle accelerator (1) according to claim 8, wherein the centres of said pump holes lie on a straight line. A tandem particle accelerator (1) according to claim 8 or 9, wherein said pump holes lie on the central axis of the equipotential surfaces having a circular section. A tandem particle accelerator (1) according to claim 8, wherein said pump holes lie off the centre of said equipotential surfaces. A tandem particle accelerator (1) according to claim 11, wherein said pump holes are arranged spiral-wise round the centre of said vacuum tube (13).

Inventors: KOUDIJS REIJER, KOUDIJS, REIJER
Applicants: HIGH VOLTAGE ENG EUROP, HIGH VOLTAGE ENGINEERING EUROPA B.V.
Application year: 1996
Main classification: H05H5
IPC codes: H05H5