GB2578293A

GB2578293A - A set of pumps, and a method and system for evacuating a vacuum chamber in a radioactive environment

Info

Publication number: GB2578293A
Application number: GB1816975.5A
Authority: GB
Inventors: David Chew Andrew
Original assignee: Edwards Ltd
Current assignee: Edwards Ltd
Priority date: 2018-10-18
Filing date: 2018-10-18
Publication date: 2020-05-06
Also published as: WO2020079428A1; EP3867529A1; GB201816975D0

Abstract

A set of pumps for providing a high vacuum within a chamber 40 in a radioactive environment, comprising (i) a primary pump 10 configured for operation outside the radioactive environment and connected via a conduit 12 to the chamber to evacuate it to a first pressure, preferably between 1 and 10-3 mbar; (ii) a radiation-resistant intermediate pump 20 configured for operation within the radioactive environment, to further evacuate the chamber to a second lower pressure, preferably between 10-3 and 10-6 mbar; and (iii) a radiation-resistant high vacuum pump 30 configured for operation within the radioactive environment and to further evacuate the chamber to a third operational vacuum pressure, preferably below 10-7 mbar. Preferably the chamber comprises an accelerator (42, fig. 5) and the radiation-resistant pumps are getter pumps or other pumps with no active electronics and no moving parts, such as ion pumps, diffusion pumps or vapour booster pumps. The primary pump may be located outside a radiation-shielded enclosure 50 containing the chamber and the radiation-resistant pumps or, alternatively, within this enclosure but inside its own radiation-shielded compartment (60, fig. 6). The intermediate pump may be a relatively high pressure getter pump configured for pulsed electric discharge.

Description

A SET OF PUMPS, AND A METHOD AND SYSTEM FOR EVACUATING A

VACUUM CHAMBER IN A RADIOACTIVE ENVIRONMENT

FIELD OF THE INVENTION

The field of the invention relates to pumps for evacuating a vacuum system in a radioactive environment.

BACKGROUND

A high vacuum may be required in regions of high radioactivity and these to preclude the use of many rotary pumps such as turbomolecular pumps which are controlled by active electronics. Ion or Getter pumps can provide high or ultrahigh vacuums and are resistant to high levels of radiation and thus, can be used to maintain vacuums in such regions. These pumps include ion getter, non-evaporable getter and Ti sublimation pumps.

Such regions include those surrounding an accelerator. An accelerator operates at ultra-high vacuums and during operation and for a period of time following operation generate levels of radiation in the shielded region housing the accelerator that are too high for personnel or non-radiation resistant pumps to operate. In this regard non-radiation resistant pumps are only operational at radiation levels below 10 to 100 Grays. Thus, were there to be a vacuum failure of such an accelerator during operation, the reactivation of the radiation resistant ion or Getter pumps that evacuate the accelerator would have to wait for the radiation levels to fall to an acceptable level for personnel and non-radiation resistant pumps to enter the shielded region and start to evacuate the accelerator to a vacuum level at which the ion or Getter pumps can start operation. This may take several hours.

It would be desirable to be able to provide a pumping set capable of evacuating a 3o radiation region to a high vacuum from ambient pressure without an undue delay. -2 -

SUMMARY

A first aspect provides a set of pumps for providing a high vacuum within a radioactive environment, said set of pumps comprising: a primary pump configured for operation outside of said radioactive environment and configured to connect to a chamber within said radioactive environment via a conduit, said primary pump being configured to evacuate said chamber to a first pressure; a radiation resistant intermediate pump configured for operation within said radioactive environment and configured to evacuate said chamber from said first io pressure to a second lower pressure; and a radiation resistant high vacuum pump configured for operation within said radioactive environment and configured to evacuate said chamber from said second lower pressure to a third operational vacuum pressure.

The inventor of the present invention recognised that where there is a vacuum failure within a radioactive environment then re-establishing the vacuum may be time-consuming. Primary pumps that are operational at higher pressures are often not radiation resistant as they generally have rotating parts and active electronics. Thus, conventionally before these pumps can be used to evacuate a chamber to a level low enough for a radiation resistant pump to be operational, the radiation level within any shielding will need to fall to a level where an operator can enter with these pumps and establish the required low pressure.

One solution of providing a pump outside of the radioactive environment and connected to the chamber via a conduit has its limitations as gas conductance falls with pressure. Thus, although pumping the chamber down to an intermediate low pressure would be possible using a primary pump located remotely from the shielded environment and using a conduit to connect the primary pump to the chamber, this pressure may not be low enough for effective 3o operation of a radiation resistant pump. This is because, as the pressure falls the conductance of the gas falls, and thus, the rate of gas flow along the conduit drops. Such an arrangement therefore only operates efficiently down to a certain -3 -pressure. Conventionally this pressure has not been low enough for operation of a radiation resistant pump. The inventor has addressed this by providing two radiation resistant pumps, and using one at a higher pressure, this higher pressure being one that is attainable by a remote primary pump, the other radiation resistant pump being operable at a high vacuum.

There is a prejudice against operating radiation resistant pumps such as ion or Getter pumps at increased pressures as the capture material such as the material on the chemically active cathode will be depleted quickly and the pump io will become non-operational and need to be replaced, leading to substantial downtime of the apparatus. This has been addressed by the use of an additional intermediate pump which operates in the region that conventionally a turbomolecular pump would have operated. This pump is only operational during pump down and thus, although it may deteriorate more quickly when operational than a pump operated at a lower pressure, as it is only used occasionally following downtime and for a shorter period of time its overall lifetime is prolonged.

Furthermore, by providing two pumps, one may be configured specifically for a higher pressure operation and thus, be more resistant to this deterioration. In this regard there are radiation resistant pumps whose designs allow them to operate at increased pressures compared to conventional radiation resistant pump. "Development of diode/triode ion pump" by Bance in Vacuum volume 40 number 5 pages 457 to 460 for example discloses such a pump.

Providing the radiation resistant pumping capability as two pumps also allows the higher pressure radiation resistant pump to be attached to the chamber in a more easily accessible place allowing its replacement, if required, to be more easily achieved. In this regard, the pump that is operational at the higher vacuum will 3o be most affected by low gas conductance issues and as such will advantageously be located at a point selected for optimal or at least preferred evacuation efficiency, and this may render it inaccessible. The intermediate pump operates -4 -at a higher pressure and the gas conductance issues are therefore less and as such, accessibility can be a factor when locating the pump. Thus, the system can be optimised for both ease of intermediate pump replacement and high vacuum evacuation. Thus, although the provision of two radiation resistant pumps may be counterintuitive owing to the additional expense that this entails, the inventor recognised that by providing the pumping capability in this way, pumps that were able to bridge the pressure gap between a primary pump that is not radiation resistant and a radiation resistant pump configured for high vacuum use could be achieved in a practical manner, allowing vacuum chambers in radiation io environments to be evacuated following a vacuum loss without the need to wait for radiation levels to fall.

Although, the vacuum chamber may have a number of forms, in some embodiments it comprises an accelerator. The accelerator may be an accelerator used in research and development, in a University for example, or it may be a large accelerator used in research such as the accelerator at CERN. Alternatively, it may be an accelerator used in a hospital such as one used for radiotherapy. Such accelerators generate radiation during use, but when they are no longer active, the radiation decays over a few hours. The accelerator requires a high vacuum for operation and thus, is evacuated to a high vacuum and this vacuum is maintained using radiation resistant pumps. Where the vacuum is lost for some reasons then before the accelerator can be evacuated to the operational pressures again, there is conventionally a wait for the radiation levels to fall to an acceptable level such that personnel can access the accelerator and non-radiation resistant pumps can be used for the initial pump down. Embodiments are particularly suitable for evacuating accelerators as the vacuum can be reestablished without the need to wat for the radiation levels to fall.

3o Although the radiation resistant pumps may have a number of forms, generally they have no active electronics which are sensitive to radiation and in some embodiments they will have no rotating parts. -5 -

In some embodiments, the radiation resistant pumps comprise Getter or ion pumps. In other embodiments they may comprise diffusion pumps or vapour booster pumps. In effect, any pump that is operational without active electronics, and generally without rotating parts may be radiation resistant and may be operable within a radiation environment.

In some embodiments, said radiation resistant intermediate pump comprises a high pressure Getter pump configured for pulsed electric discharge.

The radiation resistant intermediate pump may be configured for higher pressure use such that it bridges the gap between the high operational vacuum of a more conventional ion or Getter pump and the vacuum achievable by a primary pump located at a distance from the chamber via a conduit. Such an intermediate pump may have a pulsed electric discharge which provides some control of the speed at which the active material is depleted up at these higher pressures.

In some embodiments the radiation resistant intermediate pump comprises a releasable connector at a gas input for releasably connecting to an output of the chamber. Although the intermediate pump is configured for higher pressure operation and although it is operational for less time than the high vacuum pump it may still deteriorate more quickly and need to be replaced more often than the high vacuum pump. To address this, it may be releaseably connected to the chamber rather than being built into the system such that it can be replaced if required. In effect, an easily replaceable pump may be used to bridge the gap between the pressures achievable by a primary pump outside of the radiation environment and a high vacuum radiation resistant pump.

In some embodiments, said first pressure comprises a pressure between 1mbar 3o and 10 mbar. -6 -

The primary pump may be operable to a pressure of 1 mbar and in some cases down to as low as 10-3mbar. In some embodiments, the primary pump may be operable to a pressure between 1x10-2mbar and 5 x 10-2mbar. The primary pump may be a scroll pump or it may be a roots pump with a booster.

In some embodiments, said second pressure comprises a pressure between 10-3 mbar and 10-6 mbar, preferably said second pressure comprises a pressure between 5X10-4 mbar and 5 X 10-5 mbar.

io In some embodiments, said third operational vacuum pressure comprises a pressure below 10-5 mbar, preferably below 10-7 mbar and in some embodiments below 10-3 mbar.

A second aspect provides a system comprising a chamber within a radiation shielded enclosure and a pumping set according to a first aspect, said pumping set being configured to evacuate said chamber, wherein said primary pump is located at a location shielded from said chamber and connected thereto via a conduit; and said radiation resistant pumps are located within said radiation shielded enclosure adjacent to said chamber.

In some embodiments, said primary pump is located within said enclosure within a shielded unit.

In other embodiments, said primary pump is located outside of said radiation shielded chamber, said conduit passing from said primary pump into said chamber.

The primary pump is not a radiation resistant pump and is operational at pressures which allow it to be located at a distance from the chamber being 3o pumped and this ability to function at a distance from the chamber connected thereto via a conduit allows it to be shielded from the radiation environment. In some embodiments this is achieved by placing it outside of the shielded -7 -enclosure. In other embodiments, it may be closer to the chamber within the shielded enclosure but with its own shielding to protect it from the radiation. In this regard, the shielding required to protect the pump may not be as great as the shielding required to protect personnel operating the device and thus, using additional shielding for shielding the pump within the enclosure may allow the conduit connecting it to the chamber to be shorter and thus, the speed for pumping down the chamber to be increased and them required for this reduced. Additional shielding does of course incur significant expense and thus, in some cases it may be preferred for the pump to be connected via a longer conduit and io be located outside of the enclosure with no requirement for additional shielding.

The third aspect provides a method of evacuating a chamber within a radioactive environment to a high vacuum comprising: evacuating said chamber to a first pressure using a primary pump, said primary pump being located in a region that is shielded from said radioactive environment and being connected to said chamber via a conduit; subsequently evacuating said chamber to a second pressure using a radiation resistant intermediate pump located within said radioactive environment; subsequently pumping said enclosure to an operational high vacuum pressure using a high vacuum radiation resistant pump located within said radioactive environment Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.

Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function. 3o -8 -

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which: Figure 1 and 2 show the time taken to evacuate a chamber using a screw booster 5 pump connected to the chamber via a conduit; Figure 3 shows a graph indicating the time taken to evacuate a chamber using a turbomolecular pump connected to the chamber via a conduit; Figure 4 shows a system for evacuating a chamber within a shielded enclosure according to an embodiment; io Figure 5 shows a system for evacuating an accelerator within a shielded enclosure according to an embodiment; and Figure 6 shows a system for evacuating an accelerator within a shielded enclosure according to a further embodiment.

DESCRIPTION OF THE EMBODIMENTS

Before discussing the embodiments in any more detail, first an overview will be provided.

The radiation levels within an accelerator and related devices preclude the use of non-radiation resistant pumps such as turbomolecular TMP pumps in the local high radiation environment. This results in time delays during the re-evacuation of a system following a vacuum failure which has significant impact on the instruments use. The radiation level needs to decay to a level at which both the TMP and a human are allowed to enter the system's locality before the re-evacuation procedure can recommence.

This has been addressed in embodiments by a set of pumps including a non-radiation resistant pump and two radiation resistant pumps, the two pumps being configured to operate in different pressure regions In this way a non-radiation 3o resistant primary pump located outside of the radiation environment and connected to it via a conduit, can evacuate the system to a first pressure, and then the radiation resistant pump configured to operate in the higher of the low -9 -pressure regions, in effect in a region similar to that operated in by a TMP, can evacuate the system to a lower pressure whereupon the high vacuum radiation resistant pump can become operational.

This allows a system to be restarted without the use of a TMP and avoids or at least reduces intervention by personnel in regions of increased radioactivity and improves recommissioning times and reduces operating costs.

Figure 1 shows a graph of pressure against time during the evacuation of a 50 litre section of an accelerator that is being pumped via a 20 metres conduit using a remote primary pump set comprised of a screw pump and a booster pump. As can be seen from the initial graph the time taken to evacuate the 50 litre section to below 0.1 mbar is short. However as Figure 2 shows, the time taken to evacuate the section to lower pressures is significantly longer and indeed the pump is unable to evacuate to below 10-2 mbar. Thus, as can be seen using such a primary pump at a location remote from the chamber being pumped is effective at achieving a first low pressure but is limited in the low pressure that it can achieve.

Figure 3 shows the pressure against time of the same section being pumped by a turbomolecular pump at the same location and via the same conduit. As can be seen although this pump is able to eventually achieve a lower pressure, due at least in part to the low conductance of the gas at these lower pressures, the time to do so is prohibitively long. Thus, using such a pump at a remote location will not be effective at reducing the recommissioning times for such a system following a vacuum failure.

The above figures illustrate the problem associated with evacuating a radioactive environment to a high vacuum using a remote pump. In particular, although the 3o primary pump of this example located outside of the radioactive environment is able to achieve a first low pressure of between 1 mbar and 10-2 mbar, evacuating -10 -the chamber to a lower pressure that may be required for a conventional ion or Getter pump to start operation is more challenging.

Embodiments have addressed this by providing two radiation resistant pumps within the radioactive environment, one of which is operable in the pressure region that is usually covered by a turbomolecular pump. In particular, a radiation resistant pump that is able to pump at pressures above 10-3 mbar down to 10-4 or even 10-5 mbar is used and then a conventional radiation resistant high vacuum pump takes over.

In this regard, there are diffusion and vapour booster pumps that are radiation resistant and able to pump in this intermediate range and there are also modern ion pumps, for example ones such as the Getter pump described describes in Russian patent application 2017126531 and the ion pump described in patent application US 2018/0068836 that are able to pump in these higher pressure regions.

Furthermore, by providing the radiation resistant pumping capability in the form of multiple radiation resistant pumps the lifetime of the pumping system may be extended. The system is configured such that the pump operating at the higher pressure is operational for less time that is only during the pump down operation, while the high vacuum pump is operable for longer periods during normal operation but with a lower load due to the decreased pressure. Thus, although the higher pressure pump may be depleted more quickly when operational, its actual lifetime may be extended due to its short operational periods, while the high vacuum radiation resistant pump's lifetime will also be protected by only operating it at the higher vacuums.

The radiation resistant pumps may be Getter pumps or ion pumps. Ion pumps, 3o also commonly known as ion getter pumps or sputter ion pumps comprise an array of cylindrical anode tubes arranged between two cathode plates. An electrical potential is applied between the anode and cathode at the same time as magnets on opposite sides of the cathode plates generate a magnetic field aligned with the axes of the anode cylinders. The ion pump operates by trapping electrons within the cylindrical anodes, gas molecule entering one of the anodes being struck by the trapped electrons causing the molecule to ionize. The resulting positively charged ion is accelerated by the electrical potential towards one of the cathode plates leaving the stripped electrons in the cylindrical anode to be used for further ionization of other gas molecules. The positivity charged ion is trapped at the oppositely charged electrode, an event in which it causes material from the cathode to be sputtered into the vacuum chamber of the pump.

The sputtered material coats surfaces within the anode and acts to capture additional molecules moving within the pump. It is the sputtering effect that leads to a finite lifetime of the pump as the surface being sputtered will gradually be depleted. As can be understood operation at higher pressures with more molecules will increase the rate at which the active surface is used up. Operating at higher pressures for reduced amounts of time and using specially designed pumps that mitigate this depletion effect will enable a pump operating at these higher pressures to have an improved lifetime. Furthermore, where it is recognised that the higher pressure radiation resistant pump may have a reduced lifetime, it may be configured and located such that it is easily replaceable, while the higher vacuum pump may be located at a more inaccessible location that is optimised for evacuation of the chamber, which is important when operating at pressures where gas conductance is very low.

Figure 4 shows a schematic diagram of a system according to an embodiment.

The system comprises a primary pump 10 that is connected via conduit 12 to chamber 40 that is to be evacuated. Pump 10 is located outside the shielded radiation enclosure 50. Within the shielded environment there is a high vacuum Getter pump 30 and an intermediate Getter pump 20. The radiation enclosure 50 comprises a door 52 allowing access to the enclosure by personnel when 3o radiation levels have fallen sufficiently to make this viable.

-12 -The primary pump 10 is operable to pump the chamber 40 down from atmosphere to a first pressure in the region of 10-2 mbar. The intermediate pump 20 then becomes active and pumps chamber 40 from the first pressure to a second intermediate pressure below 10-5 mbar whereupon the high vacuum pump 30 takes over and pumps it down to the operational pressure and maintains it at this pressure. In this way the time of operation of both pumps 10 and 20 are reduced when compared to that of pump 30.

Figure 5 shows an alternative embodiment where an accelerator 42 with target T io is being evacuated. Although in this embodiment the accelerator is evacuated by one set of pumps, in some embodiments, where the accelerator is a larger device, it may be divided into sections with each section having its own set of pumps and being evacuated separately. Operation of this set of pumps is similar to that of Figure 4. That is evacuation following a vacuum failure proceeds by evacuation to a first pressure using primary pump 10, then evacuation to an intermediate low pressure is achieved using pump 20, whereupon the final evacuation to the high vacuum occurs using pump 30, this vacuum then being maintained by this pump during operation of the accelerator. As can be seen pump 30 is within the main part of the accelerator and is difficult to access but owing to its high vacuum use has a long lifetime. Pump 20 is more accessible and may have a shorter lifetime but its replacement is more straightforward.

Figure 6 shows an alternative embodiment where the primary pump 10 is within the shielded enclosure 50, but shielded from the high radiation levels by its own shielded compartment 60. This allows the conduit 12 to be shorter and recognises that the radiation levels at which a non-radiation resistant pump may safely operate are higher than those that a person may safely enter. Thus, the pump can be located at a region of increased radiation and the shielding used to protect it may be less than that used for enclosure 50.

Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the -13 -invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

-14 -

REFERENCE SIGNS

primary pump 12 conduit 20 intermediate radiation resistant pump high vacuum radiation resistant pump vacuum chamber 42 accelerator radiation shielded enclosure 52 door shielded compartment T target

Claims

-15 -CLAIMS1. A set of pumps for providing a high vacuum within a radioactive environment, said set of pumps comprising: a primary pump configured for operation outside of said radioactive environment and configured to connect to a chamber within said radioactive environment via a conduit, said primary pump being configured to evacuate said chamber to a first pressure; a radiation resistant intermediate pump configured for operation within said to radioactive environment and configured to evacuate said chamber from said first pressure to a second lower pressure; and a radiation resistant high vacuum pump configured for operation within said radioactive environment and configured to evacuate said chamber from said second lower pressure to a third operational vacuum pressure.
2. A set of pumps according to claim 1, wherein said chamber comprises an accelerator.
3. A set of pumps according to any preceding claim, wherein said radiation resistant pumps comprise pumps having no active electronics.
4. A set of pumps according to any preceding claim, wherein said radiation resistant pumps comprise pumps having no moving parts.
5. A set of pumps according to any preceding claim, wherein said radiation resistant pumps comprise Getter pumps.
6. A set of pumps according to claim 5, wherein said radiation resistant intermediate pump comprises a high pressure Getter pump configured for pulsed 3o electric discharge.
-16 - 7. A set of pumps according to claim 5 or 6, wherein said radiation resistant intermediate pump comprises a releasable connector for connecting to said chamber.
8. A set of pumps according to any preceding claim, wherein said first pressure comprises a pressure between 1 mbar and 10-3 mbar.
9. A set of pumps according to claim 8, wherein said first pressure comprises a pressure between 1 X 10-2 mbar and 5 X 10-2 mbar.
10. A set of pumps according to any preceding claim, wherein said second pressure comprises a pressure between 10-3 mbarand 10-6 mbar.
11. A set of pumps according to claim 10, wherein said second pressure 15 comprises a pressure between 5X10-4 mbarand 5 X 10-5 mbar.
12. A set of pumps according to any preceding claim, wherein said third operational vacuum pressure comprises a pressure below 10-5 mbar.
13. A set of pumps according to claim 12, wherein said third operational vacuum pressure comprises a pressure below 10 -7 mbar.
14. A system comprising a chamber within a radiation shielded enclosure and a set of pumps according to any one of claims 1 to 13, said set of pumps being 25 configured to evacuate said chamber, wherein said primary pump is located at a location shielded from said chamber and connected thereto via a conduit; and said radiation resistant pumps are located within said radiation shielded enclosure adjacent to said chamber.
15. A system according to claim 14, wherein said chamber comprises an accelerator.
-17 - 16. A system according to claim 14 or 15, wherein said primary pump is located within said enclosure within a shielded unit.
17. A system according to claim 14 or 15, wherein said primary pump is located outside of said radiation shielded enclosure, said conduit passing from said primary pump into said enclosure.
18. A method of evacuating a chamber within a radioactive environment to a io high vacuum comprising: evacuating said chamber to a first pressure using a primary pump, said primary pump being located in a region that is shielded from said radioactive environment and being connected to said chamber via a conduit; subsequently evacuating said chamber to a second pressure using a radiation resistant intermediate pump located within said radioactive environment; subsequently evacuating said chamber to an operational high vacuum pressure using a high vacuum radiation resistant pump located within said radioactive environment.
19. A method according to claim 18, wherein said first pressure comprises a pressure between 1m bar and 10-3 mbar.
20. A method according to claim 18 or 19, wherein said first pressure comprises a pressure between 1 X 10-2 mbar and 5 X 10-2 mbar.
21. A method according to any one of claims 18 to 20, wherein said second pressure comprises a pressure between 10-3 mbarand 10-6 mbar.
22. A method according to claim 21, wherein said second pressure comprises 30 a pressure between 5X10A mbar and 5 X 10-5 mbar.
-18 - 23. A method according to any one of claims 18 to 22, wherein said third operational vacuum pressure comprises a pressure below 10-5 mbar.
24. A method according to claim 18 to 23, wherein said third operational vacuum pressure comprises a pressure below 10 -7 mbar.