JPS6350554B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to cryopumps. Specifically, the present invention relates to an improvement of a cryopump, and is a cryopump with an improved chevron structure that reduces the heat load on the cryopanel caused by adhesion of spatter particles caused by an ion beam or the like to the blackened chevron. Regarding.

In general, in a cryopump that evacuates gas using a cryopanel of 20K or less, it is important to keep the heat load on the cryopanel sufficiently small in order to keep the temperature of the cryopanel low. For this reason, a blackened chevron with an emissivity close to 1 and an opening angle of 90 to 120 degrees is installed on the front of the cryopanel to absorb room temperature radiation from equipment and walls outside the cryopump. This reduces the heat load due to radiation reaching the cryopanel. The opening angle and size of this chevron are selected so as to suppress the heat load due to the radiation and to increase the conductance to the gas to be exhausted as much as possible.

When such a cryopump is used as a vacuum pump for a nuclear fusion experimental device, an experimental accelerator, an industrial ion beam generator, an industrial electron beam generator, etc., it is necessary to use the cryopump for a charged particle beam in order to improve the pumping efficiency. It is desirable to install the chevron near the space through which the neutral particle beam passes, so that the chevron faces the beam. A conventional method of installing a chevron will be explained based on FIG. 1 shows the entire cryopump. Reference numeral 2 denotes a source of spatter particles caused by beam collision, such as a beam limiter or a beam target, and its position is near the periphery of the beam axis 3.

The cryopump 1 is composed of a chevron 4, a cryopanel 5 that evacuates by adsorbing gas, and a shield plate 6 that prevents room temperature radiation from entering from the back and side surfaces thereof.

If such a cryopump is placed near the beam passing position, spatter particles emitted from the beam limiter or beam target will adhere to the chevron. 7 indicates the surface of the chevron to which the sputter particles adhere. In addition, since almost all of the sputter particles travel straight, the adhesion surface 7
is equal to the surface of all the faces of Chevron that directly looks at the source of the spatter particles. Since the emissivity of the surface to which the spatter particles are attached decreases, the reflectance increases, and when room temperature radiation from the outside of the cryopump enters the surface to which the sputter particles are attached, the thermal load on the cryopanel 5 increases. The radiation path 8 shows an example of such radiation.

On the other hand, it has been found that most of the heat load on the cryopanel due to radiation at room temperature is due to radiation that reaches the cryopanel after just one reflection from Chevron. Therefore, if spatter particles adhere to a surface that can reach the cryopanel with only one reflection, the thermal load on the cryopanel increases significantly.
FIG. 2 shows the areas of the reflective surface where such radiation can exist. Note that parts given the same reference numerals as in FIG. 1 indicate the same or corresponding parts. In this figure, 9 and 10 are examples of optical paths of radiation in which radiation from the outside reaches the cryopanel with only one reflection. Reflective surface 12 indicates a region of the reflective surface where such an optical path can occur. On the other hand, radiation incident on a surface other than the reflecting surface 12 cannot reach the cryopanel without experiencing two or more reflections. The optical path 11 shows an example in which radiation incident on a surface other than the reflecting surface 12 reaches the cryopanel after experiencing two reflections.

Comparing Figures 1 and 2, we find that in the example in Figure 1, the area of the part where spatter particles adhere is not only large in the chevron located above the beam axis, but also in the example in Figure 2. Sputter particles adhere to a portion of the reflective surface 12. Therefore, the heat load caused by the radiation reaching the cryopanel by one reflection increases significantly. In addition, even for radiation that is reflected more than once, the drop in emissivity due to adhesion of sputter particles contributes to an increase in reflectance, leading to an increase in the heat load on the cryopanel. The load will increase. Therefore, in FIG. 1, the amount of radiant heat passing through the chevrons located above the beam axis is significantly larger than that located below.

To summarize the above, when a conventional cryopump like the example shown in Figure 1 is used for a long time, the emissivity of Chevron decreases due to spatter particles.
The drawback was that the heat load on the cryopump increased significantly and the vacuum evacuation function was no longer possible.

The present invention has been made to improve such deficiencies.

The present invention will be explained with reference to FIG. In addition, the first
Parts with the same reference numerals as in the figures indicate the same or corresponding parts. The opening direction of the chevron 4 is reversed above and below the beam axis 3. Therefore, the area of the surface 7 to which the spatter particles adhere is smaller than in the case of FIG. 1.

Note that the central surface 12 of the chevron facing the beam axis has a large area where spatter particles adhere, but since this part has no effect on the radiation reaching the cryopanel, the emissivity has decreased. However, it is not a problem. Furthermore,
1 in Figure 2, where room temperature radiation can reach the cryopanel with only one reflection from Chevron.
Sputter particles do not adhere to the surface corresponding to 2. Therefore, the thermal load on the cryopanel due to adhesion of spatter particles can be significantly reduced.

As described above, according to the present invention, when a cryopump is used in an environment where charged particle beams, neutral particle beams, etc. It is possible to realize a cryopump in which pumping performance does not deteriorate over a long period of time.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between a conventional cryopump and a spatter particle generation source, and FIG. 2 shows a surface where room temperature radiation can reach the cryopanel with only one reflection. FIG. 3 shows one embodiment of the invention. 1... Cryopump, 2... Source of spatter particles, 3... Beam axis, 4... Chevron, 5
...Cryopanel, 6...Shield plate, 7...
Adhering surface of spatter particles (dotted line area), 8...An example of room temperature radiation being reflected by Chevron and reaching the cryopanel, 9...Room temperature radiation from outside the system reaching the cryopanel after just one reflection from Chevron An example, 10... An example of room temperature radiation from the outside reaching the cryopanel after being reflected once by Chevron, 11... An example of room temperature radiation from the outside reaching the cryopanel by reflecting twice by Chevron, 12... A surface where room temperature radiation from the outside can reach the cryopanel by just one reflection from Chevron.

Claims (1)

[Claims] 1. A cryopump placed around a charged particle beam or a neutral particle beam, which is characterized in that the direction of the aperture of the cheepron is reversed above and below the beam axis.