Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Patent
1981-06-16
1983-08-02
Anderson, Bruce C.
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
315 17, H01J 326
Patent
active
043968414
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the accelerator technology, and more particularly to devices for scanning a beam of charged particles.
DESCRIPTION OF THE PRIOR ART
Unlike accelerators intended for scientific investigations, industrial accelerators used, for example, for radiation treating of materials must provide irradiation fields of a considerable extent equal at least to the width of the material to be irradiated. Treating the entire surface of the material is accomplished by transferring the material lengthwise through the field of irradiation.
Forming extended fields of irradiation by forming a beam of an appropriate cross-section immediately in the accelerating structure of an accelerator presents considerable difficulties and has practically passed out of existence by now. There is a wide use of the method for forming extended fields of irradiation based on scanning a beam of charged particles, i.e. displacing a beam of a small cross-section over the surface to be irradiated by deflecting it by a time modulated field, such as magnetic field, for example.
There is known a device for scanning a beam of charged particles (Cf., for example, an article by Akulov V. V. et al. "Promyshlennye uskoriteli serii "Electron" dlja radiatsionnoi himii", NIIEFA pre-print No. -0198, Leningrad, 1974, p. II), comprising a scanning electromagnet and a scanning current generator to energize this electromagnet. The field generated by current flowing through the windings of the electromagnet provides periodical deflection of the beam of charged particles along the exit window of the vacuum chamber, through which the charged particles are rejected into the atmosphere.
Two criteria are considered to adopt the minimal frequency of the scanning current. Firstly, the beam paths on the material to be irradiated must overlap, upon motion of the latter, or at least contact each other, otherwise different points of the material to be irradiated would obtain dissimilar doze of irradiation. From this it follows, that the higher the speed of motion of the material to be irradiated and the lesser the diameter of the beam, the greater the frequency of its scanning.
Secondly, upon passing of the charged particles through the foil of the exit window, a portion of their energy is lost and is transformed into heat which heats the foil. When the beam passes over the foil, the local temperature of the foil in the site where the beam passes through it exceeds the average temperature of the foil so greater, the lesser the speed of displacement of the beam over the foil is. Local overheating of the foil is dangerous considering both the possibility of its melting and the everlasting heat deformations of the foil resulting in its rupture even at temperatures much below its melting point. Therefore, to prevent the foil from local overheating the frequency of scanning must be as high as possible.
With a frequency of scanning high enough, when there is practically no local overheating, the steady-state temperature of the foil will depend on the ratio of the charged particle beam current to the foil area over which the beam is scattered.
The beam path on the exit window, when using the device described, represents a narrow band having a width equal to the beam diameter and a length determined by the size of the objects to be irradiated. This being so, inasmuch as the current density on the beam axis is much more higher than that on its circumference, it is the mid-width portion of the foil having the most difficult heat removal that carries the major heat load. Uneven distribution of heat load over the foil can result in overheating of some spots of the foil and its malfunction.
Moreover, with increasing beam current density the heat load upon the foil increases and, in order to decrease it, one needs to enlarge the area of the exit window over which the beam is scattered. In the above device, however, because of the beam being deflected in one direction only, the operating area of the foil, i.e. the area over which the
REFERENCES:
patent: 2977500 (1961-03-01), Boeker
patent: 3066238 (1962-11-01), Arndt
patent: 3109931 (1963-11-01), Knowlton et al.
Razin Gennady I.
Valtman Dmitry S.
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