Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
2001-12-11
2003-05-27
Nguyen, Kiet T. (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S492300
Reexamination Certificate
active
06570162
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electron beam irradiating apparatus used for processing exhaust gas and the like discharged from a thermal power plant, for example, or an electron beam irradiating apparatus for large current irradiation used to refine the quality of substances such as cross-linking of resins. The present invention particularly applies to a method and apparatus for irradiating an electron beam in which the electron beam is moved in a scanning motion while being emitted into the atmosphere through a window foil for ejecting electrons.
BACKGROUND ART
It is currently thought that SOx, NOx, and other components found in flue gas that is discharged from thermal power plants and the like is the cause of such global problems as global warming and acid rain that have been linked to air pollution. Methods of desulfurization and denitration remove these toxic components SOx, NOx, and the like through the irradiation of an electron beam on the flue gas are well known in the art.
FIG. 1
shows an example of an electron beam irradiating apparatus used for the above application. This device used for processing flue gas mainly comprises a power source
10
for generating a DC high voltage; an electron beam irradiating apparatus
11
for irradiating an electron beam on the flue gas; and a channel
19
through which the flue gas is transported. The channel
19
is disposed along a window foil
15
that serves as an outlet for the electron beam irradiated from the electron beam irradiating apparatus
11
. The window foil
15
is formed of a thin plate constructed of titanium or the like. An electron beam emitted externally through the window foil
15
irradiates such molecules as oxygen (O
2
) and water vapor (H
2
O) in the flue gas. These molecules become such highly oxidative radicals as OH, O, and HO
2
. These radicals oxidize the noxious components of SOx, NOx, and the like and generate the intermediate products of sulfuric acid and nitric acid. These intermediate products react with ammonia gas that has been introduced in advance to produce ammonium sulfate and ammonium nitrate, which can be recovered and used in fertilizer. Accordingly, this type of exhaust gas processing system can remove harmful components, such as SOx and NOx from the flue gas and can recover such useful materials as ammonium sulfate and ammonium nitrate as by-products.
The electron beam irradiating apparatus
11
comprises a thermionic generator
12
such as a thermionic filament; an accelerating tube for accelerating the electrons emitted from the thermionic generator
12
; a deflecting coil
16
(electromagnet) for deflecting the electron beam in the widthwise direction by applying a magnetic field using a square wave current; and a scanning coil
17
(electromagnet) for moving the controlled electron beam in a lengthwise scanning direction by applying a magnetic field to the electron beam. Of these, the electron beam generator, accelerating electrode, and deflecting/scanning magnetic poles are accommodated in vacuum vessels
18
a
and
18
b
and maintained in a high vacuum atmosphere of approximately 10
−6
Pa. By supplying an electric current to the deflecting coil
16
and scanning coil
17
and forming a magnetic field using the electromagnets, the high-energy electron beam is injected in a prescribed range through the window foil
15
onto a prescribed area of the channel
19
, while deflecting the beam and moving the same in a scanning direction.
As described above, this type of electron beam irradiating apparatus must eject an electron beam highly accelerated in a vacuum environment into the atmosphere. Generally, in order to achieve a high electron transmission efficiency when ejecting an electron beam, a window foil formed of a pure titanium membrane or a titanium alloy membrane having a thickness of several tens of micrometers, for example 40 &mgr;m, is used. This window foil is mounted on the end of the vacuum vessel
18
a
via a mounting flange. The window foil is large, for example 3×0.6 meters. A pressure of approximately 1,000 hPa, which is atmospheric pressure, is applied to the outer surface of the window foil having an inner vacuum pressure in the vacuum vessel of 10
−6
Pa.
Next, deflection and scanning of the electron beam will be described.
A triangular wave generator
22
supplies a triangular wave current as shown in
FIG. 2A
to the scanning coil
17
in order to move the electron beam to scan in the Y-direction shown in
FIG. 3. A
square wave generator
21
supplies a square wave current as shown in
FIG. 2B
that is synchronized to the triangular wave to the deflecting coil
16
in order to move the electron beam to scan in the X-direction orthogonal to the Y-direction shown in FIG.
3
. As both coils
16
and
17
become excited by the currents, the electron beam is accelerated by an accelerating tube
13
and enters the deflection/scanning section to scan along a rectangular path as shown in FIG.
3
. The electron beam passes through the window foil
15
and irradiates the target matter.
Here, the path Y
1
shown in
FIG. 3
is formed when the square wave current between times T
1
and T
2
in
FIGS. 2A and 2B
is fixed at +Q and while the current from the triangular wave generator changes from +P to −P. The path X
1
is formed when the triangular wave current peaks at −P (time T
2
) and the square wave current changes instantaneously from +Q to −Q. Similarly, the path Y
2
is formed when the triangular wave current changes from −P to +P between times T
2
and T
3
, while the path X
2
is formed at time T
3
, when the square wave current changes instantaneously from −Q to +Q.
FIG. 4
shows the magnetic hysteresis characteristics of the scanning coil
17
. When the scanning coil
17
moves the electron beam in the scanning direction, the relationship between the current I and the magnetic flux density B of the scanning coil
17
has hysteresis characteristics at the reversing points in both Y-directions, or in terms of the scanning coil current, at the point of transition when the beam point on the triangular wave current begins to drop or rise. At these points, the flux density B cannot follow the current I, thereby slowing down the scanning rate of the electron beam. Hence, whenever the peak values (+P and −P) of the triangular wave current I enter the saturation region of the flux density B, the flux density B does not change even when the size of the current I changes, thereby causing the scanning rate of the electron beam to change. Accordingly, the amount of electron beam irradiation becomes uneven.
Referring back to the hysteresis characteristics of I and B in
FIG. 4
, the flux density B does not drop or rise in proportion to rises and falls in the current I, but rather remains relatively uniform for a short time. As a result, the electron beam stagnates during this period. Therefore, the dose at the starting points of each Y scan indicated with hatching in
FIG. 3
is increased, causing a non-uniform distribution.
FIG. 5A
is a graph plotting the distribution of electron dose along the Y-direction during this time. The graph shows the combined state of Y
1
and Y
2
. As can be seen, there is an unbalanced amount of stress added to the window foil in the irradiating window. This stress causes the temperature at specific areas of the foil surface to rise abnormally, thereby further decreasing the life of the window foil. Further, a uniform electron beam is not applied to the targeted matter beneath the window foil.
Therefore, a method has been proposed for achieving uniformity in the electron irradiation dose that considers the hysteresis delay in the flux density during the drop of the triangular wave. This method performs irradiation with a delta function step (superimposing a kick pulse) near the peak of the triangular wave.
However, simply using a triangular wave with a superimposed kick pulse to even the electron dose does not cancel the non-uniformity of th
Kiga Masahiro
Nakamura Atsushi
Ebara Corporation
Nguyen Kiet T.
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