JPH11224630A - Photo-electron generation method and device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain low emittance when generated electrons are RF accelerated. SOLUTION: When photo-electrons are generated from the front face of a photo cathode 12 by radiating a light beam 42 from a slant direction against the surface of the photo cathode 12, the cross section of a light beam 42 being incident on the photo cathode 12 is formed into elliptical short in a beam incident direction so that the light beam shape on the surface of the photo cathode 12 becomes a perfect circle or an approximate circle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for generating photoelectrons from the front of a photocathode by irradiating a photocathode surface with a light beam from an oblique direction. The present invention relates to a method and an apparatus for generating photoelectrons capable of obtaining a low emittance when accelerated by radio frequency (RF).

[0002]

2. Description of the Related Art As electron sources in electron accelerators including linear accelerators such as linacs and rotary accelerators such as cyclotrons, thermions generated from a thermal cathode have conventionally been generally used. pulse,
It was difficult to reduce the emittance.

Accordingly, in recent years, a method utilizing photoelectrons generated from a photocathode has been used because it is possible to shorten the generated electron flux to a short pulse. As one of such photoelectron generators, there is a radio frequency (RF) gun 10 for accelerating photoelectrons generated by laser irradiation with a microwave, as shown in FIG.

[0004] In this RF gun 10, a short pulse laser beam 8 is placed on the surface of a photocathode 12 as a target.
Irradiates the photoelectrons from the surface of the photocathode 12.

[0005]

However, usually,
Since the laser beam 8 is not perpendicular to the surface of the photocathode 12 but is obliquely incident at a certain incident angle θ,
For example, if the cross section of the laser beam 8 is circular,
As shown in FIG. 8, the light beam shape on the surface of the photocathode 12 becomes an elliptical shape that is long in the beam incident direction (vertical direction in the figure). When the generated electrons are accelerated by RF, a low emittance is obtained. There was a problem that it could not be done.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems. When the generated electrons are accelerated by RF,
It is an object to obtain a low emittance.

[0007]

According to the present invention, there is provided a photocathode which emits a light beam from an oblique direction to a photocathode surface to generate photoelectrons from the front of the photocathode. The above-mentioned problem has been solved by making the cross-sectional shape of the laser beam into an elliptical shape in which the beam incident direction is short and making the light beam shape on the photocathode surface a perfect circle or a substantially circular shape.

According to the present invention, there is also provided a similar photoelectron generator, wherein a beam elliptic means for making a cross-sectional shape of a light beam incident on the photocathode an elliptical beam with a short beam incident direction is provided. The above-mentioned problem is also solved by making the shape of the light beam of a circle or a substantially circle.

Further, a means for monitoring a sectional shape of the light beam immediately before the photocathode is provided.

Further, the beam elliptical means is a Brewster prism.

Furthermore, at least four Brewster prisms are used so that the optical axis of the light beam does not change before and after the prism group.

Further, a plurality of prisms are connected by a gear or a belt so that the angle can be adjusted in conjunction with each other.

Further, the beam elliptical means is at least one pair of cylindrical lenses so that the optical axis of the light beam does not change before and after the cylindrical lenses.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings.

As shown in FIG. 1, a laser optical system for photocathode irradiation according to the present embodiment includes a laser light source 40 for generating a laser pulse beam 42 having a circular cross-sectional shape, and a laser beam whose direction has been changed by a reflection mirror 44, for example. A transmission optical system 50 for focusing the pulse beam 42 and entering the photocathode 12 of the RF gun 10 from an oblique direction is mainly provided.

The transmission optical system 50 is, for example, a laser pulse beam having a sectional intensity distribution with respect to a position r in a direction perpendicular to the beam progression, as shown in FIG. A spatial filter (for example, a pinhole aperture) 52 for making 42 a distribution as shown in FIG. 2B, a beam expander 54 for expanding the beam diameter of the laser pulse passing through the spatial filter 52, A mask 56 for shielding both end portions of the intensity distribution shape as shown in FIG. 2C expanded by the beam expander 54, and a cross section having passed through an opening of the mask 56 and having a substantially rectangular cross section. A convex lens 58 for converging a laser pulse beam having a shape and irradiating the photocathode 12 with a narrow sectional intensity distribution as shown in FIG. By changing the direction of the laser beam 42 that is focused by the convex lens 58, and a reflecting mirror 60 for irradiating the photo cathode 12 of the RF gun 10.

Conventionally, in such a transmission optical system 50, the cross section of the laser pulse beam 42 is circular at any location, whereas in the present invention, the laser pulse beam 42 is located at an arbitrary location in the transmission optical system 50. As shown in FIG. 3, a beam elliptical device 68 including, for example, two Brewster prisms 70 and 72 is inserted into the laser pulse beam 42.
Is an elliptical shape in which the beam incident direction (the vertical direction in FIG. 3) is short.

For example, one Brewster prism 7
When a laser beam having a diameter a is perpendicularly incident on the prism 70 using 0, the beam diameter b in the vertical direction in FIG. 3 at the exit side of the prism 70 is expressed by the following equation.

B = (cos θb / cos θ) * a (1)

Here, θb is the Brewster angle, and θ is the vertex angle of the prism. In this case, since the width in the horizontal direction remains a, the ellipticity α is expressed by the following equation.

Α = b / a = (cos θb / cos θ) (2)

Similarly, as shown in FIG. 3, when two Brewster prisms 70 and 72 are used, the ellipticity α
Is as shown in the following equation, and an elliptical beam having a higher ellipticity can be obtained.

Α = c / a = (cos θb / cos θ) ² (3)

Here, c is two prisms 70 and 72
Is the width of the emitted laser beam (in the short axis direction) after exiting.

The number of Brewster prisms to be inserted into the transmission optical system 50 is not limited to one or two. For example, four Brewster prisms can be used in combination as shown in FIG. In this case, the ellipticity α is as shown in the following equation.

Α = e / a = (cos θb / cos θ) ⁴ (4)

Here, e is the four prisms 70 and 7
It is the width of the emitted laser beam (short axis direction) after exiting from 2, 74 and 76.

For example, when the refractive index of the glass used for the prism is n = 1.5, the Brewster angle θb = 56 °, and when the apex angle of the prism θ = 34 °, the ellipticity α is 0.207. Become.

As shown in FIG. 4, when four prisms are combined, the outgoing optical axis can be returned to the same as the incident optical axis, and the insertion and removal of the prism from the conventional transmission optical system becomes easy. .

The number of prisms is not limited to two or four, but may be one, three, five or more.

In the present embodiment, since the Brewster prism is used, the flatness can be easily changed.

When a plurality of prisms are used as described above, fine adjustment of the angle is difficult, but as shown in FIG. 5, a prism angle interlocking device 78 having gears 80 and 82.
Are provided, and the prisms 70 and 72 are connected by the gears 80 and 82 so that they can be interlocked with each other when the angle is finely adjusted, thereby facilitating the adjustment. In the example shown in FIG. 5, a gear is used, but a combination of a pulley and a belt can be used instead of the gear.

A beam ellipticizer for making the cross-sectional shape of the light beam incident on the photocathode 12 into an elliptical shape in which the beam incident direction is short is not limited to the one using the Brewster prism. Illustrated beam elliptical device 8
4, for example, two cylindrical lenses 86, 8
8 can be combined.

In this case, since the optical axis does not change, the arrangement with respect to the conventional transmission optical system is easy.

Further, as shown in FIG.
A beam cross-section monitor device 90 having a half mirror 92 is provided immediately before 2 and the light beam extracted by the half mirror 92 is projected onto a screen 94,
It is also possible to monitor the cross-sectional shape of the light beam applied to the photocathode 12.

In this way, the cross-sectional shape of the light beam on the surface of the photocathode 12 can be surely approximated to a circle.

It is to be noted that the spatial filter 52 is a Gaussian filter having a different transmittance depending on the position in the direction perpendicular to the traveling direction of the beam, and the sectional shape of the laser beam passing through the spatial filter 52 is shaped into a clean Gaussian distribution. Can also. The type of the spatial filter inserted into the optical path is not limited to the pinball aperture or the Gaussian filter, and the cross-sectional shape of the laser beam can be shaped into a top flat shape or another free shape.

In the above embodiment, the present invention is applied to an RF gun for injecting photoelectrons into an electron accelerator such as a linac, but the present invention is not limited to this.

[0039]

According to the present invention, a light beam to be irradiated becomes a perfect circle or a substantially circular shape on the surface of the photocathode. Therefore, when the generated electrons are accelerated by RF, for example, a low emittance can be obtained.

[Brief description of the drawings]

FIG. 1 is an optical path diagram showing an overall configuration of a laser optical system for photocathode irradiation according to an embodiment of the present invention.

FIG. 2 is a diagram showing a change in beam intensity distribution at various points in the laser optical system for photocathode irradiation.

FIG. 3 is a plan view and a cross-sectional view showing a configuration example and operation of a beam elliptical apparatus using two Brewster prisms according to the present invention inserted into a transmission optical system of the laser optical system for photocathode irradiation.

FIG. 4 is a plan view and a cross-sectional view showing a configuration example of a beam elliptical apparatus similarly using four prisms.

FIG. 5 is a plan view showing a configuration example of the prism angle interlocking device.

FIG. 6 is a plan view showing a configuration example of a beam ellipse using the same pair of cylindrical lenses.

FIG. 7 is an optical path diagram showing a configuration example of a beam cross section monitoring apparatus.

FIG. 8 is a cross-sectional view showing a state in which laser light is incident on a conventional RF gun.

[Explanation of symbols]

 Reference Signs List 8 laser light 10 radio frequency (RF) gun 12 photocathode 40 laser light source 42 laser pulse beam 44 reflecting mirror 50 transmission optical system 52 spatial filter 54 beam expander 56 mask 58 convex lens 68 84: Beam cross section elliptical apparatus 70, 72, 74, 76: Brewster prism 78: Prism angle interlocking apparatus 80, 82: Gear 86, 88: Cylindrical lens 90: Beam cross section monitoring apparatus 92: Half mirror 94: Screen

Claims (7)

[Claims] When a light beam is irradiated from an oblique direction to a photocathode surface to generate photoelectrons from the front of the photocathode, a cross-sectional shape of the light beam incident on the photocathode is changed in a beam incident direction. Wherein the shape of the light beam on the photocathode surface is a perfect circle or a substantially circular shape. 2. A photoelectron generator for irradiating a photocathode surface with a light beam from an oblique direction to generate photoelectrons from the front of the photocathode. A photoelectron generator, comprising: a beam elliptic unit having an elliptical shape having a short incident direction, wherein a light beam shape on a photocathode surface is a perfect circle or a substantially circular shape. 3. The photoelectron generator according to claim 2, further comprising means for monitoring a cross-sectional shape of the light beam immediately before the photocathode. 4. The photoelectron generator according to claim 2, wherein said beam elliptical means is a Brewster prism. 5. The photoelectron generator according to claim 4, wherein at least four Brewster prisms are used, and the optical axis of the light beam does not change before and after the group of prisms. 6. The photoelectron generator according to claim 4, wherein the plurality of prisms are connected by a gear or a belt, and the angles can be adjusted in conjunction with each other. 7. The photoelectron generator according to claim 2, wherein said beam elliptical means is at least one pair of cylindrical lenses.