JPS63241375A - Current measurement of electron beam - Google Patents

Abstract

PURPOSE:To enable measurement of intensity and distribution of current in an electron beam safely irrelevant to electromagnetic noise and light noise, by detecting Cherenkov light generated with an optical fiber irradiated by a relativistic electron beam. CONSTITUTION:A 20cm-long part of an optical fiber 1 with the core dia. of 600mum and fiber dia. of 750mum at one end thereof is housed into a dark box 7 and is irradiated with an electron beam 20 at right angles to the length of the optical fiber 1. When a free electron 2 has energy exceeding a fixed value, Cherenkov light is generated in the optical fiber 1 and propagates through a core 1a of a quartz optical fiber 1. Then, the light is detected with a PIN photo diode 5 connected to the other end of the optical fiber 17 meters ahead and an output thereof is inputted into a CRT 6. This allows the detection of a current value of the electron beam by checking the intensity of the light. This also enables the detection of a current distribution by checking an output of the light with the positioning of the optical fiber partially at an electron beam projection area.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for measuring the current value of a relativistic electron beam.

[Prior art and problems to be solved by the invention] There is a desire to detect the current value of an electron beam in order to know its energy or for other purposes, but there has been no effective method to measure this. , which was not possible, especially for high-velocity relativistic electron beams.

The present invention was made in view of these circumstances,
Current intensity of relativistic electron beam detected by Cherenkov light generated in optical fiber.

This makes it possible to measure distribution.

[Means for solving problems]

A current measuring method of an electron beam according to the present invention is a method for measuring a current value of a relativistic electron beam, in which the electron beam is irradiated onto an optical fiber to generate Cerenkov light, and the Cherenkov light is generated at the end of the optical fiber. It is characterized by detecting the electron beam and determining the current of the electron beam from this light.

[Effect]

When an optical fiber is irradiated with a relativistic electron beam, Cerenkov light is generated within the optical fiber. This light is totally reflected on the circumferential surface of the optical fiber and is confined within the optical fiber. This is detected at the end of the optical fiber. Since the detected light intensity includes information regarding the current of the electron beam, the current value 1 distribution can be determined from this information.

[Principle of the invention]

First, the principle of the present invention will be explained.

Now, as shown in FIG. 1, free electrons 2 are irradiated onto the optical fiber 1 at right angles to the longitudinal direction of the optical fiber 1. When these free electrons have energy above a certain value, Cerenkov light is generated in the optical fiber 1, and the angle θ between the emission direction of the Cerenkov light and the free electron irradiation direction is θ= cos-' c / v・However, C: Speed of light in vacuum V: Speed of free electrons ε: Represented by the refractive index of the optical fiber. Now, if y - + C, then ε = 1.45 in the core 1a of the silica optical fiber 1, so 1, θζ46
．． It becomes 4. Light at this angle is incident on the interface between core 1a and cladding 1b at the same angle θ, but since the total reflection angle at this interface is 46°, Cherenkov light is propagated within the core without going outside. become.

[Example] Figure 2 shows one end 20 of an optical fiber 1 with a core diameter of 600 μm and a fiber diameter of 750 μm placed in a dark box 7, and an electron beam 20 (acceleration voltage 250 kV, current density 30
0A/cd, pulse width 50ns, aperture 3X20c
An example is shown in which the light is detected by a PIN photodiode 5 connected to the other end 17 m ahead, and the output of the photodiode 5 is detected by a CRT 6.

FIG. 3 shows the photodiode output in this case, and a short pulse output with a peak value of 35 mV and a half-value width of 40 ns was obtained. This waveform is similar to the output waveform of the electron beam 20, and the spectrum of the output light from the optical fiber 1 is 30
This is clear from the fact that the wavelength is widely distributed from 0 to 450 nm.

FIG. 4 is a graph showing the relationship between the irradiation length of the electron beam and the light intensity detected by the photodiode 5, with the irradiation length (CIll) on the horizontal axis and the intensity (arbitrary scale) on the vertical axis. As is clear from this figure, there is a linear relationship between the irradiation length and the emission intensity.

To summarize the above, Cerenkov light is generated in an optical fiber by electron beam irradiation, and the intensity of the generated light has a linear relationship with electron beam current density x irradiation length (or area). Therefore, by detecting the light intensity obtained from the end of the optical fiber, the current value of the electron beam can be detected.
Furthermore, by positioning an optical fiber partially in the electron beam projection area and detecting its optical output, it is also possible to detect the current distribution. Of course, when measuring current, it is necessary to obtain a calibration curve between light intensity and current value in advance.

Note that the optical fiber is preferably coated to block external light, thereby improving the S/N ratio.

Further, since the optical fiber only needs to satisfy the condition that Cerenkov light is totally reflected, the optical fiber may have only a core without a cladding.

Further, the direction in which the electron beam is projected onto the optical fiber need not be perpendicular, but may be inclined. This is because total reflection of Cerenkov light becomes more likely to occur.

〔effect〕

According to the present invention as described above, it is possible to measure the current of a relativistic electron beam and its distribution, which has not been possible until now.

Also, since optical fiber is an electrical insulator, it has a voltage of several hundred kV.
Even if placed in the electron beam irradiation area at a high voltage above, there is no danger to the photodetection of the photodiode 5, etc.
Furthermore, even in a field where gaseous light is emitted by high-current electron beam irradiation, the current in this field can be measured independently of electromagnetic noise and optical noise. There is no effect of optical noise because the present invention detects light generated inside the optical fiber.
This is because external light cannot enter its propagation mode.

[Brief explanation of drawings]

Fig. 1 is a diagram explaining the principle of the present invention, Fig. 2 is a schematic diagram showing an embodiment of the invention, Fig. 3 is a waveform diagram of detection light, and Fig. 4 shows the relationship between irradiation length and detection light intensity. It is a graph. 1... Optical fiber 5... Photodiode 6...
・CRT 20... Relativistic electron beam patent application filed 1) Noriyuki and one other representative Patent attorney Noboru Kono I Ib Figure 1 Figure 3 Irradiation length (am)

Claims (1)

[Claims] 1. In the method of measuring the current value of a relativistic electron beam, the electron beam is irradiated onto an optical fiber to generate Cerenkov light, the Cerenkov light is detected at the end of the optical fiber, and the electron beam is determined from this light. A current measurement method characterized by determining current.