Free-electron laser oscillator

Abstract: In a free-electron laser oscillator, two laser beams (LB₁, LB₂) having different wavelengths are irradiated to a photoemitter (3) to generate an electron beam (EB). The electron beam (EB) is density-modulated by a frequency corresponding to a difference between frequencies of the two laser beams (LB₁ LB₂), and accelerated to a vicinity of light velocity by a microwave. Such a relative theory electron beam is directed into a periodical magnetic field generated by a helical wigglar (6) and a track thereof is deflected. As a result, a synchrotron radiation light is generated from the relative theory electron beam. The radiation light generates an electromagnetic field by itself. Thus, electromagnetic interaction takes place among the electromagnetic field, the periodical magnetic field and the relative theory electron beam, and the laser oscillation is started. A power of the laser beam (LB) irradiated to the photoemitter (3) may be reduced because the two laser beams (LB₁ LB₂) having different wavelengths are irradiated to the photoemitter (3) and the electron beam (EB) is bunched in accordance with the difference between frequencies.

Description: BACKGROUND OF THE INVENTION (Field of Invention) The present invention relates to a free-electron laser (FEL) oscillator which uses an electron beam as a laser medium, particularly used for one having an electron beam source constructed by a photoemitter. (Related Background Art) The free-electron laser oscillator usually uses the electron beam as the laser medium to attain a laser oscillation by mutual action of the electron beam and an externally applied electric field, magnetic field or laser beam. A high current, high brightness and long life free-electron source or electron beam source is desired. Where a linear accelerator is used to accelerate the electron beam, the use of the photoemitter as the electron beam source has been considered in order to produce a bunched beam which conforms to a frequency of a microwave applied to the accelerator. For example, the use of the photoemitter as the electron beam source is discussed in "High-Brightness Photoemitter Injector for Electron Accelerators" by J.S. Fraser et al, IEEE Transactions on Nuclear Science, Vol.NS-32, No. 5 October 1985, and "Pulsed Photocurrents From Lanthanum Hexaboride Cathodes in the ns Regime" by M.Moussoukay et al, Nuclear Instruments and Methods in Physics Research A264 (1988) 131-134, North-Holland Amsterdam. A principle of operation of the FEL oscillator is explained in detail in "Wiggler and Free Election Laser" by J-Kondo, Study Report No. 200, Electro-Technical Research Lab. AIST, MITI, Japan May 1979. According to the article, the conventional FEL oscillator is constructed as described below. A laser beam from a mode locked YAG laser is reflected by a mirror and directed to a photoemitter in a cavity through a window. Since a target of the photoemitter is made of a material having a photoelectron emitting property such as cecium-antimony, photoelectrons are emitted by the application of the laser beam and the emitted photoelectrons are accelerated by a voltage applied across the photoemitter and an acceleration electrode. The relative theory electron beam which has been accelerated to a vicinity of light velocity by the microwave accelerator has a track thereof changed by a magnetic field generated by a deflection coil and is directed to a periodical magnetic field generated by a helical wiggler. The laser beam used in the FEL oscillator to emit the photoelectrons is a laser beam having a pulse train of a given duration. It is a pulsed laser beam having a pulse train of 350 pico-seconds(ps) or 770 ps duration which is generated by mode locking in accordance with the period of the microwave used to accelerate the electron beam. As a result, a pulse train of the electron beam corresponding to the pulse train of the laser beam, that is, a bunched electron beam generated from the photoemitter is accelerated by the microwave to become the relative theory electron beam which is close to the light velocity. A method for generating the bunched electron beam is described in "High Quantum Yield from Photofield Emittors" by M. Boussoukaya et al, Nuclear Instruments and Methods in Physics Research A279 (1989) 405-409 North-Holland Amsterdam. When the relative theory electron beam thus generated is directed to the periodical magnetic field generated by the helical wiggler, the track of the electrons is deflected by the periodical magnetic field so that a synchrotron radiation light is generated. Since the synchrotron radiation light interacts with the electrons to generate an electromagnetic field and the electrons interact with the periodical magnetic field, a laser oscillation which uses the free-electrons as a laser medium is started. The conventional FEL oscillation is generated by a secondary effect of the mutual action of the relative theory electron beam, the periodical magnetic field and the synchrotron radiation light. Accordingly, a high oscillation gain is not attained. In order to attain a high free-electron laser beam output, it has been proposed to use a high brightness electron beam source which permits a high current output of, for example, 50 to 100 A/cm². However, in the electron beam source which uses the photoemitter, the damage of the photoemitter is high because a power of the mode locked laser beam irradiated to emit the photoelectrons is high. Accordingly, the lifetime of the photoemitter is reduced to one to several days. The conventional free-electron laser oscillator uses a condition that the quantity of emission of the synchrotron radiation light from the electrons which are in a deceleration phase in the periodical magnetic field slightly exceeds the quantity of absorption of the light by the electrons which are in an acceleration phase in the same periodical magnetic field, that is, it utilizes the secondary effect of the laser oscillation. Accordingly, if the wavelength of the oscillation laser beam is reduced to attain a high energy, the oscillation gain will further decrease. As a result, the scale of the device of the free-electron laser oscillator for generating the short wavelength laser beam increase. It is, therefore, a first object of the present invention to provide a free-electron laser oscillator which allows a long life of an electron emission material which forms a target of an electron beam source and assures a high oscillation gain, and an oscillation method therefore. It is a second object of the present invention to provide a free-electron laser oscillator which allows to reduce the size of the device. SUMMARY OF THE INVENTION According to the invention from one aspect, there is provided a free-electron laser oscillator comprising: a laser beam source for emitting two laser beams having different wavelengths; an electron beam source having electron emission means for emitting electrons by the irradiation of said two laser beams, for emitting an electron beam having an electron density thereof modulated in accordance with the difference between the frequencies of said two laser beams; microwave acceleration means for accelerating the electrons of said electron beam to a vicinity of light velocity to change said electron beam to a relative theory electron beam, wherein said laser beam source is mode-locked in accordance with a period of an acceleration microwave applied to said microwave acceleration means; and laser oscillation means for effecting the laser oscillation at the same wavelength as the density modulation wavelength of said electron beam while using the electron beam as a laser medium, said laser oscillation means including a laser oscillation magnet for generating a magnetic field having a period set such that the density modulation wavelength of said electron beam and the laser oscillation frequency are equal to each other. According to the invention from another aspect, there is provided a method for oscillating a free-electron laser comprising the steps of: a first step of irradiating an electron emission means with two laser beams having different wavelengths to generate an electron beam density-modulated in accordance with the difference between the frequencies of said two laser beams; a second step of accelerating said electron beam to a vicinity of light velocity to generate a relative theory electron beam; and a third step of directing the relative theory electron beam into a periodical magnetic field having a period set such that the density modulation wavelength of said electron beam and the laser oscillation frequency are equal to each other to generate the laser oscillation at the same wavelength as the density modulation wavelength of said electron beam. In accordance with the features of the present invention, the electron beam which is density-modulated in accordance with the frequency difference in generated without significantly damaging the target, by irradiating the two laser beams having different wavelengths to the electron emission material which forms the electron beam source. The electron beam is then oscillated by the primary effect of the mutual action while utilizing the electron beam as the laser medium so that the laser oscillation at the same wavelength as the density modulation wavelength of the electron beam is attained with a high gain. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a configuration of a free-electron laser oscillator in accordance with one embodiment of the present invention, Figs. 2A, 2B and 2C show distributions of photoelectron density, and Fig. 3 illustrates a principle of operation of the free-electron laser oscillator. DESCRIPTION OF THE PREFERRED EMBODIMENTS In a free-electron laser oscillator in accordance with one embodiment of the present invention shown in Fig. 1, two laser beam sources having different oscillation wavelengths from each other are provided. A first laser beam source 11 for generating a first laser beam LB₁ having an angular frequency ω₁, and a second laser beam source 12 for generating a second laser beam LB₂ having an angular frequency ω₂ = ω₁ + Δω are combined to form a laser beam source for emitting photoelectrons. The first laser beam LB₁ is reflected by a total reflection mirror M1 and passes through a half-mirror HM. On the other hand, the second laser beam LB₂ is reflected by the half-mirror HM. The first laser beam LB₁ and the second laser beam LB₂ are combined into a single laser beam LB by the actions of the mirrors M₁ and HM. The laser beam LB = LB₁ + LB₂ is reflected by another total reflection mirror M₂ and directed to a cavity 2 having the inside thereof vacuumed, through a light transmissive window 21. The laser beam LB is directed to a photoemitter 3 having a target which is formed by a photoelectron emission material. When the laser beam LB comprising the first laser beam LB₁ and the second laser beam LB₂ having different wavelengths is directed to the photoemitter 3, an electron beam EB which is density-modulated in accordance with a difference between the frequencies of the two laser beams is produced and it is accelerated by an acceleration voltage E applied across the photoemitter 3 and an acceleration electrode 4. The accelerated electrons are guided into a microwave accelerator 7 where they are accelerated to a vicinity of light velocity. The relative theory electron beam EB accelerated to the vicinity of light velocity has a track thereof deflected by a deflection coil 5 and is directed to a periodical magnetic field generated by a laser oscillation magnet 6 which may comprise an undulator and a helical wigglar. The period of the magnetic field generated by the laser oscillation magnet 6 is set such that the density modulation wavelength of the relative theory electron beam EB and the oscillation wavelength of the free-electron laser are equal to each other. As a result, the mutual action of the electron beam EB, the magnetic field by the laser oscillation magnet 6 and the synchrotron radiation light is very large, and the laser oscillation with a high gain is attained by the primary effect. Since it is sufficient for the electron beam source to generate the density-modulated electron beam EB, a photoelectron emission source or photoemitter of a low current type may be used. When the two laser beams are irradiated to the photoemitter, the powers thereof need not be high. Accordingly, the photoemitter is not damaged. With reference to the brief explanation described above, the embodiment of the present invention is now explained in more detail. In Fig. 1, the first laser beam LB₁ and the second laser beam LB₂ are expressed by E₁ = E₁₀·cos (ω₁·t + ₁) E₂ = E₂₀·cos (ω₂·t + ₁) The photoelectron current I produced from the photoemitter 3 by the irradiation of the laser beam LB is expressed by I∝E₁² + E₂² + 2E₁E₂ cos {(ω₁ - ω₂)t+₁-₂} Assuming that the wavelength of the first laser beam LB₁ having the angular frequency ω₁ is 400 to 600 nanometers (nm), and the angular frequency of the second laser beam LB₂ is ω₂ = ω₁ + Δω, the photoelectron current I is density-modulated by the angular frequency Δω. For example, if ω₁/Δω = 2 to 200, the frequency of the electron density modulation corresponds to the light wavelength of 1 to 100 micrometers (µm). The electron beam EB from the photoemitter 3 is accelerated by the microwave accelerator 7. In order to render the electron beam EB to a bunched electron beam which is in synchronism with the acceleration microwave applied to the microwave accelerator 7 and which has the same wavelength as the acceleration microwave, the laser beam sources 11 and 12 may use mode locked YAG laser oscillators. When both the first laser beam LB₁ and the second laser beam LB₂ are mode-locked, the laser intensity and the emitted electron density on the photoelectron emission plane of the photoemitter 3 are controlled as shown in Fig. 2A, in which T corresponds to the period of the acceleration microwave. In an S band, the duration T = 350 ps, and in an L band, the duration T = 770 ps. In Fig. 2A, Δt corresponds to the angular frequency difference Δω = ω₂ - ω₁, and Δt = 2π/ Δω. When the relative theory electron beam EB is directed into the periodical magnetic field having the period λ₀, the free-electron laser oscillation is attained in a manner shown in Fig. 3. An upper curve in Fig. 3 shows the electron beam EB which has a track thereof deflected by the periodical magnetic field generated by the laser oscillation magnet 6 and travels helically, and a lower curve shows an electromagnetic field of the synchrotron radiation light generated by the relative theory electron beam EB. When the electron beam EB shown in Fig.3 satisfies a resonance condition of a formula(3) , it interacts with the periodical magnetic field and the synchrotron radiation light and receives a force F E due to the electric field and a force F L due to the magnetic field. At points A and B, confirmed directions of the forces F E and F L which each electron of the electron beam EB receives are opposite to each other along the direction of travel of the relative theory electron beam EB which is close to the light velocity. Accordingly, at the point A which is in an acceleration phase, the electrons are continuously accelerated to absorb the light, and at the point B which is in a deceleration phase, the electrons are continuously decelerated to emit the light. Resonance condition; λ₀ = L a ·(1+k²)/2γ² where λ₀ is the laser oscillation wavelength and λ₀ω₀ = 2πc where c is the light velocity, L a is a distance between magnets of the undulator which generates the periodical magnetic field, k is a constant and k ≒ 1, and γ is a Lawlentz factor of electron. If the density distribution of the electron beam EB is uniform in the order of wavelength of the oscillated laser beam, the absorption and the emission of the light are substantially equal to each other at the time when the electron beam EB is directed to the periodical magnetic field, and as the result, no free-electron laser oscillation is attained. However, when the electron beam EB interacts with the periodical magnetic field, the electron beam EB is gradually density-modulated. When the energy of the electron beam EB is slightly higher than the resonance condition, the number of electrons which are in the deceleration phase is larger than the number of electrons which are in the acceleration phase and as the result the laser oscillation is attained. The laser oscillation gain G is expressed by G = G os - α·G ab where α is a ratio of the number of electrons which are in the light absorption phase (point A) to the number of electrons which are in the oscillation phase (point B), and G os and G ab are terms of oscillation and light absorption, respectively. The gain G is more specifically expressed by G≃A·I [g(ω₀/ω ω- -1) - α·g (ω₀/ω ω+ -1)] Where A is a constant, I is a photoelectron density, the function g is g(X) = (Nπ²)⁻¹ {sin²Nπx/x²}, and ω± is ω± = ω₀{1±(ℎω₀)/(γmc²)}. Accordingly, ω is divided into the first term and the second term of the formula(5). Since the second term (ℎω₀)/(γmc₂) « the first term ω₀, the function g is developed to obtain the laser oscillation gain G G≃B·I {(1-α) + (ℎω₀)/(γmc²)} where B is a constant. Where the electron beam EB is not density-modulated, α=1 in the formula (6) and the gain G is obtained from only the second term of the formula (6). The second term of the formula (6) is (energy of the oscillated laser beam)/(energy of electron beam EB). Specifically, assuming that γmc² is 10 to 100 mega electron volts (MeV) and hω₀ is 0.1 to several eV, the gain G amounts to only the order of 10⁻⁹ to 10⁻⁷. On the other hand, where the electron beam EB is density-modulated as it is in the embodiment of the present invention, α≠1 and the first term of the formula (6) remains. As a result, due to the primary effect of the mutual action, the high gain laser oscillation is attained even with the small current density I. In the free-electron laser oscillator of the embodiment of the present invention, the wavelength limit of the laser oscillation appears in a short wavelength region, that is, a visible region. In a long wavelength region or an infrared region, the limit may be variable if the frequency difference Δω = ω₂ - ω₁ between the first laser beam LB₁ and the second laser beam LB₂, and the energy of the electron beam EB are properly set. On the other hand, the following two are considered to be causes of the oscillation limit in the short wavelength region. First, the spread of the electron in time on the photoelectron emission plane of the photoemitter 3, and second, the distortion of the electron beam EB due to a Coulomb's force among the emitted electrons. In a known ultra-high speed light measurement streak camera, a time resolution of 400 femto seconds (fs) has been attained. The limit factor in this case is the Coulomb's force among the electrons in the second factor described above, and the spread in time on the photoelectron emission plane in the first factor is considered less than 100 fs. In the streak camera device, the electron beam is extracted as several pulses of 0.1 to 1A /cm₂ and 400fs, the acceleration electric field on the photoelectric plane is in the order of 10 kV/mm, and the accelerated electrons are scanned at a constant velocity of several cm. The discussion of the time resolution in such a streak camera is described in "Time Resolution of an Image Converter Camera in Streak Operation" by V.V. Korobkin et al, the Journal of Photographic Science, Vol.17, 1969. In the free-electron laser oscillator of the embodiment of the present invention, the electrons are accelerated even in an area where the electrons travel at a constant velocity in the streak camera because the velocity of the electrons is not sufficient if only the accelerated electric field is used. As a result, the density modulation of the electron beam is improved over that in the known streak camera. In the present invention, if the condition of 1 - α > (ℎω₀)/(γmc²) is met in the formula (6), the laser oscillation by the primary effect is attained. Accordingly, the oscillation at a fairly short wavelength is attained. Specifically, the oscillation up to the wavelength of 30µm which corresponds to 100 fs is attained. In the free-electron laser oscillator of the embodiment, the changes of the electron density from the photoelectron emission plane along the acceleration space direction x are shown in Figs. 2B and 2C. Fig. 2B shows a case where the Coulomb's force among the electrons do not act greatly, that is, where the current is low. A density wave E₁₁ of the electrons immediately after the emission changes to a density wave E₁₂ as the electrons are accelerated. The density-modulated wavelength Δx of the electron beam EB is expressed by Δx≃(2πv) / Δω where v is the electron velocity. Assuming that v ≒ c, then Δx≃(2πc)/Δω The value Δx is equal to the wavelength of the oscillated laser beam. Fig. 2C shows a case where the Coulomb's force among the electrons acts to deform the density wave of the electron beam EB. A density wave E₂₁ emitted from the photoelectric plane has the electron density thereof unified by the Coulomb's force as the electrons are accelerated, and changes to a density wave E₂₂. Accordingly, in the free-electron laser of the present invention, the limit of the short wavelength is determined by a response characteristic of the photoelectron emission plane to the laser beam. When the response characteristic is 100 fs, the wavelength limit is 100 fs × c (light velocity) = 30 µm. In the free-electron laser of the present invention, the electron emission material which forms the target of the photoemitter as the electron beam source may be selected from various materials so long as they have sufficient quantum efficiency, for example, 0.1 to 10 % for the laser beam irradiation. For example, cecium-antimony or gallium arcenide which has been widely used as well as a superconductive material may be used as the target material. A thermal cathode or an electric field radiation type cathode other than the photoemitter may be used as the electron beam source so long as they exhibit a sensitivity to the laser beam. For example, a dispenser cathode used for the thermal cathode has a quantum efficiency of 0.1 to 1 % to an ultraviolet ray having a wavelength of 300 to 2000 nm. Thus, it may be used where the current density is in order of 0.1 to 1 A/cm².

Claims: A free-electron laser oscillator comprising: a laser beam source (11, 12) for emitting two laser beams (LB₁, LB₂) having different wavelengths; an electron beam source (3) having electron emission means for emitting electrons by the irradiation of said two laser beams (LB₁, LB₂), for emitting an electron beam (EB) having an electron density thereof modulated in accordance with the difference between the frequencies of said two laser beams; microwave acceleration means (7) for accelerating the electrons of said electron beam (EB) to a vicinity of light velocity to change said electron beam to a relative theory electron beam, wherein said laser beam source is mode-locked in accordance with a period of an acceleration microwave applied to said microwave acceleration means (7); and laser oscillation means (6) for effecting the laser oscillation at the same wavelength as the density modulation wavelength of said electron beam while using the electron beam as a laser medium, said laser oscillation means including a laser oscillation magnet (6) for generating a magnetic field having a period set such that the density modulation wavelength of said electron beam and the laser oscillation frequency are equal to each other. A free-electron laser oscillator according to claim 1, wherein said laser beam source includes two laser oscillation devices (11, 12) for emitting laser beams (LB₁, LB₂) having different wavelengths. A free-electron laser oscillator according to claim 2, wherein said two laser oscillation devices (11, 12) are mode locked. A free-electron laser oscillator according to claim 2, wherein said laser beam source includes light path conversion means (M1, HM) for directing said two laser beams to said electron emission means as a single laser beam. A method for oscillating a free-electron laser comprising the following steps : a first step of irradiating an electron emission means (3) with two laser beams (LB₁, LB₂) having different wavelengths to generate an electron beam (EB) density-modulated in accordance with the difference between the frequencies of said two laser beams; a second step of accelerating said electron beam (EB) to a vicinity of light velocity to generate a relative theory electron beam, and a third step of directing the relative theory electron beam into a periodical magnetic field having a period set such that the density modulation wavelength of said electron beam and the laser oscillation frequency are equal to each other to generate the laser oscillation at the same wavelength as the density modulation wavelength of said electron beam.

Inventors: YANAGISAWA YUTARO, YANAGISAWA, YUTARO
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1990
Main classification: H01S3
IPC codes: H01S3