High-efficiency plasma confining method, laser oscillating method and laser oscillator

A high-efficiency plasma confining method, and a laser oscillating method of oscillating a laser, confine a plasma generated by entering a laser beam via an entry of a superconducting cylinder within the cylinder. The methods include the step of inhibiting diffusion or expansion in the radial or axial direction of the generated plasma by reducing the inside diameter of the cylinder between the entrance and an exit thereof. A magnetic line of force may be generated in the axial direction by winding a coil around the superconducting cylinder, to thereby confine the plasma. A laser oscillator for carrying out the method includes a superconducting cylinder, a supply mechanism for supplying a gas or a solid into the cylinder, and a laser irradiating mechanism. The inside diameter of the cylinder is reduced between an entry and an exit thereof, or the inside diameter of the cylinder is reduced at the entry and the exit thereof.

FIELD OF THE INVENTION 
The present invention relates to a high-efficiency plasma confining method, 
a laser oscillating method based thereon, and a laser oscillator. 
PRIOR ART 
A plasma which is generally known as the fourth state of substance has been 
studied from various points of view because of the circumstances in which 
a mode covering a collective motion of a number of particles and a mode of 
individual motion including individual particles independently exerting 
mutual actions are intertwined in a complicated manner. Material 
processing at high temperatures using unique properties of plasma, plasma 
chemistry and other practical applications have therefore been widely 
carried out and development technology related thereto is actively 
conducted. 
A plasma exhibits diverse and various behaviors under the effect of an 
electromagnetic field acting from outside. The effect of an internal 
electromagnetic field within a plasma generated by component particles 
themselves is also remarkable, and is characterized by behaviors which 
cannot be observed in any other state of substance. 
While there exist many such plasmas in the natural world, natural plasmas 
cannot be utilized in a laboratory. It is therefore necessary to provide 
an apparatus which generates a plasma by some means or other and thus 
permits experimental or practical use thereof. In such an apparatus, a 
plasma comprising atoms or molecular ions of a required kind must be 
confined for generating and holding in response to a particular purpose. 
Because control parameters of generated plasma varies with the generating 
mechanism, various contrivances have been worked out for confining, 
including the magnetic field confining method and the inertia confining 
method. 
In spite of these conventional efforts and contrivances, however, a plasma 
generated with a laser has in general a high density and a high 
temperature (up to 10.sup.21 cm.sup.-3, from 20 eV to 2 keV), and 
confining is difficult by a method based on magnetic field. 
Such a high-density and high-temperature plasma usually disappears through 
expansion, and it is difficult to hold it in a stable manner. Even for a 
low-temperature plasma (under 20 eV), the generated plasma is easily 
susceptible to a spontaneous distortion or strain, and suffers from a 
diffusion or expansion in the radial direction. 
Under these circumstances, there has been an increasing demand for a method 
permitting efficient and stable confining of a plasma such as a 
high-density and high-temperature one generated with the use of a laser. 
SUMMARY OF THE INVENTION 
As means for solving these problems as described above, the present 
invention provides a high-efficiency plasma confining method of confining 
a plasma generated by entering a laser beam via an entry of a 
superconducting cylinder within the cylinder, which comprises the step of 
inhibiting diffusion or expansion in the radial direction of the generated 
plasma by reducing the inside diameter of the cylinder between the entry 
and an exit thereof, a high-efficiency plasma confining method of 
confining a plasma generated by entering a laser beam via an entry of a 
superconducting cylinder within the cylinder, which comprises the step of 
inhibiting diffusion or expansion in the axial direction of the generated 
plasma by reducing the inside diameter of the cylinder at the entry and an 
exit thereof, and the method as described above, which comprises the step 
of generating a magnetic line of force in the axial direction by winding a 
coil around the superconducting cylinder, thereby confining the plasma. 
As a laser oscillating method made possible by the foregoing confining 
methods, the present invention provides a laser oscillating method of 
oscillating a laser by confining a plasma generated by entering a laser 
beam via an entry of a superconducting cylinder within the cylinder, which 
comprises the step of inhibiting diffusion or expansion in the radial 
direction of the generated plasma by reducing the inside diameter of the 
cylinder between the entry and an exit thereof, thereby confining the 
plasma at a high efficiency, a laser oscillating method of oscillating a 
laser by confining a plasma generated by entering a laser beam via an 
entry of a superconducting cylinder within the cylinder, which comprises 
the step of inhibiting diffusion or expansion in the axial direction of 
the generated plasma by reducing the inside diameter of the cylinder at 
the entry and at an exit thereof, thereby confining the plasma at a high 
efficiency, and the foregoing laser oscillating method, which comprises 
the step of generating a magnetic line of force in the axial direction by 
winding a coil around a superconducting cylinder, thereby confining a 
plasma. 
In addition, the present invention provides also a laser oscillator 
comprising a superconducting cylinder, supply means of a gas or a solid 
into the cylinder, and laser irradiating means, wherein the inside 
diameter of the cylinder is reduced between an entry and an exit thereof, 
or the inside diameter thereof is reduced at the entry and the exit 
thereof, the foregoing laser oscillator wherein a coil is wound around the 
superconducting cylinder to form a mirror resonator, and also the 
foregoing laser oscillator wherein a mirror is arranged near each of the 
entry and the exit of the superconducting cylinder to form a mirror 
resonator.

DETAILED DESCRIPTION OF THE INVENTION 
In the present invention, diffusion or expansion of a generated plasma in 
the radial direction or in the axial direction is inhibited by utilizing a 
cylindrical superconductor as described above. Spontaneous distortion or 
strain of plasma is also prevented. This means that the plasma can be 
aligned with the axis. This in turn makes it possible to oscillate a laser 
of a short wavelength at a high efficiency. 
Now, embodiments of the present invention will be described below further 
in detail. It is needless to mention that the present invention is not 
limited to these embodiments. 
Embodiment 1 
FIG. 1 illustrates a fundamental configuration of a high-efficiency plasma 
confining laser oscillator based on a cylindrical superconductor of the 
present invention. An incident laser beam having a divergent angle a is 
irradiated into a gas in the cylinder after being appropriately stopped 
through a lens having a focal length f and a number of openings A. In this 
case, a curvature of the inner wall of the cylinder is determined to match 
with the laser beam to be slopped. More specifically, in the 
high-efficiency plasma confining laser oscillator of the present 
invention, the inside diameter of the thinnest center portion of the cone 
should be, depending upon the focal radius of the incident laser, 100 
.mu.m on the minimum, and 5 mm on the maximum. This is no limitation on 
the outside diameter so far as the thickness of the cylinder is at least 1 
mm. In such a cylindrical superconductor, the laser beam is irradiated 
into the gas in the cylinder to generate a plasma. The incident laser beam 
may be of a short pulse of nano, pico or femtosecond order or a repetition 
pulse, and any of various gases including a rare gas such as He may 
appropriately be selected as the gas in cylinder. 
The generated plasma has initially a density of the order of that of the 
gas or the solid density, and a higher pressure leads to an earlier 
occurrence of expansion or diffusion. A self-magnetic field having been 
produced at this point cannot however enter into the superconductor under 
the influence of Meissner effect which is one of the fundamental 
properties of superconductivity. As a result, the magnetic field is 
confined in a space between the superconductor and the plasma, and 
compressed. The plasma is confined in the cylinder under the pressure of 
the compressed magnetic filed. 
The generated plasma is a laser medium, and the inside diameter r0 and the 
length W are associated with the population inversion number (i.e., the 
laser output) of particles between two levels of the medium. More 
specifically, the generated gas plasma has a radius r.sub.0 =f.alpha., a 
plasma length W=4Ar.sub.0, and a volume V=4.alpha.Af.sup.3 .alpha..sup.3, 
and the distance from the cylinder inner wall depends upon the temperature 
and density of the plasma. When using a gas such as a rare gas, 
furthermore, it is possible to achieve a configuration improving the 
population inversion by forming a magnetic line of force in the axial 
direction by winding a coil around the cylindrical superconductor. 
In the plasma generated in such a cylindrical superconductor, diffusion or 
expansion thereof in the radial direction or in the axial direction is 
inhibited, and distortion or strain is hardly caused, with correction 
thereof accomplished spontaneously. It is therefore possible to stable 
hold the plasma. 
The plasma thus confined at a high efficiency is therefore further utilized 
for a laser oscillator. The inside diameter r0 of the plasma in this case 
is dependent on the incident laser intensity necessary for the saturation 
of the upper level of the oscillation transitions. A longer W at the point 
of saturation leads to a higher laser beam output. 
Embodiment 2 
FIG. 2 illustrates a fundamental configuration of the high-efficiency 
plasma confining laser oscillator of the present invention, using a solid 
target. This is applied in a vacuum by any of the following methods: 
(1) Piercing a slender hole (or holes) at a center portion of the 
cylindrical superconductor, and inserting a target rod; and 
(2) Inserting a target along the inner surface, and irradiating a laser. 
Embodiment 3 
FIG. 3 illustrates another embodiment of the high-efficiency plasma 
confining laser oscillator of the present invention when building a mirror 
oscillator. In this embodiment, the incident laser beam enters while 
slightly shifting from the optical axis. Since the oscillated 
short-wavelength laser resonates on the optical axis under the effect of 
the mirror, a gain of laser output is available. 
Embodiment 4 
FIG. 4 illustrates further another embodiment of the high-efficiency plasma 
confining laser oscillator of the present invention in a case where the 
inside diameter of the cylindrical superconductor is reduced at the entry 
and the exit. Axial expansion of the generated plasma can be more 
effectively inhibited by adopting this configuration. 
Embodiment 5 
The superconducting cylinder in the embodiments 1, 2, 3 and 4 is 
accompanied by a cooling section having a construction as shown in FIG. 5 
(the configuration shown is a combination of the equipment shown in FIG. 4 
and a cooling section). This is to obtain a superconducting effect. If a 
room-temperature superconductor is available, therefore, it is not 
necessary to provide a cooling section based on liquid nitrogen. 
In the embodiment shown in FIG. 5, expression of the superconducting effect 
is accomplished by bonding a superconductor to the inner surface of a 
container-shaped copper cylinder with an epoxy resin adhesive excellent in 
electric conductivity, and performing cooling by introducing liquid 
nitrogen, for example, from the opening. 
According to the present invention, as described above in detail, it is 
possible to stable conduct effective confining which has been difficult in 
the conventional confining methods, This effective confining permits 
achievement of a compact and simple high-efficiency short-wavelength laser 
oscillator. 
It is needless to mention that confining of a plasma provides effect, not 
only on a high-density and high-temperature plasma, but also on a 
low-temperature plasma.