JP2001035691A - Forming method of plasma channel for laser beam by z pinch discharge - Google Patents

Abstract

PROBLEM TO BE SOLVED: To form a plasma channel with high reproducibility, uniform in the axial direction by forming a plasma medium gradually decreasing refractive index from the center toward the periphery with the shrinkage phase of high speed Z pinch discharge in gas. SOLUTION: Beams of a titanium sapphire laser 1 are focused with an off- axis parabola mirror 2 on the inlet of a discharge capillary 3 in which a hollow hole is formed in the center of a ceramic rod. Electrodes 4 are installed in the discharge capillary 3 to constitute a discharge tube. Helium gas is supplied to the discharge tube from a gas introduction opening 5, and exhausted from a turbo molecular drag pump. Differential exhaust is conducted and the inside of the discharge tube is held in helium gas atmosphere. A recessed electron- density distribution is formed on the central axis of the discharge tube by interaction of current shrinking by high speed Z pinch discharge action generated between the electrode 4 and an impulse wave driven with the current. An optical waveguide device with very long life is constituted and plasma can be compressed before magnetohydrodynamic instability is grown.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a laser accelerator,
X-ray laser, plasma X-ray source, high-order harmonic generator,
It can be used for applications such as high-intensity physics experiments that require high-intensity laser to be propagated over a long distance while being focused in plasma.

[0002]

2. Description of the Related Art A laser accelerator, an X-ray laser, a plasma X-ray source, a high-order harmonic generator, and the like need to transmit a high-intensity laser in a plasma for a long distance while being focused. However, the distance over which the laser beam can propagate while being focused is limited by the diffraction length, so that the laser beam cannot be propagated over a long distance while maintaining the focused intensity in vacuum or plasma of uniform density.

[0003] In a medium such as a distributed refractive index optical fiber, in which the refractive index is large at the center of the path of the laser and becomes small toward the periphery, the laser propagates with a constant radius. In the case of plasma, by forming a channel having a concave distribution of plasma density along the path of the laser so as to have waveguide characteristics, the laser light can be propagated over a long distance while being focused (channeling).

Various methods have been proposed for channeling the laser, but there are roughly two methods. There are a self-channeling method in which a density channel is formed by the self-focusing effect of the propagating laser itself, and a pre-channel method in which a waveguide is formed in some way before the propagating laser passes. In the pre-channel method, it was general to mainly use a laser for channel generation, but recently, a generation method using vacuum discharge in a capillary has been reported.

[0005] In the prior art, a pre-channel method utilizing a vacuum discharge in a capillary, an optical waveguide forming method utilizing a vacuum discharge in a capillary,
A channel is formed by performing vacuum discharge in a small tube having an inner diameter of several hundred microns called a capillary made of plastic. The inside of the capillary is evacuated, and a pulse current of about several hundred amperes flows through the tube wall. The current turns the plastic wall into a plasma, and a plasma gas layer is formed near the wall. As a result, the plasma density distribution inside the tube has a concave coaxial shape with a large area near the wall and a small area on the central axis. A plasma column having a concave plasma density distribution formed inside the capillary serves as a channel for guiding the laser light like an optical fiber.

[0006]

Problems with the prior art optical waveguide forming method utilizing vacuum discharge in a capillary are as follows. First, the life of the optical waveguide device is several to ten shots. It is very short. This is because the capillary tube wall is melted by the driving current and the plasma is supplied, so that the diameter of the capillary tube increases with each shot, the discharge condition changes, and the diameter of the formed channel also increases with each shot. The second problem is that the plasma channels formed are non-uniform in the axial direction, which is fatal if longer channels are to be formed. In addition, discharge in a vacuum starts with dielectric breakdown called tracking of a part of a wall surface, and thus a clean cylindrical channel is not formed.

[0007]

The present invention relates to a plasma channel forming method belonging to a pre-channel method using a discharge, but uses a discharge called Z-pinch, and its channel generation mechanism is different from the conventional one. to differ greatly. Z-pinch is a method in which a large current flows in the axial direction (Z direction) of a low-pressure gas-filled discharge tube, and the current is generated by an azimuthal self-magnetic field generated by this current
(Plasma column) is a phenomenon in which itself is compressed.

In a high-speed Z-pinch discharge driven by a high-speed rising current, a shock wave is driven in front of a coaxial current layer that contracts at a high speed, and the current layer and the shock wave form a concave shape within 100 μm inside the core of the plasma column. A plasma density distribution is formed. The internal structure of the plasma formed on the Z axis in the contraction process (implanting phase) of this high-speed current drive Z-pinch discharge is used for guiding laser light.

A ceramic having a diameter of one to several millimeters or a capillary made of glass is used for the discharge tube.
Add gas of ~ several Torr. As the driving current, a rising number of about several tens ns and a peak value of about several tens kA are used. Further, before the drive current flows, a current of several millimeters to several amperes is passed through the capillary to pre-ionize the sealed gas. By using high-speed current driving by a low-inductance power supply and preliminary ionization of gas, the plasma inside the capillary can be quickly separated from the tube wall, and the erosion of the wall by the plasma can be extremely reduced.

Therefore, an optical waveguide device having a much longer life than the conventional technology is obtained. Further, the plasma can be compressed before the magnetohydrodynamic instability grows, and a cylindrical plasma channel having uniform axial reproducibility and good reproducibility can be formed.

The high-speed Z-pinch discharge means that the rising speed is fast.
This is a Z-pinch discharge driven by a current (about several-several + nanoseconds). The high-speed Z-pinch discharge of the present invention has the following features. 1. Preliminary ionization of the loaded gas is performed in advance.

The reason for this is that in order to form a uniform plasma channel in the axial direction with good reproducibility, it is necessary to suppress instability of the electromagnetic fluid. Pre-ionization of the load gas improves the initial uniformity of the plasma and makes the initial distribution of the drive current uniform. Thereby, the instability of the electromagnetic fluid is suppressed.

2. Discharge is driven by a fast-rising current (about several to several + nanoseconds). This is necessary because
In order to create a channel with a large refractive index gradient, it is necessary to create a strong shock wave.
(Several minus several plus nanoseconds). 1. In order to suppress the instability of the magnetohydrodynamic fluid in the same manner as in the above, the drive is performed with a rising current faster than the growth rate of the magnetohydrodynamic instability.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Solving the Problems of the Prior Art (1) A long-life discharge optical waveguide device can be obtained.

(2) A long plasma waveguide that is uniform and long in the laser propagation direction can be formed stably and with good reproducibility. 2. Advantages newly generated by the present invention (1) The refractive index distribution of the plasma channel can be controlled.

The temporal and spatial distribution of the plasma density inside the capillary can be controlled by controlling the discharge parameters (drive current rise, peak value, initial charged gas pressure, capillary diameter). Therefore, the channel diameter, the refractive index gradient, the refractive index, and the channel holding time of the optical waveguide can also be controlled.

(2) An optical waveguide can be formed in a wide plasma density region. The density parameter required for the optical waveguide differs depending on applications such as laser acceleration, X-ray laser, and generation of higher-order harmonics, and has an optimum value and an optimum spatial distribution. (1)
For the same reason as described above, in the present invention, an optical waveguide can be formed in a wide density region (10 ¹⁶ -10 ²⁰ cm ^-3 ) by controlling discharge parameters. Therefore, an optical waveguide having a density distribution suitable for each application purpose can be formed.

(3) A plasma type can be selected. Depending on applications such as X-ray laser and high-order harmonic generation, plasma forming a waveguide may be used as a light source medium. The light generated in this case depends on the type of plasma. In the present invention, since a gas is used as the plasma source of the waveguide, the type of plasma can be selected according to the purpose.

(4) An optical waveguide having a focusing effect of a charged particle beam. The optical waveguide of the present invention uses a Z-pinch discharge. Since the Z-pinch discharge causes a large current to flow in the axial direction, a very large self magnetic field of 1 T (tesla) or more is generated in the azimuthal direction of this current, and the focusing power becomes 10,000 T / m or more.

The formed optical waveguide is surrounded by this current in a cylindrical shape. Therefore, if the charged particle beam enters the waveguide from the axial direction, the beam propagates through the channel while being focused by the lens effect due to the strong magnetic field. When this optical waveguide is used for a laser accelerator, not only a guide for a driving laser but also a focusing effect of charged particles incident in synchronization with the laser can be obtained at the same time.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on embodiments. (Example 1) The arrangement of the laser light guide experiment and the channel formation process observation experiment will be described with reference to FIG. Titanium-sapphire leather Light 1 is off-axis and a parabolic mirror 2 is used to form a capillary (discharge capillary) having a hollow hole with an inner diameter of 1 mm and a length of 2 cm formed at the center of a ceramic rod.
Light was collected at the entrance of No. 3. The capillary is provided with an electrode 4 to form a discharge tube. This discharge tube contains He
Gas is supplied from the inlet 5 and exhausted from the outlet (turbo molecular pump). The inside of the discharge tube was kept in a helium gas atmosphere by performing differential evacuation.

In the discharge tube, a concave electron density distribution is formed on the central axis of the discharge tube due to the interaction between the current contracted by the action of the high-speed Z pinch discharge generated between the electrodes and the shock wave driven by the current. Was formed. The profile of the laser beam introduced into the discharge tube is guided by the plasma channel having the formed electron density distribution, passes through the discharge tube, and then passes through the telescope lens 6, band-pass filter 7, ND (neutral density) filter. 8 and arrived at the CCD camera 9 and observed.

The present invention is a plasma channel forming method using a discharge, and uses a discharge called Z-pinch. A large current flowed in the axial direction of a low-pressure gas-filled discharge tube made of a ceramic (Al ₂ O ₃ ) capillary, and the current (plasma column) itself was compressed by a self-magnetic field generated by the current.

Further, since a high-speed Z-pinch discharge driven by a high-speed rising current is performed, a shock wave is driven in front of the coaxial current layer that contracts at a high speed, and the current layer and the shock wave cause the inside of the current (plasma column) core. , A concave plasma density distribution was formed. The internal structure of the plasma formed by the contraction process of the Z pinch discharge driven by the high-speed current was used for guiding laser light.

Further, a capillary (thin tube) having a diameter of 1 mm is used for the discharge tube, and 0.9 H (Torr) of H is contained therein.
e gas was charged. Number of rises 15n as drive current
s, a peak current of 4.8 kA, and a DC current of several milliamps was passed through the capillary before the drive current flowed to pre-ionize the He-filled gas. The plasma inside the capillary was quickly separated from the tube wall by using high-speed current drive by a low-inductance power supply and preionization of gas.

Therefore, the optical waveguide device has a very long life as compared with the prior art, and furthermore, it is possible to compress the plasma before the magnetohydrodynamic instability grows, and to obtain a cylindrical reproducible tube having uniform axial reproducibility and good reproducibility. A plasma channel could be formed.

Example 2 FIG. 2 shows the result of observation of guiding of a high intensity ultrashort pulse laser. FIG. 2A shows an intensity profile of a laser beam when a discharge channel is formed according to the present invention (a profile of a laser beam that has passed through an optical waveguide (length: 2 cm) by a high-speed Z-pinch discharge).
FIG. 2B shows the intensity profile of the laser beam when there is no discharge channel [capillary (D = 1 m
m, L = 20 mm), pressure of 0.9 Torr (He), titanium-sapphire laser wavelength 790 nm, 2TW
(Terawatts) and a light-collecting intensity of 1 × 10 ¹⁷ W / cm ² ].

FIG. 2C shows the channel N of the CCD camera.
o. (A), (b) measured by a CCD camera with respect to
It can be seen from this that the laser light passing through the channel is guided while being collected in a region of about 40 μm in the center. The energy transmittance of the laser beam increased from 30% to 65% during channel formation.

FIG. 2 shows that the high intensity ultrashort pulse laser was guided only at the time when the optical waveguide was formed by the high-speed Z-pinch discharge. (Embodiment 3) FIG. 3 (a) shows the results (time evolution) of the emission of plasma during the discharge channel formation process of the present invention (measured using a high-speed time-swept camera (streak camera)). FIG. 3B shows a radial intensity profile of plasma emission at time t = 8.5 nanoseconds, and FIG. 3C shows a discharge using a He—Ne (helium neon) laser as a probe laser. The observation result (time evolution) of the channel formation process is shown.

FIGS. 3A and 3B show that a channel having a diameter of about 70 μm is formed around time t = 8.5 nanoseconds. Further, from FIG. 3 (c), a laser beam was guided and a bright spot was observed around the discharge channel formation time t = 8.5 nanoseconds. (A), (b),
(C) shows that an optical waveguide having a diameter of about 70 μm was formed uniformly over a length of 2 cm on the central axis.

(Embodiment 4) A computer simulation result of a channel forming process using a one-dimensional magnetic fluid code is shown. FIG. 4A shows the spatio-temporal distribution (change) of the current density inside the capillary driving the high-speed Z-pinch discharge, and FIG. 4B shows the spatio-temporal distribution (change) of the electron density inside the plasma optical waveguide. . The calculation conditions were a helium gas of 0.9 torr and a drive current of 4.8 kA, and the same discharge parameters as in the experiment of Example 1 were used. The time change of the current density well explains the light emission distribution (FIG. 3A) obtained by the experiment. Further, the calculation results clearly show that when the Z pinch contracts (at time t = about 8.5 nanoseconds), a concave density distribution with a concave center is formed.

FIGS. 4A and 4B show that a channel is formed during the contraction process of the Z pinch (the optical waveguide is formed with a concave density distribution), and the current shown in FIG. The density distribution agrees well with the emission distribution obtained in the experiment (FIG. 3A). From FIG. 4 (c), the optical waveguide is formed around the discharge channel formation time t = 8.5 nanoseconds.
Good agreement with experimental results.

(Embodiment 5) FIGS. 5, 6, and 7 show a high-speed pulse power supply, a circuit diagram thereof, and a discharge tube device used in a test experiment for the present invention. In the waveguide forming method of the present invention, it is necessary to start a large current at a high speed. Further, in order to form a cylindrical plasma with good reproducibility, the electric current must flow axially symmetrically. To this end, as shown in FIGS. 5 and 7, the discharge tube and the pulse power source use four coaxial cables, and the cables are arranged coaxially symmetrically.

The pulse power supply device and the Z-pinch discharge tube device must have a low inductance structure in order to start current at high speed. For this reason, a coaxial cable is used to reduce the area created by the current forward path and the current return path. A ceramic capacitor was used to reduce the internal inductance of the capacitor itself. Further, as shown in FIG. 7, the area of the forward and return paths of the current is reduced by using four coaxial cables in order to reduce the inductance of the discharge tube.

In order to improve the reproducibility of the plasma, it is necessary to reduce the pre-pulse of the current caused by the switching characteristics of the thyratron element. In order to solve this, a pre-pulse gab filled with gas was used.

In the present invention, the circuit shown in FIG.
It is important to use such a power supply to drive the discharge tube device with a preionization circuit and a fast rising current.
That is, in order to form a stable plasma optical waveguide with good reproducibility, the pre-ionization of the load gas and the drive current at the high speed are important. In addition, in this high-speed rise, it is necessary to reduce the inductance of the load section (resistance) and the power supply section. For this reason, the power supply is made coaxial to reduce the area of the current forward path and return path.

Further, in the discharge tube of the present invention, a low-inductance load is required to perform high-speed rising current driving. A coaxial cable is used, and the discharge tube device has a geometrically low inductance. Structured.

The low-inductance structure comprises a low-inductance power source and a low-Z discharge load. In the power source, a ceramic capacitor having a small internal inductance is used as shown in FIG. This is done by reducing the inductance determined by That is, as shown by the arrow in FIG. 8, the current flows through the thyratron, the ceramic capacitor, the pre-pulse gap, and the coaxial cable. The geometrically determined inductance is reduced by arranging this area as small as possible.

In the Z discharge load portion (capillary discharge tube), as shown in FIG. 9, a current flows through a thyratron, a ceramic capacitor, a pre-pulse gap, and a coaxial cable. (Shaded area) is determined by the area formed by the forward path and the return path of the current. By arranging the area as small as possible, the geometrically determined inductance is reduced.

[0040]

According to the present invention, when using a laser accelerator having a peak power of 2 TW and a pulse width of 100 fs, 1
When an optical waveguide of 0 cm is used, an acceleration energy gain of 1 GeV can be obtained. This means making a very large conventional accelerator a tabletop size. The waveguide forming method of the present invention can realize a device having a very long life required for an accelerator. The reproducibility and stability of the channel have been confirmed by experiments, and are suitable for use as an acceleration tube of a laser accelerator.

X-ray laser: The X-ray laser cannot take a long amplification distance in the medium due to the refraction effect of the medium plasma. When the optical waveguide of the present invention is used for an X-ray laser medium, an amplification distance that is 10 times or more that of a conventional type can be obtained.

The optical waveguide of the present invention is a high-order harmonic generator,
In addition, it can be used for applications that require long-distance propagation while focusing a high-intensity laser in plasma, such as high-intensity physics experiments.

[Brief description of the drawings]

FIG. 1 shows an apparatus for forming a laser light guide in a gas-filled capillary by Z-pinch discharge.

FIG. 2 is a diagram showing a profile of an intense laser at an outlet of a capillary discharge tube.

FIG. 3 is a diagram showing a process of forming a guide channel by rapid contraction in a high-speed Z-pinch discharge in a capillary discharge tube.

FIG. 4 is a diagram showing a computer simulation result of a channel forming process.

FIG. 5 is a diagram showing a pulse power supply for a Z-pinch discharge for forming a plasma optical waveguide.

FIG. 6 is a diagram showing a pulse circuit for Z-pinch discharge for forming a plasma optical waveguide.

FIG. 7 is a diagram illustrating a high-speed Z-pinch discharge tube device for forming a plasma optical waveguide.

FIG. 8 is a diagram showing a current flow in a power supply portion.

FIG. 9 is a diagram showing a current flow in a Z discharge load portion (capillary discharge tube).

[Explanation of symbols]

 1: Titanium-sapphire laser 2: Off-axis parabolic mirror 3: Capillary 4: Electrode 5: Gas inlet 6: Telescope 7: Bandpass filter 8: ND (neutral density) filter 9: CCD camera / streak camera

Claims (2)

[Claims] In a method of forming a plasma channel for guiding a laser beam by a Z-pinch discharge, a contraction phase of a high-speed Z-pinch discharge in a gas is used, and the refractive index is large at the center and small as it goes to the periphery. A method in which a plasma medium is formed and used for guiding laser light. 2. In a high-speed Z-pinch discharge in a gas, a concave electron density distribution is formed on a central axis of a discharge tube by an interaction between a current contracted by a self-magnetic field and a shock wave driven by the current. The method according to claim I, wherein this is used as a plasma channel for guiding laser light.