Title:
SOFT X-RAY GENERATOR ASSISTED BY LASER
United States Patent 3746860
Abstract:
A high desnity pulsed plasma generator is disposed in axial alignment with a high energy pulse laser to receive laser radiation from the laser axially to the movement of the plasma blob along the centterline of the center electrode of the plasma gun. A high Z-material is disposed between the laser output and the plasma for evaporation by the laser and subsequent injection of heavy z-ions into the plasma for enhancing x-ray radiation from the plasma. Electrrical circuit means is provided for energizing the plasma gun and the laser at predetermined times.


Inventors:
Shatas, Romas A. (Huntsville, AL)
Roberts, Thomas G. (Huntsville, AL)
Stettler, John D. (Huntsville, AL)
Meyer III, Harry C. (Huntsville, AL)
Application Number:
05/227177
Publication Date:
07/17/1973
Filing Date:
02/17/1972
Assignee:
STETTLER J,US
MEYER H,US
SHATAS R,US
ROBERTS T,US
Primary Class:
Other Classes:
376/145
International Classes:
H05G2/00; H05H1/22; (IPC1-7): G21G3/00
Field of Search:
250/84
View Patent Images:
Primary Examiner:
Dixon, Harold A.
Claims:
We claim

1. A laser assisted x-ray generator comprising a pulsed plasma generator for generating a plasma, said plasma generator having center and outer concentric electrodes; pulsed laser means for producing a laser beam for conditioning said plasma for enhanced radiation of x-rays, said laser beam being compatible with said plasma so that the plasma will absorb energy therefrom, said laser means being disposed in axial alignment with said center electrode; a member of high ionic charge ions disposed for vaporization by said laser beam for injection into said plasma for enhanced radiation of x-rays from said plasma; a lens positioned coaxially with said center electrode of said plasma generator for focusing the laser beam into said plasma generator; triggering means connected to said laser for triggering said laser; light sensing means positioned so as to detect the light energy in said plasma generator and to generate a signal upon the light energy reaching a predetermined value, said signal disposed for triggering said triggering means into operation at a predetermined time.

Description:
BACKGROUND OF THE INVENTION

Soft x-ray pulses of submicrosecond duration are needed to test materials and components of pulsed fusion reactions. Techniques presently employed to generate such pulses are (a) electron diode guns bombarding a heavy metal target, (b) underground fusion devices and (c) dense focus with high Z-material electrode tips which erode during the pulse. Electron diode guns at the required x-ray energies of fractional MeV are very inefficient because of conversion efficiency of electron beam energy into Bremsstrahlung decreases superlinearly with the decrease of electron energy for a given target anode, a fact which is well known to the designers of flash x-ray tubes. In addition, at low electron energies of fractional MeV, the space charge of electron beams is not cancelled by relativistic effects and limits severely the maximum current density of the electron beam available at the target anode. Furthermore, the electric fields at the cathode are usually not sufficient to obtain a copious electron emission by the field effect and therefore thermionic cathodes must be employed which intrinsically yield a much lower electron emission current density than field emitters. Present electron beam-Bremsstrahlung flash generators of useful x-ray fluence therefore employ electron beams in the several MeV range. They generate x-ray flashes of spectral distribution which contains most of the photon energy in the hard x-ray spectral range. Because the x-ray penetration depth decreases superlinearly with the photon energy, the deposited x-ray energy density in test materials and components is substantially different for soft and hard x-ray flashes of identical fluence at the source. Therefore, pass-fail conclusions of tests on materials, components and devices performed with many MeV energy electron beam x-ray flash generators are not directly scalable to predict the performance under a soft x-ray flash. Underground fusion flash tests suffer from the intrinsic inability to separate the various components of radiations and expansion waves generated during the test. Therefore, various radiation and blast wave effects cannot be readily differentiated and only the cumulative, gross effects are observed. Thus, the materials designer is handicapped in separating the individual contributions from each damaging radiation.

The plasma focus alone can also be used as a soft x-ray flash generator by altering the electrode design and configuration such as to increase the evaporation and erosion of certain portions of the electrodes. Because only the energy stored in the plasma focus can be used for soft x-ray production, the fluence of x-ray flash is limited. In addition, a full control of erosion of the electrodes cannot be achieved in this case. Therefore, the intensity and the spectral distribution of x-ray flashes varies from one firing to another.

In the apparatus of the present invention all of these dificiencies are either eliminated or substantially reduced by a programmed evaporation of high Z-material by laser pre-pulse, injection of heavy z-ions into the dense focus by laser-induced ion detonation wave and by heating of the high z-plasma by the main laser pulse.

SUMMARY OF THE INVENTION

The laser assisted soft x-ray generator comprises a device that utilizes laser energy deposition in the plasma of the coaxial plasma gun to create the necessary conditions for a copious emission of x-rays at kilo-volt energies. The high energy laser, dense plasma focus combination is arranged and operated in the following way: First, the dense focus gun is fired and as a consequence of an electric breakdown initiated shock wave of charged particles between the gun electrodes, a dense plasma focus is created at the center electrode of the gun. Usually, hydrogen or hydrogen-isotop gas is used to create the initial plasma in the gun. This hydrogen or hydrogen-isotope plasma is a weak source of x-rays, although it may produce a burst of thermonuclear neutrons. However, since the intensity of X-rays increases very rapidly with the effective electric charge of the highly ionized, hot plasma of the dense focus gun, a short burst of laser radiation is employed to vaporize a metered quantity of high Z-material and to inject it through the laser initiated ion burst wave into the volume element of the formation of the dense plasma focus. Once within the plasma, these ions enhance the x-ray radiation of the plasma in all of the three dominant radiation processes: Bremsstrahlung, line radiation, and electron-positive ion recombination radiation. By choosing appropriate high Z-materials, a variety of x-ray spectral distributions in these three constituent components can be obtained. However, because the energy for the x-ray radiation is extracted from the plasma focus which stores a certain amount of energy in thermal, motional and magnetic energies, the enhanced radiation would cease, upon rapidly exhausing these stored energies. Since by appropriately metering the injected high z-plasma component into the plasma focus many thousand-fold enhancement of x-ray radiation can be obtained, a separate energy source is needed to sustain the temperature of the highly radiating plasma. This is accomplished by heating the plasma focus with a high energy laser pulse which is chosen in frequency and in geometrical configuration as to be as completely as possible absorbed by the radiating plasma focus.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagrammatic view of the x-ray generator of the present invention.

FIG. 2 is a diagrammatic view of an alternate embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in the FIGS., the apparatus of the present invention includes a plasma gun 10 and a laser 12 disposed in axial alignment. A power supply 14 is provided for the plasma gun 10 and the electrical system therefor includes a charging resistor R1, condenser bank 16, starting switch 18, pulse generator 20 and switch 21. The electrical system for the laser includes a power supply 22, charging resistor R2 and an electrical laser pulser 24.

Plasma gun 10 includes an insulator 25, an outer electrode 26 and an inner electrode 28. The plasma gun is operationally connected to the laser through a light pipe 30, optical attenuator 32, photo-diode 34 and a signal delay generator 36 which is connected to the electrical laser pulser 24. A member 38 of high Z-material is disposed in plasma gun 10 along a longitudinal axis 40 extending through plasma gun 10 and laser 12.

In the embodiment illustrated in FIG. 2, wherein like numerals refer to like parts, the plasma gun 10 is arranged so the insulator 25 is away from the laser beam. Lens 50 is shown mounted in the path of the laser output within the center electrode. The positioning of the lens is arbitrary within the center electrode it only being necessary that the laser radiation is focused onto the plasma. With the laser and plasma gun in this arrangement, ions are blown back into the focus at a velocity of about 109 cm/sec.

Referring to the Figures, before the sequence of events is started both the coaxial plasma gun 10 and the high energy CO2 laser 12 are filled to the desired pressures with the gases to be used; and the power supplies 14 and 22 have charged through their respective charging resistors R1 and R2, the condenser bank 16 and the laser pulser 24 to the working voltages. The sequence of events is now started by closing starting switch 18. This causes the pulse generator 20 to close the switch 21 and the voltage of the condenser bank 16 appears across the electrodes of the coaxial dense plasma focus gun 10. The gas in the coaxial plasma gun breaks down near insulator 25 forming current sheath 37 The current sheath then propagates between the outer electrode 26 and the center electrode 28. The current sheath is driven by the magnetic pressure of its own magnetic field, and the discharge becomes more intense as the sheath propagates. When the current sheath reaches the end of the electrodes it folds back on itself and rapidly collapses the plasma toward the axis of the tube as in a z-pinch. This produces the hot plasma volume 14 where electron or ion number density may be as high as 2 × 1019 cm-3, the temperature may be as high as several times 107 ° Kelvin and the confining magnetic fields are of the order of megagauss. At this time and for a period of the order of a 100 nanoseconds neutrons are produced. As the current sheath is moving down the coaxial gun and the light intensity of the discharge increases, it is being detected by light pipe 30 which carries it to photo diode 34 after having passed through the variable optical attenuator 32. Variable optical attenuator 32 is preset so that the light intensity will not cause the signal delay generator 36 to begin operating until the current sheath has reached a predetermined location in the coaxial plasma gun. In this manner the jitter of all events prior to the time the signal delay generator is started are avoided and have no effect on the problem of synchronizing the laser firing. The signal which starts signal delay generator 36 is delayed a preset amount and is then used to actuate the laser pulser 26 which has already been pre-charged up to the required electrical energy level by the power supply 22 so that the entire voltage of laser pulser 24 appears across the discharge electrodes 40 and 42 of the high energy pulsed CO2 laser 12. The preferred embodiment of the laser is the high pressure electron beam preionized electrical discharge pumped arrangement in which the electric field intensity between the discharge sustaining electrodes 40 and 42 is chosen such as to maximize the transfer of electrical energy into molecular rotational and vibrational energy of the lasing gas. The electron beam in ionizing collisions with the lasing gas provides the positive and negative charge carriers which drift at relatively slow velocities under the sustainer field and in colliding with neutral atoms or molecules of the lasing gas transfer a part of their kinetic energy into rotational-vibration excitations. This creates a medium of very high gain between mirrors 44 and 46, disposed at opposite ends of the laser tube. Therefore, oscillations are set up between the mirrors and the energy in the inversion is extracted in a pulse of radiation at 10.6 μm. The shape of this pulse can be tailored somewhat if desired by rotating mirror 44 so that the laser is Q-switched.

The laser beam is focused onto the member 38 of high Z-material by a lens 50 which may be made of Na C1 or any other material which transmits energy at 10.6 μm. The laser radiation vaporizes member 38 and a metered quantity of high Z-material is injected into the plasma through the laser initiated ion burst wave into the volume element of the formation of the dense plasma focus for enhancement of x-ray radiation.

In the device shown in the Figures, the plasma is confined to a volume of the order of 10-2 cm3 in the presence of a magnetic field of the order of a megagauss for times of the order of 100 nanoseconds. The electron and ion densities and the temperature during this time are about 2.1019 cm-3 and 1 to 5 keV, respectively.

When the apparatus of the present invention is operated for generation of x-rays, both the absorption of the laser radiation and the production of x-rays is strongly dependent on the presence of a small amount of high z-impurity in the plasma. To illustrate this, the absorption and reradiation of the energy will be calculated for a pure hydrogen plasma and for a hydrogen plasma which contains 5 percent (atomic) of heavy ions with an effective Z of 20. For example, this could be tungsten ionized to the 20th degree.

Assume the temperature and charge density to be spatially uniform, and the electrons to have a Maxwellian distribution. The absorption is primarily describable by a free-free electron transion in the field of positive ions (inverse Bremsstrahlung). The absorption constant is

αωZ = Ne Ni /n Z2 G (Z)/G (1) A cm-1 (1)

where n = [1 - (ωp ω)2 ]1/2 and A ≉T-2/3 λ2 = 3 × 10-39 for T = 103 eV and λ= 10.6 μm.

For most hydrogen isotope plasmas at high temperatures the laser beam will penetrate the plasma and be efficiently absorbed only when the electron density is such that the plasma frequency, ωp, is near but less than the laser frequency, ω1. When ωp = ω1 an anomalous absorption may occur such that the energy is totally absorbed in a very thin layer near the surface of the plasma. Absorption coefficients and the penetration depths or e-folding distances, for the pure hydrogen plasma and the impure hydrogen plasma with 5 percent high Z material where Zeff = 20, calculated in the wings of the function n where n is approximately unity, are geven by

αH = 1.3/n cm-1 ≉ 1.3 cm-1, d ≉ 0.77 cm (2)

and

α5%Z = 32/n cm-1 ≉ 32 cm-1, d ≉ 3 × 10-2 cm. (3)

These calculations do not include the effects of anomalous absorption mentioned above; therefore, the actual penetration depths can be expected to be somewhat smaller, depending whether the electric field intensity associated with the laser flux is sufficient to drive the plasma into the anamolous region. Thus, if we neglect the enhancement of absorption by plasma instabilities the pure hydrogen plasma in the dense plasma focus considered here is optically thin to the CO2 laser radiation unless the electron density is fairly accurately controlled so that the plasma frequency remains quite close to the laser frequency. However, the hydrogen plasma with 5% Zeff = 20 ions is optically thick to the CO2 laser radiation even when n is near unity, and it may be assumed that this impure plasma in the small volume of the plasma focus device absorbs nearly all of the energy in the laser beam, e.g., we neglect reflections.

The effective ion charge of 20 was calculated assuming a 10 nanosecond duration as the time available for ionization when the temperature is of the order of 103 eV and the atomic number of the injected material is much greater than 20. If the energy to heat and ionize the high A component of the plasma is supplied by the dense plasma focus, for example, by introducing the high Z impurity during the early formative stages of the discharge, there may be more time available for ionization and the effective charge might be greater than 20. However, an early introduction of high Z material will limit the plasma temperature because of an intense line radiation.

PLASMA RADIATION

During the 100 nanosecond confinement time the primary plasma cooling mechanisms consist of radiation losses. These losses are compensated by the absorption of laser energy which is supplied at a rate tailored to the desired spectral shift of the X-rays during the simulation event. For the purposes of this discussiOn, it is taken that the laser power is equal to the total X-ray radiated power and therefore the electron temperature remains nearly constant. The radiation losses consist of a Bremsstrahlung continuum which results from free-free (ff) transitions, a recombination continuum which results from free-bound (fb) transitions and line radiation which results from bound-bound (bb) transitions.

The power density radiated in the form of Bremsstrahlung is given by j = B Ne Ni Z2 W cm-3 (4)

where B varies as T1/2 and is a constant here as postulated above. For a hydrogen isotope plasma the power radiated ty typical dense plasma focus device considered here is PffH = 1.1 × 106 W and for a 5 percent impurity with Zeff = 20 we have a Bremsstrahlung ratio of PffZ /PffH = f Z (1 + Z + Z2 f) + 1 = 42 where f = 0.05 is the fraction of high Z ions taken with respect to hydrogen. (Each of these high Z ions has a positive charge of 20. ) Therefore, the power PffZ radiated by the free-free transitions of plasma to which high Z ions have been admixed amounts to 46 × 106 W.

The recombination radiation is given by the ratio

Pfb /Pff = EI /Te (5)

where EI is the effective ionization potential and Te the electron temperature. Therefore, for a pure hydrogen isotope plasma the recombination radiation is negligible. However, for the high Z component of the impure plasma the effective ionization potential can be taken as EI = 2.2 Z2 eV and therefore the ratio is

PfbZ /PffZ ≉ 0.9.

Therefore, with our choice of high Z ions added the radiation of the plasma recombination continuum is nearly equal to the Bremsstrahlung continuum, i.e.,

PfbZ = 41 × 106 W.

The power emitted in the form of line radiation is given by the ratio

Pbb /Pff ≉ 20 × 106 exp(-EI /Te)/Z2 Te, (6)

and again the line radiation from the hydrogen plasma alone is completely negligible. However, for the high Z component of the impure plasma we have

PbbZ /PffZ ≉ 30

so that

PbbZ = 1.38 × 109 W.

The total power radiated by the pure hydrogen plasma consists essentially of Bremsstrahlung while the total power radiated by the impure hydrogen plasma is given by

PT = PffH + PffZ (1 + PfbZ /PffZ + PbbZ /PffZ) = 1.47 × 109 W.

ENERGY BALANCE

The energy which needs to be supplied by the laser in order to maintain the electron temperature nearly constant during the 100 nanoseconds of plasma confinement time amounts to the total energy loss in the form of radiation, the thermal energy required to heat the added high Z material and the ionization energy required to ionize the added high Z material. This total is given by

EX-ray (Bremsstrahlung, recombination, line) 147 Joules EThermal (heating added high Z material) 30 Joules EIonization (ionizing added high Z material) 10 Joules ETotal 187 Joules

The efficiency of converting the absorbed laser energy at 10.6 μm into soft X-ray energy is then

η= EX-ray /ETotal = 147/187 = 0.79.

In order to operate the laser assisted x-ray generator again, one must first change the gases in the coaxial plasma gun and the high energy CO2 laser by way of gas supplies 54 and 56 and pumps 58 and 60 and recharge condenser bank 16 and laser pulser 24.

The CO2 laser referred to herein may be of the type described by G. J. Dezenberg et al, IEEE J. Quantum Electron. QE-6, 652 (1970).

A typical plasma generator utilized in conjunction with the laser may be of the type developed by J. W. Mather at Los Alamos Scientific Laboratories, Los Alamos, N.M. and disclosed in Phys. Fluids 8, 366 (1965).