Apparatus and method for generating soft X-ray lasing action in a confined plasma column through the use of a picosecond laser

Abstract: Soft X-ray lasing action may be obtained by focusing a high power, picosecond laser pulse on a confined plasma column. The plasma column is created by focusing a CO₂ laser pulse on a target such as carbon. A strong cylindrical magnetic field contains the plasma and causes it to assume the shape of a column. While the plasma column is cooling rapidly due to radiation losses, a second high power laser with a duration time of approximately 1 picosecond is focused along the central part of the plasma column. The powerful picosecond laser provides strong selective excita­ tion of the ions for enhancement population inversion and the creation of high gain for X-ray lasing action. According to alternative embodiments, other multi-Z elements such as molybdenum, aluminum and krypton can be used to create a lasing medium in the plasma column.

Description: The invention comprises a method and apparatus for generating soft X-ray lasing action by focusing a powerful picoseond laser on a confined plasma column. Soft X-ray lasers have gained recent attention because of the interesting scientific potential for such devices. It is believed that X-ray lasers would have the capability of forming holographic interference images which could, for example, be used to study the detailed three-dimensional structure of small molecules such as DNA. Other significant applications are anticipated. The potential for X-ray lasers is described in the October 30, 1984 editions of THE NEWARK STAR LEDGER and THE WALL STREET JOURNAL, as well as the April 2, 1985 edition of THE NEW YORK TIMES. New technology has made it possible to create very powerful excimer lasers having pulse duration times in the neighborhood of 1 picosecond and powers in the neighborhood of 10¹⁵ - 10¹⁷ W/cm² and above. Since high power excimer lasers have only recently been possible, there isn't much in the way of prior art that appears relevant to the concept. Of some interest is Patent No. US-A-4,058,486 entitled PRODUCING X-RAYS which discusses the formation of a plasma by a low power precursor pulse which is followed up by a subsequent higher power main pulse with the object of creating X-rays from the interaction. A variety of low-Z and high-Z materials are described as potential laser targets. The use of multi-Z plasmas is also discussed in in a different laser environment. According to an alternative embodiment of the present invention the CO₂ laser could be replaced by a neodymium glass laser. The use of Nd:G glass lasers is discussed in a different enviroment in US-A-4,058,486. Also of possible interest is a publication entitled "Soft X-Ray Pumping of Metastable Levels of Li Plus", Appl. Phys. Lett. 42(1) 1 Jan. 1985, 9-11. An approach to a soft X-ray laser with a single laser and rapid plasma cooling after the laser pulse was disclosed in a publication entitled "Population Inversion and Gain Measurements for Soft X-Ray Laser Development in a Magnetically Confined Plasma Column," by Suckewer, et al. Quantum Electronics, Vol. QE19, December 1983. In one aspect the invention comprises apparatus for use with apparatus for generating soft X-ray lasing action as set out in claim 1. In another aspect the invention comprises a method for generating population inversion and soft X-ray lasing action as set out in claim 9. An example of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a general overall schematic diagram of the preferred embodiment of the invention. Figure 2 is a detail view of the reaction chamber portion of the preferred embodiment illustrated in Figure 1. Figure 3A is a cross-sectional view of the helical magnet. Figure 3B is an axial view of the helical magnet illustrated in Figure 3A showing the location of the gap therein. Figure 4 illustrates the magnetic field distribution and conductor configuration of the helical magnet. Figure 5 is a schematic diagram of the powerful picosecond laser. Figure 6 illustrates in perspective view the elements of the CO₂ laser. Figure 7 illustrates the gain for a 182 Angstrom line created by the interaction of the powerful picosecond laser of Figure 5 with CVI ions in a plasma column as created by the CO₂ laser in Figure 6. During the course of this description like numbers will be used to identify like elements according to the different figures which illustrate the invention. The preferred embodiment of the soft X-ray laser apparatus 10 is illustrated in Figure 1. A target material 12 , such as carbon fiber is housed within a vacuum vessel 14. Alternatively a carbon foil could be employed or a fiber or foil of a multi-Z material. A pair of differential vacuum pumps 30 and 32 bring the vacuum in chamber 14 down to the neighborhood of 1.33 x 10⁻⁵ Pa (10⁻⁷ Torr). A CO₂ laser 18 produces a beam 20 focused by mirror 22 on carbon fiber target 12. Beam 20 preferably comprises a pulse having a duration of 20 - 30 nanoseconds and an energy level of 1.5 KJ. While a pulse duration of 20 - 30 nanoseconds is preferred, a time scale in the neighborhood of 10 - 100 nanoseconds would be acceptable. Similarly, while a CO₂ laser is preferred other types of lasers such as a Nd:Glass laser would also be acceptable. The cross-section of the CO₂ laser beam 20 typically has a length approximately the length of the carbon fiber target 12 and a width slightly wider than target 12. Therefore for a target 12 1-2 cm long and 35 to 100 microns thick the cross-section of the CO₂ laser beam might be 1-2 cm long and 100-200 microns wide. CO₂ laser beam 20 creates a plasma which is confined to a column shape by helical magnets 16. The plasma column 15 formed thereby is typically 1 - 2 centimeters long and about 1 millimeter in diameter. The magnetic field created by the helical magnet 16 has a strength in the neighborhood of 10 - 15 TesPa (100-150KGauss). The plasma column 15 confined by the magnetic field B z cools rapidly by radiation losses and becomes a very good medium for creating relatively high gain in the soft X-ray region of 1-2 x 10⁻⁸m (100 - 200 Angstroms). It has been found that radiation losses for the plasma column 15 are more efficient for fast cooling than through cooling by adiabatic expansion. It is important that the confined plasma column 15 be relatively long, and uniform along its axis and that it have a higher density than the freely expanding column at the time when the highest population inversion (gain) is expected. Shortly after the plasma column 15 is formed by the Co₂ laser beam 20, the plasma column 15 is subjected to a high power picosecond laser pulse 26. Laser pulse 26 is created by high power picosecond laser 24 and focused by mirror 28 onto the plasma column 15. The high power picosecond pulse 26 is focused on the plasma column 15 approximately 20 to 100 nanoseconds after the CO₂ laser pulse 20 has created the plasma column 15. High power loser pulse 26 has a pulse duration time of approximately 1 picosecond and a power output in the range of 10¹⁵ - 10¹⁷ W/cm². The powerful picosecond laser beam 26 is smaller in cross-section than CO₂ laser beam 20 and is focused along the central part of the plasma column 15 which is approximately 50 micrometers in diameter. Power for the helical magnet 16 is provided by a pulse power system 38. Laser beams 20 and 26 are strongly focused in one direction as shown in Figure 2 and slightly focused in the other perpendicular direction as shown in Figure 1. Therefore laser beams 20 and 26 can be accurately described as line focused beams since they are focused along the length of the target 12 in order to create an elongated plasma column 15 with small radial dimensions. Both beams 20 and 26 are near-perpendicular to plasma column 15 as can be appreciated from Figure 2. A variety of instruments are used to monitor the total radiation beam 34 from the apparatus 10. On one side the beam 34 shown in Figure 1 is divided by beam splitter mirror 40 into two beams 44 and 46. Beam 44 is analyzed by XUV spectrometer 50 (X-ray part of the beam) whereas beam 46 is reflected by mirror 42 into air monochrometer 48 (UV part of the beam). The outputs from the air monochrometer 48 and XUV spectrometer 50 provide some of the inputs to data acquisition system circuit 52. Another portion of the beam 34 passes to another beam splitter 40 where a portion of the beam 44 is monitored by XUV monochrometer 62 (X-ray part of the beam). The other portion of the beam 46 is, reflected off mirror 42 into Pin diode detector 60 (UV part of the beam). The outputs of Pin diode detector 60 and the XUV monochrometer 62 are likewise forwarded to data acquisition system 52. A streak camera 54, a vacuum XUV spectrometer 56, and an air UV spectrometer 58 also monitor the conditions inside of vacuum vessel 14 and forward the data to data acquisition system 52. Figure 2 is a detail mechanical illustration of the vacuum vessel 14 and the associated elements which are shown more generally in Figure 1. Optical fiber probes 66 are directed towards the target area 12 where the plasma column 15 is formed. The output from optical probes 66 forms the input for the streak camera 54. The vacuum XUX spectrometer 56 and the air UX spectrometer 58 and other instruments are connected to vacuum chamber 14 of apparatus 10 and can be arranged to view the plasma 15 through a system of conventional mirrors and lenses as well as through optical fibers 66. The diameter DR of the vacuum vessel 14 is preferably in the neighborhood of 5 centimeters. Figures 3A, 3B and 4 all illustrate various aspects of the helical magnet 16. Figure 3A is a cross-sectional view of the helical solenoid magnet 16. Magnet 16 includes a core aperture 68 having a diameter DR in the neighborhood of 5 centimeters as previously described. A pair of end plates 70 held together by tie rod 76 support the windings 71. Power is supplied to windings 71 by the pulse power system 38 illustrated in Figure 1. Windings 71 create a strong magnetic field having a strength of approximately B = 10 Tesla (100KGauss). However, magnetic fields with strengths in the neighborhood of B = 15 to 20 Tesla (150 to 200KGauss) are also contemplated. Because windings 71 generate a great deal of heat they are cooled by water cooled platens 72 and water cooled spacers 74. Figure 3B is an end view of the helical solenoid magnet 16 illustrated in Figure 3A. The left hand portion of Figure 4 illustrates the relative magnitude of the magnetic field B z with respect to the long axis Z cm of the apparatus 10. The magnitude of the field B z is greatest near the center of the helical magnet 16 (B z MAX = 18 Tesla (180KGauss)) and lowest near the portion where the plasma column is formed at Z cm =0 (B z MIN = 14 Tesla (140 KGauss)). The right hand portion of Figure 4 shows the location of the conductors 80 that form parts of the winding 71 illustrated in Figure 3A. The right hand side of Figure 4 also shows the preferred dimensional relationship between the helical magnetic section 16 and the radial axis R cm and the longitudinal axis Z cm . Those preferred dimensions are as follows: D₁ = 2.5 centimeters D₂ = 7.6 centimeters D₃ = 2.5 centimeters The dimension D₃ is equal to 2.5 centimeters which is one-half of the width of gap 68 which has a diameter DR=5.0 centimeters. Figure 5 illustrates in schematic fashion the details of the powerful picosecond laser 24. Picosecond laser 24 is typical of the type of lasers that can now be constructed. There are other techniques that could be used as well. This particular technique is offered only to show the preferred embodiment for use with this invention and is not meant to limit the scope of other powerful picosecond lasers that could be used as well. Initially a mode locked Nd:YAG laser 82 produces an output 84. The output 84 goes to a KTP doubler 85 whose output is split and forms the input to a dual jet dye laser 86 and an Nd:YAG amplifier 92. The output from the dual jet dye laser 86 forms input to cavity dumper 88 the output of which forms the input to a three stage dye amplifier 90. Similarly the output from Nd:YAG amplifier 92 passes through a KD*P doubler 94 which pumps the three stage dye amplifier 90. The output of the KD*P doubler 94 is optically delayed by device 96 to correlate with the peak gain of the final dye amplifier 98 for which it provides pumping input. The output from the three stage dye amplifier 90 is the input for final dye amplifier 98. The output from final dye amplfier 98 is doubled in doubler crystal 100 and forwarded to mixer crystal 102. The output from mixer crystal 102 forms the input to preliminary KrF amplifier 104 which feeds the final KrF amplifier 106. The laser output 84 from the mode lock Nd:YAG laser 82 originally had a pulse duration of t p = 100 picoseconds and a wavelength of 1063nm. The apparatus 24 converted that output to a powerful picosecond laser beam 26 having a pulse width of t p = 1 picosecond and a wavelength of 248nm. The output energy of the powerful picosecond laser burst 26 is in the neighborhood of E = 1J. The elements just described which form part of the powerful picosecond laser apparatus 24 are conventional elements and, the method and apparatus for forming the powerful picosecond laser output 26 are believed to be within the abilities of one of ordinary skill in the art. Likewise, the CO₂ laser 18 illustrated in Figure 6 is believed to be generally conventional. The elements of the CO₂ laser 18 can, for example, be purchased individually from Maxwell Laboratory Inc., 8888 Bell Bowa Avenue, San Diego, California 92128. Typically a Marx generator 108 provides the initial power for the E-beam chamber 112. Sustainer bank oil tank 110 contains high voltage elements to provide a high voltage discharge in laser chamber 114. The electron beam formed in E-beam chamber 112 passes through laser chamber 114 for initiation discharge in laser chamber 114. The discharge in laser chamber 114 creates lasing action and the resulting CO₂ laser beam 20 is subsequently focused by optical system 116. The CO₂ beam 20 is shown schematically as it is emitted from the optical system 116. It preferably has a pulse width t p of 20 to 30 nanoseconds and an energy level of 1.5KJ. While the powerful picosecond laser 24 illustrated in Figure 5 and the lower power CO₂ laser 18 illustrated in Figure 6 have been described in sufficient detail for one of the ordinary skill in the art to make and use lasers 18 and 24, it will be appreciated that a detailed understanding of their operation structure. is not necessary for an understanding of the overall method and apparatus of the preferred embodiment of the invention 10. In operation a target 12, such as carbon is placed in the vacuum vessel 14 which is then evacuated. CO₂ laser 18 is then activated causing laser beam 20 to impinge upon carbon target 12. The 1.5 K joule, 20 - 30 nanosecond pulse causes the carbon target 12 to ionize. The powerful helical solenoid magnets 16 forms a magnetic field having a strangth of B = 10 Tesla (100KGauss) which causes the plasma to assume the shape of a column 15 having a length of 1 -2 centimeters and a diameter of approximately 1 millimeter. Approximately 20 - 100 nanoseconds after laser beam 20 has created the plasma column 15, it is subjected to the powerful picosecond laser beam 26 which has a pulse duration t p of approximately 1 picosecond and power in the neighborhood of 10¹⁵ - 10¹⁷ W/cm². Powerful picosecond laser beam 26 is focused along the central part of the plasma column which is approximately 50 micrometers in diameter. In general prior art efforts have been directed towards the interaction of powerful lasers with cold gas or solid targets. In contrast to the prior art approach, the present invention 10 comprehends interacting a very powerful picosecond laser with a plasma column. The unique plasma column 15 formed according to the present invention 10 has very favorable conditions for population inversion and gain due to its fast radiation cooling. Hence a significantly increased population inversion by fast selective excitation and ionization of ions by multiphoton processes induced by powerful picosecond laser leads to high gain and lasing action. Figure 7 illustrates the nature of the output pulse 120 which occurs with the interaction of the powerful picosecond laser pulse 26 with CVI ions in a confined plasma column 15. A maximum gain of approximately 11 measured in terms of K(cm⁻¹) is shown approximately 7 picoseconds after the powerful picosecond laser pulse 26. Soft X-ray laser beam 120 has a wavelength of 182 Angstroms. While the output laser beam 120 is shown in the soft X-ray lasing range of 1 - 2 x 10⁻⁸m (100 - 200 Angstroms), it is likely that higher gains can also be produced creating wavelengths in the shorter 0.6 - 0.7 · 10⁻⁸m (60 - 70 Angstrom) region using other than carbon target (e.g. higher Z materials). Moreover, theoretical modelling suggests that it is possible to predict and produce lasing action in the 0.1 - 0.2 · 10⁻⁸m (10 - 20 Angstrom) range using the same general method and apparatus but using more powerful lasers and high-Z materials as targets. The following table gives the preferred value of selected parameters of the invention 10 along with approximate ranges for producing acceptable results. TABLE OF VALUES PARAMETER PREFERRED VALUE APPROXIMATE RANGE D₁ 2.5 cm 2 - 3 cm D₂ 7.6 cm 6 - 8 cm D₃ 2.5 cm 2 - 3 cm D₄ 10.0 cm 5 - 15 cm DR 5.0 cm 4 - 6 cm Laser 18 Energy 1.5K J 1 - 2 K J Laser 18 Power 50 GW 40 - 80 GW Laser 18 Pulse FWHM 20-30 nanosecond 10-100 nsecond Laser 26 Energy 1J 0.5 - 2J Laser 26 Power 1000 GW 500 - 2000 GW Laser 26 Power Density 10¹⁶W/cm² 10¹⁵-10¹⁷ W/cm² Laser 26 Pulse FWHM 1 picosecond 1 - 2 picoseconds Time between Laser Pulses 18 & 26 50 nsecond 20-100 nsecond Plasma column length 1.5 cm 1 - 2 cm Plasma column diameter 1 mm 0.5 - 1.5 cm Plasma column electron density 10¹⁹cm⁻³ 10¹⁹-10²⁰cm⁻³ Plasma column ion density 1.5 x 10¹⁸cm⁻³ 1.5 x 10¹⁸-10¹⁹cm⁻³ Plasma column temperature during lasing action 20eV 10 - 50 eV Target 12 Carbon O, Ne, Ar, Al, Si, Kr, Mo Strength of Magnetic Field B = 10 Tesla (100 KGauss) 10 - 20 Tesla (100-200 KGauss) Vacuum in Pressure Vessle 14 1.33 x 10⁻⁵ Pa (10⁻⁷ Torr) 1.33 x 10⁻⁴ - 1.33 x 10⁻⁶ Pa (10⁻⁶ - 10⁻⁸ Torr) While the preferred embodiment of the invention relates to the use of a carbon target 12, it will be appreciated that large population inversions are also possible in multi-electron high-Z ions as well as in H- and Li-like ions of low-Z elements. For picosecond laser powers in excess of 10¹⁵ to 10¹⁷ W/cm² an increase in gain by selective population of the level n = 3 in CVI and OVII is expected. Similar results are anticipatable for Li-like ions NeVIII, AlXI and SiXII to increase the gain at wave-lengths of 2.92, 1.57 and 1.30 x 10⁻⁸m (292 Angstroms, 157 Angstroms and 130 Angstroms) respectively. Even better results are anticipated for the interaction of a powerful picosecond laser 26 with multi-electron ions in a recombining plasma column. Present information indicates that a powerful picosecond laser 26 with an energy level less than 10¹⁵ W/cm² can ionize multi-electron atoms (e.g. Ar, Kr, Xe, U) much more effectively than atoms with a few electrons (e.g. H, He, C). It is possible to create a recombining plasma column consisting of, for example, Ar VIII ions (or for example, Kr VIII or Kr IX ions) which can be excited by a powerful picosecond laser 26. Multiphoton ionization of Ar, Kr, Xe, U up to ArVII, KrVII, XeIX and UXI has already been observed. It is therefore believed possible to create a very high gain in the region of 1 - 2 x 10⁻⁸m (100-200 Angstroms) by the interaction of a powerful picosecond laser 26 near resonant multiphoton excitation frequencies of Ar VIII as well as Kr VIII or Kr IX. One of the problems with prior art approaches is that efforts were made to interact powerful lasers with cold gases. Under those circumstances the laser first had to ionize the atoms to be proper stage of ionization and then generate a large population inversion. However, according to the method and apparatus of the present invention, if the ions of the proper stage of ionization can be created independently (i.e. by the CO₂ laser beam 20) then the role of the powerful picosecond laser beam 26 is simplified to providing the high population inversion. Doppler line broadening in a plasma is significantly larger than in a cold gas. However, in a cold gas the gain is also limited by Stark line broadening, and this can be comparable to Doppler broadening in a recombining plasma.

Claims: An apparatus for use with apparatus for generating soft-X-Ray lasing action comprising: a target (12); first laser means (18) for producing a first laser beam (20) having a pulse duration between 10 and 100 nanoseconds and a power of between 40 and 80 GW for interacting with said target (12) and for generating a plasma having ions therein; magnetic field means (16) for confining said plasma and shaping it substantially into the form of a column (15); second laser means (24) for producing a second laser beam (26) having a pulse duration of between 1 and 2 picoseconds and a power in excess of 10¹⁵ w/cm² for interacting with said plasma column (15) to provide strong selective excitation of said ions to create population inversion and gain for soft X-ray lasing action. Apparatus as claimed in claim 1 further including: control means for controlling said second laser means (24) so that said second laser beam (26) interacts with said plasma column (15) between 20 and 100 nanoseconds after said plasma was generated by said first laser beam (20). Apparatus as claimed in any one of claims 1 to 2 wherein said magnetic field means (16) creates a magnetic field having a strength of 10 Tesla (B=100KGauss). Apparatus as claimed in any one of claims 1 to 3 wherein said plasma column (15) is approximately 1 millimeter in diameter. Apparatus as claimed in any one of claims 1 to 4 wherein said ions in said plasma column (15) comprise at least one member selected from the group consisting of C, O and Ne. Apparatus as claimed in claim 5 wherein said ions comprise ions of carbon. Apparatus as claimed in any one of claims 1 to 6 wherein said ions in said plasma column (15) comprise at least one member selected from the group consisting of Al, Si, Ar, Kr and Mo. Apparatus as claimed in any one of claims 1 to 7 wherein said first laser means (18) comprises a CO₂ laser. A method for generating population inversion and soft X-ray lasing action comprising the steps of: generating a first laser beam (20) having a pulse duration between 10 and 100 nanoseconds and a power between 40 and 80 GW; creating a plasma having ions therein by impinging said first laser beam (20) on a target (12); generating a magnetic field (16) to confine said plasma and shape said plasma substantially into the form of a column (15); generating a second laser beam having a pulse duration of between 1 and 2 picoseconds and power in excess of 10¹⁵ W/cm²; cooling said plasma column (15) by radiation cooling and subsequently interacting said second laser beam (26) with said plasma column (15) to provide strong selective excitation of said ions to create population inversion and gain for soft X-ray lasing action. The method as claimed in claim 9 wherein said ions are formed from material comprising at least one member selected from the group consisting of C, O and Ne. The method as claimed in claim 10 wherein said ions are carbon ions. The method as claimed in claim 9 wherein said ions are formed from material comprising at least one member selected from the group consisting of Al, Si, Ar, Kr and Mo. The method as claimed in any one of claims 9 to 12 wherein said first laser beam (20) is generated by a CO₂ laser.

Inventors: SUCKEWER SZYMON, SUCKEWER, SZYMON
Applicants: UNIV PRINCETON, PRINCETON UNIVERSITY
Application year: 1986
Main classification: H01S4
IPC codes: H01S3, H01S4, H05H1