Patent Number: 
Section: description

A photon source in accordance with a feature of the invention operates in two stages to produce X-ray or extreme ultraviolet radiation. In a first stage, a central plasma is formed using multiple ion beams directed at a central plasma discharge region as described below. In a second stage, a heating current pulse is passed through the central plasma in order to heat and compress the plasma, raising its temperature and density. The astron source is a source of photons comprising a discharge chamber, a plurality of ion beam sources in the discharge chamber, each electrostatically accelerating a beam of ions of a working gas toward a plasma discharge region, and a neutralizing mechanism for at least partially neutralizing the ion beams before they enter the plasma discharge region. The neutralized beams enter the plasma discharge region and form a hot plasma that radiates photons. The astron principle that operates in the first stage of the photon source described above is illustrated in FIGS. 1A and 1B. The embodiment of the source shown in FIGS. 1A and 1B has a two gap ion acceleration structure 100. Acceleration structure 100 includes concentric spherical electrode shells 112, 113 and 114. The electrode shells 112, 113 and 114 have a plurality of sets of holes aligned along axes which pass through a central plasma discharge region 120. Thus, for example, holes 122, 123 and 124 in electrode shells 112, 113 and 114, respectively, are aligned along an axis 126 that passes through plasma discharge region 120. Each set of holes, such as holes 122, 123 and 124, defines an acceleration column 128. The spaces between electrode shells 112, 113 and 114 constitute acceleration gaps for electrostatic acceleration of ion beams. Thus, each acceleration column has two gaps in the embodiment of FIGS. 1A and 1B. The embodiment of FIGS. 1A and 1B includes 36 acceleration columns 128, arrayed in three sets of 12. Thus, the acceleration structure directs 36 ion beams toward plasma discharge region 120. However, different numbers of ion beams may be utilized within the scope of the invention. The electrode shells 112, 113 and 114 may be supported by insulating spacers 130. A plenum 132 having ports 134 encloses acceleration structure 100. A working gas is introduced, either in a pulsed mode or continuously, through ports 134 into a space 144 behind the outermost electrode shell 114. Some of the working gas flows down the acceleration columns 128. When the appropriate gas density is present in the acceleration columns, a pulsed voltage may be applied between electrode shells 112 and 114, with the polarity of electrode shell 114 being positive with respect to electrode shell 112. In the configuration of FIGS. 1A and 1B, provided the appropriate gas density is present and provided that sufficient voltage is applied, a pseudospark discharge develops simultaneously in each of the acceleration columns 128. The pseudospark discharge is characterized by the development of oppositely directed electron and ion beams that can have extremely high intensity. The ion beam exits from the negative polarity end of the acceleration column 128 at electrode shell 112 and progresses toward the central plasma discharge region 120. By correct adjustment of the working gas density at an exit region 146 of each of acceleration columns 128, most of the ions can be neutralized by resonant charge exchange, so as to form a neutral beam that propagates without deflection to the plasma in plasma discharge region 120. Those ions that are not neutralized contribute excess positive charge to each of the ion beams, causing electrons to be attracted from the nearby surface of electrode shell 112, which is already primed as a cathode due to the breakdown into a pseudospark discharge. Thus, the neutral atoms are accompanied by a nearly charge-balanced beam plasma, including the remaining unneutralized ions and electrons. The slow ions resulting from resonant charge exchange define tracks that are favored for a conduction of a high current heating pulse in the second stage of device operation, as described below. Additional details and embodiments of the astron photon source are described in the aforementioned application Ser. No. 09/815,633, which is hereby incorporated by reference. In the second stage of device operation, the newly formed plasma is heated and compressed, or pinched, by passage through the plasma of a pulse of electric current. A first embodiment of a photon source which incorporates both the first stage of operation, wherein beams of ions are electrostatically accelerated toward a plasma discharge region and are at least partially neutralized, and the second stage, wherein an electric current is passed through the plasma discharge region, is shown in FIGS. 2A and 2B. FIG. 2A is a simplified cross-sectional side view of the photon source, and FIG. 2B is a cross-section defined by revolution of line Axe2x80x94A in FIG. 2A around axis 200. In FIGS. 2A and 2B, a central cathode shell, corresponding to electrode shell 112 in FIG. 1A, is divided into two half shells 202 and 204 that are electrically connected to a pulse voltage source 205. The anode shell of the photon source is divided into two half shells 212 and 214 which are electrically connected by a conductor 216. The working gas is introduced at low pressure through ports 218 and flows through passages 220 to enter hollow anode volumes 222 within anode half shells 212 and 214. Cathode half shells 202 and 204 are electrically isolated by insulator 225. The cathode half shells are electrically isolated from the respective anode half shells by insulators 227 and 229. A pulse voltage source 215 is connected between cathode half shells 202 and 204 and anode half shells 212 and 214. During the first phase of operation of the photon source shown in FIGS. 2A and 2B, a pulsed voltage V1 from pulse voltage source 215 is applied between anode half shells 212 and 214 and cathode half shells 202 and 204. In the absence of any applied voltage V2 from pulse source 205, the potential difference between cathode half shells 202 and 204 remains at zero. The combined cathode half shells are therefore pulsed negatively by voltage V1 relative to the combined anode half shells, and a discharge develops as described above in connection with FIGS. 1A and 1B. Neutralized beams from this discharge pass through a plasma discharge region 224 to form a small spherical plasma. At the same time, the passage of ions and energetic neutral atoms forms ionized tracks 230 between cathode half shell 202 and plasma discharge region 224, and ionized tracks 232 between cathode half shell 204 and plasma discharge region 224. The ionized tracks 230 and 232 lie on the surfaces of two cones that have their vertices located at plasma discharge region 224 and provide conducting paths between cathode half shells 202 and 204. During the second phase of operation, a pulsed voltage V2 from pulse voltage source 205 is applied between cathode half shells 202 and 204. The circuit is completed by conduction through the conical configuration of ionized tracks connecting cathode half shells 202 and 204. The current flows through the plasma in plasma discharge region 224, heating and compressing it via the magnetic pinch effect. The plasma temperature and density rise to the point where the desired X-ray or extreme ultraviolet radiation is emitted copiously. The radiation escapes the photon source in conical beams 234 that are relayed to the point of use by collecting optical surfaces (not shown in FIGS. 2A and 2B). The working gas pressure in the central part of the acceleration structure is preferably maintained in a range of about 1.0 to 100 millitorr. As noted above, one suitable working gas is xenon. Other suitable working gases include, but are not limited to, hydrogen, lithium, helium, nitrogen, oxygen, neon, argon and krypton. The ion beams may be pulsed or continuous, and the ion acceleration voltage V1 may be from 2 kV to 20 kV, but is not limited to this range. Voltage V1 may have a typical pulse duration of 100 nanoseconds to 10 microseconds, but may also be applied continuously. The heating voltage V2 is applied typically within 100 nanoseconds to 10 microseconds of the initial application of voltage V1 . The amplitude of voltage V2 is typically in the range of 100 volts to 10 kV, and the width of this pulse typically ranges from 10 nanoseconds to 1 microsecond. A second embodiment of a photon source in accordance with the invention is shown in FIGS. 3 and 4. FIG. 3 is a simplified cross-sectional side view of the photon source, and FIG. 4 is a cross-section defined by revolution of line Axe2x80x94A in FIG. 3 around axis 200. Like elements in FIGS. 2A, 2B, 3 and 4 have the same reference numerals. The embodiment of FIGS. 3 and 4 differs from the embodiment of FIGS. 2A and 2B by the addition of a transformer 211 for coupling pulsed electrical current from pulse voltage source 205 to cathode half shells 202 and 204. Transformer 211 includes multiple primary windings 208, a toroidal core 210, which may be of non-magnetic or magnetic material, and an armature or secondary 206, also having a toroidal configuration. Primary windings 208 are connected to pulse voltage source 205, and secondary 206, which may have a single turn, is connected between cathode half shells 202 and 204. During the first phase of operation, a pulsed voltage V1 is applied between the anode half shells 212 and 214 and secondary winding 206 that is connected to cathode half shells 202 and 204. In the absence of any applied voltage V2 from pulse voltage source 205 to primary windings 208, the electric potential between the cathode half shells 202 and 204 remains at zero. The combined cathode half shells are therefore pulsed negatively by voltage V1 relative to the combined anode half shells 212 and 214, and a discharge develops as described above. The neutralized beams of this discharge pass through plasma discharge region 224 to form a small spherical plasma. At the same time, the passage of ions and energetic neutral atoms forms ionized tracks 230 and 232 as described above. During the second phase of operation, a pulsed voltage V2 is applied simultaneously and in parallel across all the primary windings 208, with the result that a voltage is induced between cathode half shells 202 and 204 that are connected to opposite ends of transformer secondary 206. The transformer secondary circuit is completed by a conduction through the ionized tracks 230 and 232 connecting cathode half shells 202 and 204. The secondary current flows through the plasma in plasma discharge region 224, heating and compressing it via the magnetic pinch effect. As described above, the plasma temperature and density rise to the point where the desired X-ray or extreme ultraviolet radiation is emitted. An embodiment of a system for generating photons in accordance with the invention is shown schematically in FIG. 5. An acceleration structure 500 may correspond to the acceleration structure shown in FIGS. 2A and 2B, the acceleration structure shown in FIGS. 3 and 4, or in any other acceleration structure within the scope of the present invention. In the system of FIG. 5, acceleration structure 500 is a modification of acceleration structure 100 shown in FIGS. 1A and 1B and described above. Like elements in FIGS. 1A, 1B and 5 have the same reference numerals. Acceleration structure 500 includes concentric spherical electrode shells 112, 113 and 114, each of which is divided by an insulator 503 into electrode half shells. A pulse voltage source 540 is connected between inner electrode half shells 112a and 112b. A pulse voltage source 530 is connected between outer electrode half shells 114a and 114b and inner electrode half shells 112a and 112b.  Acceleration structure 500 is enclosed within a housing 502 that defines a discharge chamber 504. A top aperture 140 of acceleration structure 500 is coupled through a screen 510 to a collection region 514 that is defined by an enclosure 516. Enclosure 516 contains collection optics 518 for relaying a photon beam 150 to a remote point of use. Screen 510 allows propagation of photons from discharge chamber 504 to collection region 514 but impedes flow of gas from discharge chamber 504 to collection region 514. A gas source 520 coupled to housing 502 supplies a working gas through inlets 522 and ports 134 in plenum 132 to acceleration structure 500. A bottom aperture 142 of acceleration structure 500 is coupled to a vacuum pump 524. An outlet 526 of vacuum pump 524 is connected to gas source 520 to form a gas recirculation system. The gas source 520 and the vacuum pump 524 are connected to housing 502 in a closed loop configuration that permits recirculation of the working gas through discharge chamber 504. Gas source 520 may include elements for removing impurities and particulates from the working gas. The system may include a detector 550 located in collection region 514, a control circuit 552 and a flow controller 554 for a feedback control of the rate of flow of the working gas into the discharge chamber 504 in response to a measured spectrum of the radiated photons. The system of FIG. 5 operates with first and second phases as described above in connection with FIGS. 2A and 2B. In particular, pulse source 530 applies a pulsed voltage between inner electrode half shells 112a and 112b and outer electrode half shells 114a and 114b, causing neutralized beams to be directed toward plasma discharge region 120. In the second stage, the plasma in discharge region 120 is heated and compressed by passage of a pulse of electric current. The neutralized beams form ionized tracks between cathode half shells 112a and 112b and plasma discharge region 120. Application of a pulse to cathode half shells 112a and 112b by pulse source 540 causes electrical current to flow along the ionized tracks through plasma discharge region 120, heating and compressing the central plasma. The plasma temperature and density rise to the point where the desired x-ray or extreme ultraviolet radiation is emitted. The radiation escapes the acceleration structure 500 as conical photon beam 150. A third embodiment of a photon source in accordance with the invention is shown in FIGS. 6A and 6B. Like elements in FIGS. 1A, 1B, 6A and 6B have the same reference numerals. The embodiment of FIGS. 6A and 6B differs from the structure of FIGS. 1A and 1B by the addition of an external electrode 600 for supplying an electrical heating current to plasma discharge region 120. A pulse voltage source 601 is connected between inner electrode shell 112 and outer electrode shell 114. A pulse voltage source 602 is connected between inner electrode shell 112 and external electrode 600. Insulators 603 and 604 electrically isolate the connections to electrode shells 112 and 114, respectively. External electrode 600 may have a cylindrical configuration and may be positioned in the bottom aperture of the acceleration structure in spaced relationship to plasma discharge region 120. During the first phase of operation, a pulse voltage V3 from voltage source 601 is applied between electrode shells 112 and 114. A discharge develops as described above, and the neutralized beams of the discharge pass through plasma discharge region 120 to form a plasma. At the same time, the passage of ions and energetic neutral atoms forms ionized tracks as described above. During the second phase of operation, a pulse voltage V4 from voltage source 602 is applied between external electrode 600 and electrode shell 112. The circuit is completed by conduction through the ionized tracks connecting electrode shell 112 and plasma discharge region 120 and through a glow region 605 between plasma discharge region 120 and external electrode 600. The current flows through the plasma in plasma discharge region 120, heating and compressing it. The plasma temperature and density rise to the point where the desired x-ray or extreme ultraviolet radiation is emitted. The plasma tends to be elongated in the direction of external electrode 600. The working gas extends to electrode 600 at approximately the same pressure as inside electrode shell 112. While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.