CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This Patent Application claims priority from (a) U.S. Provisional Patent Application o. 61/411,390, filed on November 8, 2010 and (b) U.S. Provisional Patent Application No. 61/425,198, filed February 1, 2011. The disclosure of each of these provisional applications is incorporated by reference herein in its entirety for all purposes.

[0002] This application is related to PCT Patent Application No. (Attorney Docket No.

91920-823094), filed on ,. The disclosure of which is incorporated by reference herein in its entirety for all purposes. This application is also related to (a) U.S. Provisional Patent Application No. 61/537,789 filed on September 22, 2011, and (b) U.S. Provisional Patent Application No. 61/537,794, filed on September 22, 2011.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

[0003] The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the U.S. Department of Energy and Lawrence Livermore National Security, LLC, for the operation of Lawrence Livermore National Laboratory.

BACKGROUND

[0004] Precise alignment and positioning of a target in a fusion reaction chamber is important in order to ensure that maximum energy is transferred to the target in order to start and sustain a fusion reaction.

[0005] Despite progress being made in methods and systems for target injection and tracking, there is a need in the art for more improved method and systems for target injection and tracking.

SUMMARY

[0006] The present invention generally relates to fusion reactors. Some embodiments of the present invention provide a system that includes an injection mechanism configured to load a fusion target, a transportation mechanism coupled to the injection mechanism, a fusion chamber disengageably coupled to the barrel, a tracking mechanism coupled to the fusion chamber and configured to (a) determine an expected location for the fusion target near the center of the fusion chamber and (b) determine a time at which the fusion target will reach the expected location. The system also includes an engagement mechanism configured to determine one or more locations on the target for focusing one or more laser beams.

[0007] Some embodiments of the present invention provide a tracking system. The tracking system includes a first laser transmitter and a first laser receiver disposed on opposite sides of a fusion chamber along a lateral direction. The first laser transmitter generates a first laser beam that is incident on the first laser receiver. The tracking system also includes a second laser transmitter and a second laser receiver disposed on the opposite sides of the fusion chamber along the lateral direction. The second laser transmitter generates a second laser beam that is incident on the second laser receiver. The second laser beam is spaced apart in a vertical direction from the first laser beam by a first distance. The tracking system further includes a processor configured to compute a velocity of a fusion target travelling in a direction orthogonal to the first and the second laser beam.

[0008] Certain embodiments of the present invention provide a method that includes focusing a first light beam on a fusion target disposed in a fusion chamber and collecting scattered light from the fusion target using one or more optical assemblies. The method further includes generating an image of the fusion target using the scattered light, identifying a location on the fusion target using the image, and determining coordinates for a current focus point for a laser beam. Thereafter the method includes determining a difference between the coordinates of the current focus point and the identified location, determining a correction parameter based on the difference, and providing the correction parameter to a laser control system. In some embodiments, the laser control system controls the laser beam to focus on the identified location on the fusion target.

[0009] Numerous benefits are realized by the systems and methods described herein over conventional systems. For example, use of gas-assisted injection ensure proper and vibration-free acceleration of the target to the intended velocity as it enters the fusion chamber. Accurate tracking of the target after it enters the fusion chamber helps to align the laser systems to ensure that when the target reaches its intended location within the chamber, the lasers hit the target simultaneously at desired locations on the target to initiate the fusion reaction. [0010] These and other embodiments of the invention along with many of its advantages and features are described in more detail in conjunction with the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Fig. 1 is a schematic illustrating a system according to an embodiment of the present invention.

[0012] Fig. 2 is a schematic illustrating a target injection system according to an

embodiment of the present invention. [0013] Fig. 3 is a schematic illustrating details of the gas-gun injector mechanism according to an embodiment of the present invention.

[0014] Fig. 4 is a cross-sectional view of a barrel including a target travelling inside the barrel according to an embodiment of the present invention.

[0015] Fig. 5 is a schematic illustrating a steering mechanism for the target according to an embodiment of the present invention.

[0016] Fig. 6 is a schematic illustrating a shutter assembly according to an embodiment of the present invention.

[0017] Fig. 7 A is a schematic illustrating a target tracking system according to an embodiment of the present invention. [0018] Fig. 7B is a schematic illustrating details of tracking a target according to an embodiment of the present invention.

[0019] Fig. 8 is a schematic illustrating a target engagement system according to an embodiment of the present invention.

[0020] Fig. 9 is a flow diagram of a process for tracking a target according to an embodiment of the present invention.

[0021] Fig. 10 is a flow diagram of a process for engaging a target according to an embodiment of the present invention. DETAILED DESCRIPTION

[0022] The present invention generally relates to a fusion reactor. More specifically, embodiments of the present invention relate to systems and methods for injecting a fusion target into a fusion chamber. Certain embodiments of the present invention relate to tracking the target once the target enters the fusion chamber and providing feedback on its position to an engagement system. Other embodiments of the present invention relate to mechanisms for aligning the lasers using the position information from the tracking systems to ensure that all the lasers hit the target at their intended location on the target and at the intended time.

[0023] Fig. 1 is a simplified schematic illustrating a system 100 according to an

embodiment of the present invention. System 100 includes a fusion chamber 102. A target injection barrel 104 is coupled to fusion chamber 102. A fusion target 116 is loaded from a distal end of barrel 104 and travels through barrel 104 and enters fusion chamber 102. The details of the structure and function of target 116 are provided in a co-pending U.S. Patent Application No._( Attorney Docket No. 91920-823094). [0024] One or more laser systems 108 generate multiple laser beams 112 that are directed into fusion chamber 102 to be incident on target 1 16 at a predetermined time. In some embodiments, laser systems 108 include (a) the necessary laser generation mechanisms that are configured to generate laser beams that deliver a certain power and (b) control mechanisms that are configured to receive location information and properly orient the laser beams so that they hit target 1 16 at the desired time and at desired locations. In some embodiments, laser beams 1 12 can be incident on target 116 from more than one direction. In some embodiments, up to 500 laser beams can be used to energize target 116 when it reaches the desired location within fusion chamber 102.

[0025] An target injection mechanism 106 may be used to accelerate target 116 through barrel 104 in order to achieve a predetermined velocity for target 116 as it enters fusion chamber 102. Details of injection mechanism 106 are provided below. Tracking and engagement system 110 can be configured to track the target as it travels through fusion chamber 102 and toward it's intended location. Tracking and engagement system 110 can provide feedback on the target's speed and position to laser systems 108. Details of the tracking and engagement system are provided below.

[0026] In operation, target 116 may be loaded from the distal end of barrel 104. Injection mechanism 106 may be used to inject target 116 and then accelerate it to a predetermined velocity. In some embodiments, the predetermined velocity is about 250 m/sec. Once target 116 enters fusion chamber 102, tracking and engagement system 110 can track target 116 as it travels from the entry point into fusion chamber 102 to its final destination, which is somewhere near the center of fusion chamber 102. In some embodiments, tracking and engagement system 110 determines the velocity and position of the target as it travels within fusion chamber 102. Tracking and engagement system 110 provides this information to laser systems 108. In response to that, laser systems 108 determine when to turn on laser beams 112 and the intended destination of the laser beam so that all laser beams 112 concurrently hit the target at the intended time and at a predetermined location on the target.

[0027] Once the target reaches its predetermined location within fusion chamber 102, laser systems 108 generate multiple laser beams 112 that hit target 116 thereby initiating a fusion reaction within target 116.

[0028] It will be appreciated that the system configurations and components described herein are illustrative and that variations and modifications are possible. Further, while the system is described herein with reference to particular blocks, it is to be understood that these blocks are defined for convenience of description and are not intended to imply a particular physical arrangement of component parts. Further, the blocks need not correspond to physically distinct components. Blocks can be configured to perform various operations, e.g., by programming a processor or providing appropriate control circuitry, and various blocks might or might not be reconfigurable depending on how the initial configuration is obtained. Embodiments of the present invention can be realized in a variety of devices including electronic devices implemented using any combination of circuitry and software.

[0029] It is to be noted that Fig. 1 is a simplified schematic of fusion reaction chamber. One skilled in the art will realize that there are many more systems/components that may be needed to make system 100 work. However, these additional systems and components are not illustrated and/or described herein for sake of brevity.

[0030] As described above, in an embodiment, a fusion system may include a target injection system, a target tracking system, and a target engagement system, among others. Each of these systems is described below.

Target Injection System

[0031] Fig. 2 is a schematic of a target injection system according to an embodiment of the present invention. Injection system may include a loading mechanism 202 that can be used to load targets 210 into barrel 204. A gas gun 206 may be used to generate pressure for accelerating target 210 through barrel 204. Due to the high pressure generated by gas gun 206 and the velocity of the target as it travels through barrel 204, in some instances, barrel 204 may be subject to vibration. The vibration in turn may lead to change in velocity and/or trajectory of target 210. Thus, target 210 may veer off its intended course, which is undesirable. In order to prevent or minimize the vibration, in some embodiments, barrel 204 is clamped at its muzzle by a clamping mechanism 208. In some embodiments, barrel 204 can be up to 10 meters long and can be made of materials such as steel, steel with a chromium liner, steel with a diamond-like-carbon liner, etc.

[0032] In some embodiments, target 210 is accelerated to between 4000 m/s and

10000 m/s ² as it travels through barrel 204. It would be beneficial to have low acceleration as this will likely reduce the stress on the injection system components. In an embodiment, as target 210 exists barrel 204, its velocity may be between 50 to 500 meters/sec. At such high velocity and acceleration values, if the muzzle of barrel 204 is not clamped, the ensuing vibration of barrel 204 will likely alter the trajectory of the target as it enters the fusion chamber. Vibration damping and active vibration cancelling features may be added to reduce the vibration of the barrel. In some embodiments, barrel 204 includes a high precision bore at its muzzle in order to provide a specific trajectory to the target and align the target as it enters the fusion chamber.

[0033] In order to impart the desired acceleration to the target, an embodiment of the present invention may use a gas gun 206, as described above. In some embodiments, gas gun 206 may use Helium gas under pressure in order to accelerate the target. However, other gases such as Xenon, Neon, Argon etc. may also be used. In some embodiments, the gas pressure inside gas gun 206 is between 1 psi and 200 psi. In an embodiment, the gas pressure may be about 25 psi. In some embodiments, barrel 204 is charged with the gas to a desired pressure before the target is injected into the barrel. In other embodiments, the pressure inside barrel 204 may be continuously raised as target 210 travels through barrel 204.

Regardless of the method used to achieve the desired gas pressure, target 210 accelerates as it moves through barrel 204. In some embodiments, gas gun 206 may have to be recharged again for accelerating the next target. In some embodiments, up to 15 targets may be injected into barrel 204 per second. So, gas gun 206 may have a repetition rates of between 5 Hz and 25 Hz.

[0034] As target 210 travels through barrel 204, the gas in the barrel dissipates thereby resulting in a drop in pressure across target 210 during its motion. The injection pressure or the pressure to be applied as target 210 is being loaded into barrel 204 needs to set by taking onto account this pressure drop that may result as target 210 travels through the barrel. In some embodiments, the injection mechanism may include more than one gas gun 206 for redundancy. As illustrated in Fig. 3, in an embodiment of the present invention, more than one gas gun 306 may be mounted on a turret. In this instance, if one of the gas guns 306 malfunctions, it can be replaced with another gas gun thus reducing downtime for the fusion system. The malfunctioned gas gun 306 may then be replaced during the next maintenance cycle.

[0035] In some embodiments, as the target travels through the barrel it may touch the inner walls of the barrel creating friction and potentially slowing down the target, causing damage to the target, or both. Ideally, it would be beneficial if the target could travel through the barrel without any friction thus reducing drag on the target making it easier to achieve the desired final velocity and acceleration. Fig. 4 illustrates a cross-sectional view of a barrel including a mechanism to reduce friction inside the barrel according to an embodiment of the present invention.

[0036] As illustrated in Fig. 4 a target 404 travels through barrel 402. As target 404 travels, it may contact the inner walls of barrel 402. This creates friction between target 404 and the inner walls of the barrel. Since target 404 is travelling at a high velocity this friction may cause damage to target 404, barrel 402, or both. It would not be desirable to have a damaged target enter the fusion chamber as it may lead to other potential issues including but not limited to failure of fusion reaction to occur. Also, wear on the barrel may lead to a premature mechanical failure of barrel 402, which is also undesirable and may increase maintenance cost for a fusion system that incorporates the barrel.

[0037] In an embodiment, multiple gas channels 406 may be incorporated within the inner walls of barrel 402. A gas may be injected through gas channels 406. The injected gas may form a cushion of gas around target 404 thus preventing target 404 from touching the inner walls of barrel 402. In other words, target 404 may be suspended or levitated by the surrounding gas cushion thus reducing friction between target 404 and barrel 402. In some embodiments, the gas is injected from an external source other than the gas gun. In other embodiments, the gas injected by the gas gun described above may be circulated within barrel 402 in a manner to form the aforementioned gas cushion. [0038] In another embodiment, a liquid or solid lubricant may be applied to the barrel or the parts of target 404 in contact with barrel 402. Some examples of solid lubricants that may be used include MoS ₂ , TS ₂ and graphite. Some example of liquid lubricants that may be used include polyalkylene glycols. In some embodiments, lubricants may exhibit a phase change during the acceleration process, e.g., a solid lubricant changing into a liquid or gaseous form.

[0039] Fig. 5 is a schematic that illustrates a mechanism for accelerating a target as it moves through the barrel according to an embodiment of the present invention. As illustrated in Fig. 5, target 502 travels through barrel 504 once it is injected into barrel 504, e.g. using the gas gun described above. The periphery of barrel 504 is lined with one or more electromagnets 506. When activated, electromagnets 506 generate a travelling magnetic field whose radial component induces circumferential (eddy) currents in conductive target 502. This current sets up a magnetic field, which in turn interacts with the travelling wave to propel target 502. In some embodiments, it may be beneficial to spin target 502 as it travels through barrel 504. The spinning may provide stability to target 502 as it travel through barrel at high velocities. In some embodiments, additional coils can be added on the periphery of barrel 504 to generate the desired spinning profile for the target. In some embodiments, target 502 is spun at a rate of between 1 and 3 meters per turn. [0040] It is desirable that the interaction between the target and the hot gases within the barrel be kept to a minimum. If the target is in contact with the hot gases inside the barrel for a longer period, the temperature of the target may increase causing damage to the target before it enters the fusion chamber. Also, as described above, it is beneficial if the target enters the fusion chamber at the right velocity and trajectory. It order to maintain the desired trajectory, the target may need to be steered once it enters the barrel so that upon exit from the barrel, the target is on its desired trajectory. Several techniques may be used to steer the target as it moves through the barrel. For instance, In some embodiments, the target may be diamagnetic and magnetic focusing rays may be used to steer the target through the barrel. In other embodiments, acoustic waves may be used to steer the target through the barrel. [0041] As described above, the barrel couples to the fusion chamber. As the target exits the barrel, it enters the fusion chamber where it undergoes a fusion reaction. Since about 16 targets may be injected into the fusion chamber every second, there is continuous flow of targets within the barrel even when a previously injected target is undergoing a fusion reaction. Thus, it is beneficial to protect the subsequent targets and the inner surfaces of the barrel from damage due to the by-products of the fusion reaction. In some embodiments, the fusion reaction inside the fusion chamber generates neutrons. These neutrons may be absorbed by the barrel and even the injection mechanism components upstream resulting in generation of heat. This heat needs to be dissipated and that too in a fairly short amount of time to prevent the barrel and other components from degradation. [0042] Fig. 6 illustrates a shutter mechanism that may be used to protect the barrel once the target is injected into the fusion chamber according to an embodiment of the present invention. Shutter mechanism 602 that may be rotatably coupled to the barrel 604. Shutter 602 may include one or more slots 606 through which a target may pass on its way to the fusion chamber. Shutter 602 may also include one or more blocking sections 608. Each blocking section 608 may be disposed between two adjacent slots 606. In some

embodiments, the rotation of shutter 602 may be adjusted such that after a target is injected into the fusion chamber via a slot 606, a blocking section 608 covers the opening of barrel 604 for the duration of the fusion reaction thus effectively providing shielding for the barrel and other components upstream. In some embodiments, shutter 602 may made rotated at a speed of about 450 rotations per minute (rpm). In order to absorb the neutrons generated during the fusion reaction in the fusion chamber, in an embodiment, shutter 602 may be made from materials that can absorb the neutrons, e.g., concrete. In some embodiments, it may be necessary to employ additional cooling mechanism to dissipate the heat built up into shutter 602 by the neutron absorption.

[0043] In some embodiments, in order to ensure redundancy, one or more clusters of shutters 610 may be coupled to barrel 604. Thus if one of the shutters malfunctions, another may be swapped to keep the downtime at a minimum.

Tracking System

[0044] Embodiments described above relate to loading of a target and injecting the target into the fusion chamber. Once the target enters the fusion chamber, it is beneficial to know the position of the target at any given time so that the laser systems can be controlled to align the laser beams to properly to hit the target at the desired time and location in order to initiate the fusion reaction. Failure to hit the target with the lasers at the right time and location may lead to failure of initiating the fusion reaction. In an embodiment, the desired location for the target before a fusion can be initiated may be the center of the fusion chamber or in the near vicinity of the center of the fusion chamber. Thus there is a need to effectively track the target as it enters the fusion chamber and to signal the laser systems at the proper time to energize the target with the laser beams. [0045] In some embodiments, the tracking system determines the location and velocity of the target related to the fusion chamber environment. Based on that determination, the tracking system can predict when the target will reach the center of the fusion chamber or near the center of the fusion chamber. Based on this information, the laser control system can fire the laser beams at the appropriate time. [0046] Fig. 7 A is a schematic of a tracking system according to an embodiment of the present invention. The tracking system may include one more laser beam transmitters and receivers. Each pair of laser beam generator 702 and laser beam receiver 704 are located on the opposite sides of fusion chamber 706. The laser beam generator 702 and receiver 704 are mounted on a support mechanism 714 that is mechanically isolated from fusion chamber 706 in order to prevent the ignition/injection induced motion from disturbing the tracking system components. A tracking beam 708 generated by each laser beam generator 704 traverses in a direction that is orthogonal to the direction of the target travel path. In some embodiments, multiple such laser beams 708 may be used to track a target as it travels through fusion chamber 706. In an embodiment up to five laser beams may be used to track the target. In the instance where there are multiple tracking beams 708, they are stacked vertically, as illustrated in Fig. 7A. In some embodiments, tracking beams 708 are spaced apart by a distance ranging between 0.2 m and 1 m. In some embodiments, tracking beam 708 may be generated using any of various laser sources such as Nd:YAG, diode lasers, etc. [0047] Each tracking beam 708 is aligned to a permanent set of reference points associated with fusion chamber 706. The reference points are located by precision survey in the same coordinate system as the fusion chamber. Thus, the precise coordinates of the tracking beams 708 are known in reference to fusion chamber 706. As the target moves through the fusion chamber, it passes through each tracking beam 708. When the target passes through a tracking beam 708 , it intercepts the tracking beam 708 and creates a shadow. The target's moving shadow produces a time varying signal that can be detected by one or more fast diodes 710a. The signal detected by fast diodes 710a can provide the time at which target 712 passes the known vertical location of the tracking beam 708 as illustrated in Fig. 7B.

[0048] As the target passes through successive tracking beams 708, it is possible to determine the velocity of the target since the distance between the tracking beams 708 is known. In addition, a lateral (or X) position at which the target passes through tracking beam 708 can be determined from an image captured by a line camera 710b, e.g. a linear CCD array. In some embodiments, a rapid succession of such images records the target's transverse position with respect to time. Variations in this lateral position as the target passes through the tracking beam 708 determine the target is tilted as it nears the center of the fusion chamber. If the target is indeed tilted, then the laser systems may have to be aligned so as to still hit the target at designated locations on the target.

[0049] Based on the information about the velocity of the target and the transverse (or lateral) location at which it passes through each of the laser beams, it is possible to determine a precise location at which the target will be when it gets closer to the center of the fusion chamber. In addition, the tracking system can also determine the time at which the target will reach that location. As described above, it is important that the precise end location of the target and the time the target will be at that location be known in order to properly align the laser systems.

[0050] In some embodiments, the tracking system can provide an accuracy of about 50 μηι for the horizontal target position relative to the tracking coordinate system. In a particular embodiment, each tracking beam 708 may be spaced at intervals of about 0.25 m. In this instance, the tracking system may provide velocity measurement accuracy of about

0.12 m/sec. In an embodiment, the tracking system can provide tilt measurement accuracy of about 4 mrad. In some embodiments, additional tracking beams 708 may be deployed if further accuracy is needed.

[0051] In some embodiments, an area may be defined near the center of the fusion chamber. It may be desired that the target be located within this defined area in order to ensure that a fusion reaction can be initiated successfully. In this embodiment, if the target ends up outside this area, e.g., due to excessive tilt, it may be not be possible to initiate a fusion reaction since the laser systems may not be able to point the laser beams at the desired locations on the target. Thus, in an embodiment, the tracking system may determine whether the target is expected to be in the defined area and if so, at what time. [0052] In some embodiments, the tracking beams 708 may have to be periodically adjusted to maintain their position relative to the surveyed references. Several factors may cause the alignment of the tracking beams to deviate from their original position. For example, vibration-induced mechanical relaxation and periodic maintenance of the tracking system components, can cause the tracking beams to become misaligned. Since it is important to maintain the tracking beams in precise position, it may be necessary to periodically check and realign them if needed.

Engagement System

[0053] As described above, the tracking system provides a fairly accurate estimation of where the target will end up near the center of the fusion chamber and at what time.

However, since the initiation of the fusion reaction depends on precisely hitting the designated locations on the target with the laser beams at the desired time, more detailed measurements may be needed as the target passes the last of the tracking beams and nears its expected location near the center of the fusion chamber. This finer control mechanism is referred to herein as the engagement system. In some embodiments, the engagement system is activated a few microseconds before the target is estimated to reach its location near the center of the fusion chamber. In a particular embodiment, the engagement system is activated about 23 μβ before the target is estimated to reach its location near the center of the fusion chamber. Ideally, the engagement system may be activated as late as practically possible before the target reaches its location. In some embodiments, the engagement system is activated when the target is about 7 mm away from its estimated location.

[0054] The purpose of the engagement system is to fine tune the direction of the laser beams that will eventually hit the target. It is to be noted that every target may not end up in the same location as the previous one. Thus, the laser beams may need to be adjusted for each target. Activating the engagement system moments before the target is scheduled to reach its determined location, enables the final alignment and positioning of the laser beams in order to hit the target at designated locations.

[0055] Fig. 8 is a schematic illustrating the engagement system along with the tracking system according to an embodiment of the present invention. The engagement system includes one or more engagement sources (not shown) that generate one or more engagement pulses 810. In some embodiments, the engagement source can be a laser that outputs a 355 nanometer, 10 nanosecond light pulse to illuminate the end of the target, with a fluence of about 60 uj/cm . The engagement pulse(s) 810 illuminates one or more fiducials or features on a target 802. Due to the nature of the target, the target scatters the light from the engagement pulse 810. The entry angle of the engagement beam is optimized for the collection of scattered light by a number of main laser beam lines. In some embodiments, the engagement pulse 810 enters at an angle of about 39 degrees offset from the vertical axis. In some embodiments, the scattered light passes along the same path (but in the reverse direction) as the laser beam that will eventually hit the target to initiate the fusion reaction. The scattered light is detected by one or more engagement sensors 804. Each engagement sensor 804 may be associated with a laser control system 806. It is to be noted that only a single engagement pulse and a single laser beam is illustrated in Fig. 8 for ease of explanation. One skilled in the art will realize, that in practice, there may be several engagement pulses and several laser beams that may hit the target at several locations. Also, light scattered from a single engagement pulse may enter several beam paths associated with the several lasers beams.

[0056] The scattered light from target 802 is captured by the optics assembly 808 and transported to the engagement sensor 804. In some embodiments, engagement sensor 804 includes a beam splitter that extracts a sample of the main laser alignment beam and a corner cube that retro-reflects it for comparison with a sample of the scattered engagement light, a pair of orthogonally oriented linear CCD array cameras, and optics to image the end of the target onto the line cameras using the scattered light. Once engagement sensor 804 receives the scattered light, it can compare the direction of the scattered light to the direction of the laser beam that will hit the target. Using this information along with the velocity and trajectory information described above, the engagement system can calculate the correction that may be needed to point the laser beam at the correct location. The correction value is then provided to laser control system 806, which adjusts the direction of the laser beam so that the laser beams are aligned to the final expected location of target 802. Once the target reaches is desired location, laser control system 806 fires the main laser beams to irradiate the target.

[0057] As described above, there may be more than one engagement sources that may each generate an engagement pulse. In some embodiments, up to eight engagement sources may be arranged so as to illuminate the target - four from the top and four from the bottom. It may be beneficial to have as many engagement pulses as needed to ensure that light is scattered in enough directions so as to generate accurate data regarding the precise pointing of each beam relative to the location of the target.

[0058] In some embodiments, the engagement pulse may have a different wavelength than the laser beams. In some embodiments, the light reflected from the target during the engagement operation enters the optics assembly 808 at a different angle than the laser beam coming out of optics assembly 808. This happens because the laser beam is pointed at a different location than the location the target at the time when the engagement pulse is activated. Thus, the scattered light from the engagement pulse follows a less than optimal path through optics assembly 808 back to engagement sensor 804. Optics assembly 808 is a diffraction grating, so it steers the incoming light as a function of its wavelength. By using an engagement pulse that has a slightly different wavelength than the laser beams, optics assembly 808 may steer the scattered light differently resulting in the scattered light travelling through optics assembly 808 on a more optimal path. In an embodiment, the wavelength of the engagement pulse is chosen such that the path followed by the scattered light beyond optics assembly 808 is the same as the path of the laser beam that travels through optics assembly 808.

[0059] As described above, it is important that the laser beams hit the target at intended locations in order to initiate the fusion reaction. In conventional methods, a single location on the target may be identified as a result of specular reflection from the target, e.g., a spherical target. Specular reflection is the mirror-like reflection of light from a surface, in which light from a single incoming direction is reflected into a single outgoing direction. Based on this, the lasers may all be pointed at that location. However, this may not be the optimal manner of energizing the target to initiate the fusion reaction. In embodiments of the present invention, several locations on the target are indentified and the laser beams are controlled to be incident on the identified locations at the desired time.

[0060] In some embodiments, the scattered light from the target is used to form an image of the target. The image of the target thus formed is used in determining the locations on the target on which the laser beams are to be focused. Once the locations are determined, laser control system 806 can maneuver each of the laser beams to hit one of the identified locations at the desired time. In order to make it easy to identify locations on the target, in some embodiments, one or more specific features may be included on the target. These features may help in quickly and accurately identifying the desired locations on the target. For example, the ends of the target may have a special coating on them that produces a distinguishable feature when the image of the target is constructed from the scattered light. The distinguishable feature can then be used as a reference to identify other locations on the target where the laser beams are to be focused.

[0061] As discussed above, a specular reflection from the target will produce a single bright spot on the target. This may make it more difficult to construct the image of target. Thus, it would be beneficial to avoid the specular reflection. In an embodiment, the location of the engagement source may be adjusted to reduce or eliminate specular reflection. The location of the engagement source is chosen so as to make sure that when an engagement pulse impinges on the target, there is minimal specular reflection. The shape of the target can also be designed to minimize specular reflection.

[0062] As discussed above, it would be beneficial to have the engagement pulse activated at a time closer to the time when the target is expected to reach its intended location.

However, enough time needs to be allocated for (a) the scattered light to be transported back to the engagement sensor, (b) determination of any correction parameters, (c) controlling the laser sources to point the laser beams at the intended locations, and (d) firing the laser and allowing the laser beams to hit the target. Thus, activating the engagement pulse too late may not leave enough time to perform these subsequent actions. On the other hand, activating the engagement pulse too early may not yield accurate correction parameters since the target may change its path after the scattered light is collected to determine the correction parameters. In this instance the laser beams may not focus at the intended locations on the target. In some embodiments, the engagement pulse is activated about 20-25 before the target is expected to reach its intended location. In some embodiments, an acousto-optic beam deflector may be used to steer the laser beams based on the determined correction value. It is to be noted that the engagement technique described above may be performed for each target.

[0063] Fig. 9 is a flow diagram of a process 900 for tracking a target according to an embodiment of the present invention. Process 900 can be performed, e.g., by the tracking system of Fig. 7 A and 7B. At step 902, a fusion target enters a fusion reaction chamber. At step 904 the target passes through a first tracking beam that traverses in a lateral direction across the fusion chamber. At step 906, the shadow caused by the fusion target intersecting the laser beam is captured by one or more sensors to determine the arrival time of the target, its transverse position relative to the tracking beam, and its tilt. In some embodiments, similar images may be captured concurrently by sensors on an orthogonal tracking beam. At step 908, the fusion target passes through a second tracking beam. Based on the distance between the two tracking beams and the time it takes the target to go from the first to the second tracking beam, a velocity of the target is determined at step 910. In addition, additional images of the target may be captured to determine an change in the tilt and transverse position. The velocity information is used to determine a time when the target is expected to reach a location near the center of the fusion chamber. The accumulated transverse position and tilt information combined with the velocity measurement defines the trajectory of the target (Step 912). The time information is provided to a laser control system at step 914 to enable the laser control system to prepare to fire the lasers at the correct time. The extrapolated trajectory indicates whether the target will be within the operational range of the engagement system when the target arrives near the center of the fusion chamber. [0064] It should be appreciated that the specific steps illustrated in FIG. 9 provide a particular method of tracking a fusion target according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Moreover, the individual steps illustrated in FIG. 9 may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize many variations, modifications, and alternatives. [0065] Fig. 10 is a flow diagram of a process 1000 for engaging a target according to an embodiment of the present invention. Process 1000 can be performed by, e.g., the

engagement system illustrated in Fig. 8.

[0066] A fusion target is injected into a fusion chamber. As the fusion targets nears it final destination near the center of the fusion chamber, a light pulse is focused on the target to illuminate the target (1002). As described above, multiple light pulses can be focused on the target in some embodiments. The light is scattered in several directions after it hits the fusion target. Several optics assemblies located around the fusion chamber collect the scattered light (1004) and steer the collected light back to an associated engagement sensor. Each engagement sensor and associated circuitry generates an image of the fusion target using the collected scattered light (1006). Based on the generated image, the engagement system can identify at least one location on the target where its associated laser beam is to be focused as part of initiating a fusion reaction and the future coordinates of that location when the fusion target will reach its intended location (1008). Thereafter, the engagement sensor determines coordinates for a current focus point of the laser beam (1010). In some embodiments, the coordinates of the current focus point may be the same as the future coordinates of the identified location on the fusion target. In other embodiments, they may be different.

[0067] The engagement system then determines a difference in the coordinates of the current focus point and the identified location on the target (1012) and determines a correction parameter based on the determined difference (1014). Thereafter, the correction parameter is communicated to a laser control system for controlling the laser beam to point at the future coordinates of the identified location (1016).

[0068] It should be appreciated that the specific steps illustrated in FIG. 10 provide a particular method of controlling a laser system according to an embodiment of the present invention. Other sequences of steps may also be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Moreover, the individual steps illustrated in FIG. 10 may include multiple sub-steps that may be performed in various sequences as appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular applications. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

[0069] This description of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and many modifications and variations are possible in light of the teaching above. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications. This description will enable others skilled in the art to best utilize and practice the invention in various embodiments and with various modifications as are suited to a particular use. The scope of the invention is defined by the following claims.