Patent Publication Number: US-9887076-B2

Title: Method and system for controlling convective flow in a light-sustained plasma

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is related to and claims the benefit of the earliest available effective filing date(s) from the following listed application(s) (the “Related Applications”) (e.g., claims earliest available priority dates for other than provisional patent applications or claims benefits under 35 USC § 119(e) for provisional patent applications, for any and all parent, grandparent, great-grandparent, etc. applications of the Related Application(s)). 
     RELATED APPLICATIONS 
     The present application constitutes a continuation patent application of United States Non-Provisional Patent Application entitled METHOD AND SYSTEM FOR CONTROLLING CONVECTIVE FLOW IN A LIGHT-SUSTAINED PLASMA, naming Ilya Bezel, Anatoly Shchemelinin, Matthew Derstine, Ken Gross, David Shortt, Wei Zhao, Anant Chimmalgi and Jincheng Wang as inventors, filed Mar. 25, 2014, U.S. patent application Ser. No. 14/224,945, which constitutes a regular (non-provisional) patent application of United States Provisional Patent Application entitled INVERTED ELLIPSE, naming Ilya Bezel, Anatoly Shchemelinin, Matthew Derstine, Ken Gross, David Shortt, Wei Zhao, Anant Chimmalgi and Jincheng Wang as inventors, filed Mar. 29, 2013, Application Ser. No. 61/806,739. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to plasma based light sources, and more particularly to the use of an inverted collector element to aid in controlling convective flow in a plasma of a plasma based light source. 
     BACKGROUND 
     As the demand for integrated circuits having ever-small device features continues to increase, the need for improved illumination sources used for inspection of these ever-shrinking devices continues to grow. One such illumination source includes a laser-sustained plasma source. Laser-sustained plasma light sources are capable of producing high-power broadband light. Laser-sustained light sources operate by focusing laser radiation into a gas volume in order to excite the gas, such as argon or xenon, into a plasma state, which is capable of emitting light. This effect is typically referred to as “pumping” the plasma. The orientation of collection optics below the plasma-generating volume in traditional laser-sustained plasma sources results in plasma convective flow being directed to the internal portion of the source. Traditional sources require convection control, plume capture and temperature control to be implemented within the space inside the ellipsoidal collector optics of traditional sources. In currently implemented systems, a significant amount of effort is directed to the cooling of the top part of the plasma bulb and plasma convection plume mitigation, which is limited by the geometrical constraints resulting from the upward orientation of the collections optics. Air-cooling of the top portion of the plasma bulb causes warm air to propagate inside of the volume designated for laser and plasma light propagation and causes additional noise as a result of air wiggle. In addition, current methods of cooling the top of the plasma bulb via a downward-directed air shower results in air flow counter to the direction of natural convection, which leads to blowing of hotter air on colder bulb parts. In addition, there are severe instabilities in bulb-less system designs in which the convective plume propagates inside of the volume designated for laser and plasma light propagation. Therefore, it would be desirable to provide a system and method for curing defects such as those of the identified above. 
     SUMMARY 
     An apparatus for controlling convective flow in a light-sustained plasma is disclosed, in accordance with an illustrative embodiment of the present invention. In one embodiment, the apparatus may include an illumination source configured to generate illumination. In another embodiment, the apparatus may include a plasma cell including a bulb for containing a volume of gas. In another embodiment, the apparatus may include a collector element arranged to focus the illumination from the illumination source into the volume of gas in order to generate a plasma within the volume of gas contained within the bulb. In one embodiment, the collector element may include an ellipsoid-shaped collector element. In another embodiment, the plasma cell is disposed within a concave region of the collector element. In another embodiment, at least a top portion of the collector element is arranged above a plasma-generating region of the plasma cell and is configured to focus illumination from the illumination source into the volume of gas in order to generate a plasma below at least the top portion of the collector element. In another embodiment, the collector element may include an opening for propagating a portion of a plume of the plasma to a region external to the concave region of the collector element. In another embodiment, the opening is positioned substantially in a top portion of the collector element. In another embodiment, the apparatus may include an external plasma control element positioned in the region external to the concave region of the collector element. 
     An apparatus for controlling convective flow in a light-sustained plasma is disclosed, in accordance with an additional illustrative embodiment of the present invention. In one embodiment, the apparatus may include an illumination source configured to generate illumination. In another embodiment, the collector element may include a concave region for containing a volume of gas. In another embodiment, the collector element is arranged to focus the illumination from the illumination source into the volume of gas in order to generate a plasma within the volume of gas contained by the concave region of the collector element. In one embodiment, the collector element may include an ellipsoid-shaped collector element. In another embodiment, at least a top portion of the collector element is arranged above the volume of gas and is configured to focus illumination from the illumination source into the volume of gas in order to generate a plasma below at least the top portion of the collector element. In another embodiment, the collector element may include an opening for propagating a portion of a plume of the plasma to a region external to the interior region of the collect element. In another embodiment, the opening is positioned substantially in a top portion of the collector element. In another embodiment, the apparatus may include an external plasma control element positioned in the region external to the concave region of the collector element. 
     A method for controlling convective flow in a light-sustained plasma is disclosed, in accordance with an illustrative embodiment of the present invention. In one embodiment, the method may include providing a collector element, containing a volume of gas within a plasma cell disposed within a concave region of the collector element; forming a plasma within the plasma cell by focusing illumination into the volume of gas contained within the plasma cell, and propagating a portion of a plume of the plasma to a region external to the concave region of the collector element via an opening in the collector element. 
     A method for correcting convection based aberrations is disclosed, in accordance with an additional illustrative embodiment of the present invention. In one embodiment, the method may include providing a collector element; containing a volume of gas within a concave region of the collector element; forming a plasma within the concave region of the collector element by focusing illumination into the volume of gas contained within the concave region of the collector element; and propagating a portion of a plume of the plasma to a region external to the concave region of the collector element via an opening in the collector element. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and together with the general description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The numerous advantages of the disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which: 
         FIG. 1A  is a high level schematic view of a system for controlling convective flow in a light-sustained plasma, in accordance with one embodiment of the present invention. 
         FIG. 1B  is a high level schematic view of a collector element and plasma cell of a system for controlling convective flow in a light-sustained plasma, in accordance with one embodiment of the present invention. 
         FIG. 1C  is a high level schematic view of a bulb-less system for controlling convective flow in a light-sustained plasma, in accordance with one embodiment of the present invention. 
         FIG. 2  is a flow diagram illustrating a method for controlling convective flow in a light-sustained plasma, in accordance with one embodiment of the present invention. 
         FIG. 3  is a flow diagram illustrating a method for controlling convective flow in a light-sustained plasma, in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. 
     Referring generally to  FIGS. 1A through 3 , a system and method for controlling convective flow in a light-sustained plasma are described in accordance with the present disclosure. Embodiments of the present invention are directed to the implementation of an inverted collector/reflector element in a light-sustained plasma light source. The inversion of the collector element of the plasma light source of the present invention allows for the plasma plume to propagate from the plasma region of the source to a region outside of the collector boundary via an opening in the collector element. In embodiments where the opening in the collector element is positioned at or near the apex of the collector element, the plume readily propagates (e.g., propagates within a plasma bulb or propagates in a bulb-less setting) upward through the opening into a region above and external to the interior region of the collector element. Such a configuration allows for the implementation of any number of plasma control mechanisms at a location external to the collector element. For instance, the plasma control mechanisms may include, but are not limited to, gas and/or plume cooling and/or heating, convection control, and/or plume capture and/or redirection. It is noted herein that the implementation of plasma control in the region external to the interior region of the collector element serves to remove the various plasma control devices and architecture from the optically active region of the system, thereby alleviating geometrically constraints within the system. 
       FIGS. 1A-1B  illustrate a system  100  suitable for aiding in convective flow control of plasma in a light-sustained plasma cell, in accordance with one embodiment of the present invention. The generation of plasma within inert gas species is generally described in U.S. patent application Ser. No. 11/695,348, filed on Apr. 2, 2007; and U.S. patent application Ser. No. 11/395,523, filed on Mar. 31, 2006, which are incorporated herein in their entirety. Various plasma cell designs and plasma control mechanisms are described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012, which is incorporated herein by reference in the entirety. 
     In one embodiment, the system  100  includes an illumination source  112  (e.g., one or more lasers) configured to generate illumination of a selected wavelength. In another embodiment, the system  100  includes a plasma cell  104  for generating a plasma  106 . In another embodiment, the plasma cell  104  includes a bulb  105  for containing a selected gas (e.g., argon, xenon, mercury or the like) suitable for generating a plasma  106  upon absorption of suitable illumination. In one embodiment, focusing illumination  114  from the illumination source  112  into the volume of gas  103  causes energy to be absorbed through one or more selected absorption lines of the gas or plasma  106  within the bulb  105 , thereby “pumping” the gas species in order to generate or sustain a plasma  106 . In another embodiment, although not shown, the plasma cell  104  may include a set of electrodes for initiating the plasma  106 , whereby the illumination source  112  maintains the plasma  106  after ignition by the electrodes. 
     In another embodiment, the system  100  includes a collector/reflector element  102  (e.g., an ellipsoid-shaped collector element) configured to focus illumination emanating from the illumination source  112  into the volume of gas  103  contained within the bulb  105  of the plasma cell  104 . The collector element  102  may take on any physical configuration known in the art suitable for focusing illumination emanating from the illumination source  112  into the volume of gas  103  contained within the bulb  105  of the plasma cell  104 . In one embodiment, the collector element  102  may include a concave region  109  with a reflective internal surface  111  suitable for receiving illumination  114  from the illumination source  112  and focusing the illumination  114  into the volume of gas  103  contained within bulb  105 . For example, the collector element  102  may include an ellipsoid-shaped collector element  102  having a reflective internal surface  111 , as shown in  FIG. 1B . In another embodiment, the collector element  102  is arranged to collect broadband illumination emitted by plasma  106  and direct the broadband illumination to one or more additional optical elements (e.g., homogenizer  126 ). 
     In one embodiment, the collector element  102  is arranged such that a top portion of the collector element  102  is positioned above the plasma-generating region of the plasma cell  104 , as shown in  FIG. 1B . In another embodiment, the collector element  102  is arranged to focus illumination  114  from the illumination source  112  into the volume of gas  103  in order to generate a plasma  106  below at least the top portion of the collector element  102 . For example, as shown in  FIG. 1B , at least the apex of the collector element  102  is positioned above a portion (e.g., plasma-generating portion) of the bulb  105 . In this regard, the internal surface  111  of the concave region  109  is arranged to direct illumination  114  from the illumination source  112  in a downward direction toward the bulb  105  of the plasma cell  104 . 
     In one embodiment, the collector element  102  includes an opening  108  for propagating a portion of a plume  107  of the plasma  106  generated within the bulb  105  to a region  110  external to the concave region  109  of the collector element  102 . In one embodiment, as shown in  FIG. 1B , a portion of the plasma cell  104  may be positioned so as to pass through opening  108 . For example, as shown in  FIG. 1B , the bulb  105  of plasma cell  104  may be positioned so as to pass through opening  108 . For instance, a first portion of the bulb  105  may be located within the concave region  109 , or the internal region, of the collector element  102 , while a second portion may be located in a region  110  external to the collector element  102 . In this regard, gas or plasma  106  contained within the bulb  105  may traverse from one side of the collector element  102  (e.g., inside collector element) to an opposite side (e.g., outside of collector element), allowing for convective flow between the interior and exterior regions of the collector element  102 . 
     In one embodiment, the plasma cell  104  is arranged within the opening  108  in the collector element  102 . In one embodiment, the plasma cell  104  is disposed within the opening  108  of the collector element  102 . In another embodiment, a first portion of the plasma cell  104  is placed in thermal communication with the concave region  109  of the collector element  102 , while a second portion of the plasma cell  104  is placed in thermal communication with the region  110  external to the concave region  109  of the collector element  102 . In another embodiment, a first portion of the bulb  105  of plasma cell  104  is placed in thermal communication with the concave region  109  of the collector element  102 , while a second portion of the bulb  105  of plasma cell  104  is placed in thermal communication with the region  110  external to the concave region  109  of the collector element  102 . 
     In one embodiment, the opening  108  is positioned substantially in a top portion of the collector element  102 . In another embodiment, the opening  108  is positioned at or near the apex of the collector element  102 . For example, in the case of an ellipsoid-shaped collector element  102 , as shown in  FIG. 1B , the opening  108  may be positioned at or near the apex of the ellipsoid-shaped collector element  102 . It is noted herein that the present invention is not limited to the positioning of the opening  108  at or near the apex of the collector element  102 . It is further recognized herein that the opening  108 , or openings, may be positioned at a variety of locations along the wall of the collector element  102  in order to allow the propagation of a portion of the plume  107  to the external region  110 , outside of the concave region  109  of the collector element  102 . 
     The bulb  105  of plasma cell  104  may take on any shape known in the art suitable for traversing the opening  108  between the concave region  109  and the external region  110 . For example, the bulb  105  may have, but is not required to have, an elongated shape, as shown in  FIG. 1B . 
     It is noted herein that the inverted orientation of the collector element  102  along with the positioning of the opening  108  at the top portion of the collector element  102  provides for improved thermal control of the bulb  105  of the plasma cell  104 . In this regard, the positioning of at least a portion of the bulb  105  (e.g., the top portion of the bulb  105 ) outside of the concave region  109  aids in cooling the bulb  105 . Further, the propagation of the plume  107  outside of the concave region  109  aids in mitigating the impact of the plasma plume  107 . 
     In another embodiment, the system  100  includes one or more external plasma control elements  128 , as shown in  FIG. 1B . In one embodiment, the external plasma control element  128  is disposed in the region  110  external to the concave region  109  of the collector element  102 . In one embodiment, the external plasma control element  128  is disposed within the bulb  105  of plasma cell  104 , as shown in  FIG. 1B . In another embodiment, although not shown, the external plasma control element  128  is disposed outside of the bulb  105  of plasma cell  104 . For example, the external plasma control element  128  may be affixed to the outside wall of the plasma bulb  105  or may be disposed proximate to the plasma bulb  105 . 
     In one embodiment, the external plasma control element  128  may include any plasma control element known in the art for controlling one or more characteristics of the plasma  106 . 
     In one embodiment, the external plasma control element  128  includes an external temperature control element. For example, an external temperature control element may be disposed inside or outside of the plasma bulb  105  of plasma cell  104 . The external temperature control element may include any temperature control element known in the art used to control the temperature of the plasma cell  104 , the plasma  106  and/or the plasma plume  107 . 
     In one embodiment, the external temperature control element may be utilized to cool the plasma bulb  105  of plasma cell  104  and/or the plume  107  generated by the plasma  106  by transferring thermal energy to a medium external to the concave region  109  of the collector element  102 . In one embodiment, the external temperature control element may include, but is not limited to, a cooling element for cooling the plasma bulb  105 . In one embodiment, the external temperature control element may include a heat transfer element for transferring thermal energy from the bulb  105  (or plume  107 ) to a medium external  110  to the concave region  109  of the collector element  102 . For example, the external temperature control unit may include, but is not limited to, a heat pipe (not shown) in thermal communication with one or more portions (e.g., bulb wall, electrodes within bulb and the like) of the plasma bulb  105 . Further, the heat pipe may be placed in thermal communication with a heat exchanger (not shown). In this regard, the heat pipe may transfer thermal energy from within the plasma bulb to the heat exchanger disposed at a region external to concave region  109  of the collector element  102 . The heat exchanger may be further configured to transfer the received thermal energy from the heat pipe to a medium (e.g., heat sink) external of the plasma bulb  105  and the concave region  109  of the collector element  102 . In another embodiment, the heat pipe is configured to transfer thermal energy from the plume  107  generated by rising gas from the plasma region  106  of the plasma bulb  105  to a medium external to concave region  109  of the collector element  102  via the heat exchanger. 
     In another embodiment, the bulb  105  may include one or more passive heat transfer elements coupled to one or more portions of the bulb  105 . For example, the one or more passive heat transfer elements may include, but are not limited to, baffles, chevrons or fins arranged to transfer thermal energy from the hot plasma  106  to a portion of the plasma cell  104  (e.g., top electrode of bulb) to facilitate heat transfer out of the bulb  105 . 
     In another embodiment, the external temperature control element may be utilized to heat the plasma bulb  105  of plasma cell  104 . For example, the external temperature control element may include a heater or heat transfer element (e.g., heat pipe transferring thermal energy from an external medium to the bulb  105 ) in thermal communication with the plasma bulb  105  and configured to transfer thermal energy to the plasma bulb  105 . For instance, the external temperature control element may include a heater or heat transfer element disposed inside of the plasma bulb  105  or outside of the plasma bulb  105 . 
     The utilization of heat transfer elements is generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012, which is incorporated by reference above in the entirety. The utilization of heat transfer elements is also generally described in U.S. patent application Ser. No. 12/787,827, filed on May 26, 2010, which is incorporated by reference herein in the entirety. 
     In another embodiment, the external plasma control element  128  includes an external convection control element. For example, an external convection control element may be disposed inside or outside of the plasma bulb  105  of plasma cell  104 . The external convection control element may include any convection control device known in the art used to control convection in the plasma cell  104 . For example, the external convection control element may include one or more devices (e.g., structures positioned within plasma cell  104 ) suitable for controlling convection currents within the plasma bulb  105  of plasma cell  104 . For instance, the one or more structures for controlling convection currents may be arranged within the plasma bulb  105  in a manner to impact the flow of hot gas from the hot plasma  106  of the plasma cell  104  to the cooler inner surfaces of the plasma bulb  105 . In this regard, the one or more structures may be configured in a manner to direct convective flow to regions within the plasma bulb  105  that minimize or at least reduce damage to the bulb  105  caused by the high temperature gas. 
     The utilization of convection control devices is generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012, which is incorporated by reference above in the entirety. The utilization of convection control devices are also generally described in U.S. patent application Ser. No. 12/787,827, filed on May 26, 2010, which is incorporated by reference above in the entirety. 
     In another embodiment, the external plasma control element  128  includes an external plume capture element. For example, an external plume capture element may be disposed inside or outside of the plasma bulb  105  of plasma cell  104 . The external plume control element may include any plume control device known in the art used to capture or redirect the plume  107  of plasma  106  within the plasma cell  104 . For example, the external plume capture element may include one or more devices having a concave portion suitable for capturing and redirecting a convection plume emanating from the plasma  106  within the bulb  105  of the plasma cell  104 . For instance, the external plume capture element may include one or more electrodes (e.g., top electrode) disposed within the plasma bulb  105  of plasma cell  104  having a concave portion or a hollow portion suitable for capturing and/or redirecting a convection plume emanating from the plasma  106  within the bulb  105  of the plasma cell  104 . 
     The utilization of plume capture devices is generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012, which is incorporated by reference above in the entirety. The utilization of plume capture devices is also generally described in U.S. patent application Ser. No. 12/787,827, filed on May 26, 2010, which is incorporated by reference above in the entirety. 
       FIG. 1C  illustrates a system  150  suitable for aiding in convective flow control of plasma, in accordance with one embodiment of the present invention. It is noted herein that system  150  is suitable for generating a plasma without the use of a plasma bulb. In this regard, system  150  may be referred to herein as a “bulb-less” system design. It is further noted that the various embodiments and illustrations provided previously herein with respect to system  100  should be interpreted to extend to system  150  unless otherwise noted. 
     In one embodiment, the collector element  102  is configured to contain, or at least contribute to the containment, of a volume of gas suitable for generating plasma  106 . In another embodiment, the collector element  102  is arranged to focus the illumination  114  from the illumination source  112  into the volume of gas  153  in order to generate, or at least maintain, a plasma  106  within the volume of gas  153  contained by at least the concave region  109  of the collector element  102 . In another embodiment, the collector element  102  is arranged to collect broadband illumination emitted by plasma  106  and direct the broadband illumination to one or more additional optical elements (e.g., homogenizer  126 ). 
     In one embodiment, the system  150  includes a gas containment structure  152 . In another embodiment, the gas containment structure  152  is operably coupled to the collector element  102 . For example, as shown in  FIG. 1C , the collector element  102  is disposed inside gas containment structure  152 . In another embodiment, although not shown, the collector element  102  may be disposed on an upper portion of the gas containment structure  152 . It is noted herein that the present invention is not limited to the above description or the depiction of system  150  in  FIG. 1C  as it is contemplated herein that system  150  may encompass a number of bulb-less configurations suitable for initiating and/or maintaining a plasma  106  in accordance with the present invention. A bulb-less laser sustained plasma light source is generally described in U.S. patent application Ser. No. 12/787,827, filed on May 26, 2010, which is incorporated above by reference in the entirety. 
     As previously described herein, the system  150  includes an opening  108  for propagating a portion of the plume  107  of the plasma  106  to a region  110  external to the concave region  109  of the collector element  102 . In this regard, gas or plasma contained within the collector element  102  may traverse from one side of the collector element  102  (e.g., inside collector element) to an opposite side (e.g., outside of collector element) via opening  108 , allowing for convective flow between the interior and exterior regions of the collector element  102 . 
     In another embodiment, the system  150  includes a gas circulation system  158 . For example, the gas circulation system  158  may transfer gas from the external region  110  to the interior concave region  109 . In this regard, the gas circulation system  158  may transfer cooled gas (after heat transfer from the plume  107  to a medium (e.g., heat sink)) back into the plasma generating region of the interior concave region  109 . In another embodiment, although not shown, the gas circulation system  158  may include one or more gas pumps for facilitating circulation of gas. 
     In another embodiment, the system  150  includes one or more windows  154  coupled to the gas containment structure  152  and arranged to allow incident illumination  114  from the illumination source  112  to enter the volume of the gas containment structure  152  and the concave region  109  of the collector element  102 . The window  154  may include any window material known in the art suitable for transmitting light, such as laser light, from the illumination source  112  to the inside of the gas containment structure  152 . 
     In one embodiment, although not shown in  FIG. 1C , the system  150  may include an external plasma control element. In one embodiment, as discussed previously herein, the external plasma control element may be positioned in the region  110  external to the concave region  109  of the collector element  102 . In one embodiment, the external plasma control element of system  150  may include any plasma control element known in the art for controlling one or more characteristics of the plasma  106  in a bulb-less system, such as system  150 . In one embodiment, the external plasma control element of system  150  may be disposed on, or integrated with, a portion of the gas containment structure  152 , a portion of the external wall of the collector element  102  and/or a portion of the gas circulation system  158 . 
     In one embodiment, the external plasma control element of system  150  may include an external temperature control element. For example, as discussed previously herein, the external temperature control element may include, but is not limited to, any heating element, cooling element or heat transfer element known in the art. For instance, the external temperature control element may include any heating element, cooling element or heat transfer element known in the art suitable for cooling and/heating gas or the plasma plume  107 , which propagates through opening  108  and through the external region  110 . In one embodiment, the temperature control element may include a temperature control element that is external to the concave region, but internal to the gas containment structure  152 . For instance, the temperature control element may include one or more cooling pipes disposed in region  110 , outside of concave region  109 , but within the gas containment structure  152 , and is configured to cool the hot gas and/or plume  107  as it rises from the hot plasma  106 . In another embodiment, the temperature control element may include a temperature control element that is external to the concave region  109  of the gas containment structure  152 . For instance, the system  150  may include a cooling jacket (not shown) or cooling collar (not shown) disposed around a portion of the gas containment structure  152  and configured to cool the hot gas and/or plume  107  as it rises from the hot plasma  106 . Temperature control systems and devices usable in the context of system  150  are generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012; and U.S. patent application Ser. No. 12/787,827, filed on May 26, 2010, which are both incorporated by reference above in the entirety. 
     In another embodiment, the external plasma control element of system  150  may include an external convection control element. In one embodiment, the gas circulation system  158  may contribute to the convection control of system  150  by controlling convection associated with hot gas rising from the plasma  106  into the external region  110  through opening  108 , as discussed previously herein. In one embodiment, the convection control imparted by the gas circulation system  158  may include passive convection control, whereby gas, upon cooling, naturally circulates through the gas circulation system  158  back into the concave region  109 . In another embodiment, the convection control imparted by the gas circulation system  158  may include active convection control. For example, the gas control system  158  may include a pump configured to pump gas from the external region  110  to the concave region  109 . It is recognized herein that in the bulb-less system  150  the convection control may be coupled to cooling/heating control. For example, cooling elements (e.g., cooling jacket) located at one or more positions of the gas containment structure  152 , gas circulation system  158 , the external region  110  and/or the concave region  109  may be used to control convection throughout the system  150 . Convection control systems and devices usable in the context of system  150  are generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012; and U.S. patent application Ser. No. 12/787,827, filed on May 26, 2010, which are both incorporated by reference above in the entirety. 
     In another embodiment, the external plasma control element of system  150  may include an external plume capture element. The external plume control element may include any plume control device known in the art suitable to capture or redirect the plume  107  of plasma  106  in the region  110  external to the concave region  109 . For example, the external plume capture element may include one or more devices having a concave portion suitable for capturing and redirecting a convection plume  107  propagating through opening  108  from the plasma  106 . For instance, the external plume capture element may include one or more devices (e.g., top electrode) disposed within region  110  having a concave portion or a hollow portion suitable for capturing and/or redirecting a convection plume emanating from opening  108 . Plume capture devices usable in the context of system  150  are generally described in U.S. patent application Ser. No. 13/647,680, filed on Oct. 9, 2012; and U.S. patent application Ser. No. 12/787,827, filed on May 26, 2010, which are both incorporated by reference above in the entirety. 
     In another embodiment, the system  150  includes one or more windows  156  for transmitting generated broadband light (e.g., broadband UV light) from the plasma  106  to one or more optical elements situated outside of the gas containment structure  152 . The window  156  may include any window material known in the art suitable for transmitting light, such as broadband UV light, from the plasma-generating region within the gas containment structure  152  to one or more optical elements situated outside of the gas containment structure  152 . 
     In one embodiment, systems  100  and  150  may include various additional optical elements. In one embodiment, the set of additional optics may include collection optics configured to collect broadband light emanating from the plasma  106  (e.g., plasma in bulb  105  of system  100  or plasma maintained in concave region  109  of system  150 ). For instance, the systems  100  and  150  may include a cold mirror  122  arranged to direct illumination from the collector element  102  to downstream optics, such as, but not limited to, a homogenizer  126 . 
     In another embodiment, the set of optics may include one or more additional lenses (e.g., lens  118 ) placed along either the illumination pathway or the collection pathway of system  100  or system  150 . The one or more lenses may be utilized to focus illumination from the illumination source  112  into the volume of gas  103  or  153 . Alternatively, the one or more additional lenses may be utilized to focus broadband light emanating from the plasma  106  onto a selected target (not shown). In a further embodiment, the set of optics may include one or more filters  124  (not shown in  FIG. 1C ) placed along either the illumination pathway or the collection pathway in order to filter illumination prior to light entering the plasma bulb  105  (or the concave region  109  of collector element  102 ) or to filter illumination following emission of the light from the plasma  106 . It is noted herein that the set of optics of systems  100  and  150  as described above and illustrated in  FIGS. 1A through 1C  are provided merely for illustration and should not be interpreted as limiting. It is anticipated that a number of equivalent optical configurations may be utilized within the scope of the present invention. 
     It is contemplated herein that the systems  100  and  150  may be utilized to sustain a plasma in a variety of gas environments. In one embodiment, the gas used to initiate and/or maintain plasma  106  may include an inert gas (e.g., noble gas or non-noble gas) or a non-inert gas (e.g., mercury). In another embodiment, the gas used to initiate and/or maintain a plasma  106  may include a mixture of gases (e.g., mixture of inert gases, mixture of inert gas with non-inert gas or a mixture of non-inert gases). For example, it is anticipated herein that the volume of gas  103  or  153  used to generate a plasma  106  may include argon. For instance, the gas  103  or  153  may include a substantially pure argon gas held at pressure in excess of 5 atm. In another instance, the gas may include a substantially pure krypton gas held at pressure in excess of 5 atm. In another instance, the gas  103  or  153  may include a mixture of argon gas with an additional gas. 
     It is further noted that the present invention may be extended to a number of gases. For example, gases suitable for implementation in the present invention may include, but are not limited, to Xe, Ar, Ne, Kr, He, N 2 , H 2 O, O 2 , H 2 , D 2 , F 2 , CH 4 , one or more metal halides, a halogen, Hg, Cd, Zn, Sn, Ga, Fe, Li, Na, Ar:Xe, ArHg, KrHg, XeHg, and the like. In a general sense, the present invention should be interpreted to extend to any light pump plasma generating system and should further be interpreted to extend to any type of gas suitable for sustaining a plasma within a plasma cell or within a bulb-less system, such as system  150 . 
     In another embodiment, the illumination source  112  of system  100  or system  150  may include one or more lasers. In a general sense, the illumination source  112  may include any laser system known in the art. For instance, the illumination source  112  may include any laser system known in the art capable of emitting radiation in the visible or ultraviolet portions of the electromagnetic spectrum. In one embodiment, the illumination source  112  may include a laser system configured to emit continuous wave (CW) laser radiation. For example, in settings where the volume of gas  103  or  153  is or includes argon, the illumination source  112  may include a CW laser (e.g., fiber laser or disc Yb laser) configured to emit radiation at 1069 nm. It is noted that this wavelength fits to a 1068 nm absorption line in argon and as such is particularly useful for pumping the argon gas. It is noted herein that the above description of a CW laser is not limiting and any CW laser known in the art may be implemented in the context of the present invention. 
     In another embodiment, the illumination source  112  may include one or more diode lasers. For example, the illumination source  112  may include one or more diode laser emitting radiation at a wavelength corresponding with any one or more absorption lines of the species of the gas contained within volume of gas  103  or  153 . In a general sense, a diode laser of the illumination source  112  may be selected for implementation such that the wavelength of the diode laser is tuned to any absorption line of any plasma (e.g., ionic transition line) or any absorption line of the plasma-producing gas (e.g., highly excited neutral transition line) known in the art. As such, the choice of a given diode laser (or set of diode lasers) will depend on the type of gas contained within the plasma cell  104  of system  100  or the concave region  109  of system  150  of the present invention. 
     In another embodiment, the illumination source  112  may include an ion laser. For example, the illumination source  112  may include any noble gas ion laser known in the art. For instance, in the case of an argon-based plasma, the illumination source  112  used to pump argon ions may include an Ar+ laser. 
     In another embodiment, the illumination source  112  may include one or more frequency converted laser systems. For example, the illumination source  112  may include a Nd:YAG or Nd:YLF laser having a power level exceeding 100 Watts. In another embodiment, the illumination source  112  may include a broadband laser. In another embodiment, the illumination source may include a laser system configured to emit modulated laser radiation or pulsed laser radiation. 
     In another embodiment, the illumination source  112  may include one or more non-laser sources. In a general sense, the illumination source  112  may include any non-laser light source known in the art. For instance, the illumination source  112  may include any non-laser system known in the art capable of emitting radiation discretely or continuously in the visible or ultraviolet portions of the electromagnetic spectrum. 
     In another embodiment, the illumination source  112  may include two or more light sources. In one embodiment, the illumination source  112  may include or more lasers. For example, the illumination source  112  (or illumination sources) may include multiple diode lasers. By way of another example, the illumination source  112  may include multiple CW lasers. In a further embodiment, each of the two or more lasers may emit laser radiation tuned to a different absorption line of the gas or plasma within the plasma cell  104  of system  100  or the concave region  109  of system  150 . 
       FIG. 2  is a flow diagram illustrating steps performed in a method  200  for controlling convective flow in a light-sustained plasma. Applicant notes that the embodiments and enabling technologies described previously herein in the context of systems  100  and  150  should be interpreted to extend to method  200 . It is further noted, however, that the method  200  is not limited to the architecture of systems  100  and  150 . 
     In a first step  202 , a collector element  102  is provided. For example, as shown in  FIGS. 1A and 1B , a collector element  102  having a generally ellipsoidal shape and a reflective internal surface  111  may be provided. Further, the collector element  102  may be arranged such that it directs illumination  114  from the illumination source  112  in a generally down direction to a volume of gas  103  below at least the top portion of the collector element  102 . 
     In a second step  204 , a volume of gas  103  is contained within a plasma cell  104  disposed within a concave region  109  of the collector element  102 . For example, the system  100  may include a plasma cell  104  disposed within the concave region  109  of the collector element  102 . For instance, the plasma cell  104  may include a bulb  105  suitable for containing a volume of gas (e.g., pure gas or gas mixture). 
     In a third step  206 , a plasma  106  within the plasma cell  104  is formed by focusing illumination  114  into the volume of gas  103  contained within the plasma cell  104 . For example, illumination  114  of a selected wavelength may be generated utilizing an illumination source  112 , such as a laser. In turn, the illumination  114  is focused into the volume of gas  103  in order to generate plasma  106  within the volume of gas  103 . For example, the collector element  102  may receive illumination  114  from the illumination source  112  and focus the illumination  114  into the volume of gas  103  contained within the bulb  105  of the plasma cell  104 . It is noted herein that the plasma  106  need not be initiated by the light  114  from the illumination source  112 . For example, one or more electrodes (not shown) may be utilized to initiate the plasma  106 , while light  114  is used to maintain the plasma  106 . 
     In a fourth step  208 , a portion of a plume  107  of the plasma  106  is propagated to a region  110  external to the concave region  109  of the collector element  102  via an opening  108  in the collector element  102 . For example, the bulb  105  of plasma cell  104  may be disposed within an opening  108  of the collector element  102  such that the bulb  105  is in contact with the interior concave region  109  as well as the external region  110 . For instance, the opening  108  may be arranged in the top portion (e.g., at or near the apex) of the collector element  102 . 
       FIG. 3  is a flow diagram illustrating steps performed in a method  300  for controlling convective flow in a light-sustained plasma. Applicant notes that the embodiments and enabling technologies described previously herein in the context of systems  100  and  150  should be interpreted to extend to method  300 . It is further noted, however, that the method  300  is not limited to the architecture of systems  100  and  150 . 
     In first step  302 , a collector element is provided. For example, as shown in  FIG. 1C , a collector element  102  having a generally ellipsoidal shape and a reflective internal surface  111  may be provided. Further, the collector element  102  may be arranged such that it directs illumination  114  from the illumination source  112  in a generally down direction to a volume of gas  103  below at least the top portion of the collector element  102 . 
     In a second step  304 , a volume of gas is contained within a concave region of the collector element. For example, as shown in  FIG. 1C , the concave region  109  of the collector element  102  may serve to at least partially contain the volume of gas  153 . Further, as shown in  FIG. 1C , the concave region  109  may operate, although is not required to operate, in concert with a gas containment structure  152  to contain the volume of gas  153 . 
     In third step  306 , a plasma is within the concave region of the collector element by focusing illumination into the volume of gas contained within the concave region of the collector element. For example, illumination  114  of a selected wavelength may be generated utilizing an illumination source  112 , such as a laser. In turn, the illumination  114  is focused into the volume of gas  153  in order to generate plasma  106  within the volume of gas  153 . For instance, the collector element  102  may receive illumination  114  from the illumination source  112  and focus the illumination  114  into the volume of gas  153  contained within the concave region  109  of the collector element  102 . It is noted herein that the plasma  106  need not be initiated by the illumination  114  from the illumination source  112 . For example, one or more electrodes (not shown) may be utilized to initiate the plasma  106 , while the illumination  114  is used to maintain the plasma  106 . 
     In a fourth step  308 , a portion of a plume  107  of the plasma  106  is propagated to a region  110  external to the concave region  109  of the collector element  102  via an opening  108  in the collector element  102 . For example, as shown in  FIG. 1C , the opening  108  may be arranged in the top portion (e.g., at or near the apex) of the collector element  102  such that the plume  107  generated by the generation of plasma  106  pass through the opening  108  into the region  110  external to the concave region  109  of the collector element  102 . 
     The herein described subject matter sometimes illustrates different components contained within, or connected with, other components. It is to be understood that such depicted architectures are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In a conceptual sense, any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “connected”, or “coupled”, to each other to achieve the desired functionality, and any two components capable of being so associated can also be viewed as being “couplable”, to each other to achieve the desired functionality. Specific examples of couplable include but are not limited to physically mateable and/or physically interacting components and/or wirelessly interactable and/or wirelessly interacting components and/or logically interacting and/or logically interactable components. 
     It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described is merely explanatory, and it is the intention of the following claims to encompass and include such changes. Furthermore, it is to be understood that the invention is defined by the appended claims.