Abstract:
An exemplary calorimeter includes a body configured to capture radiation generated by a source of the radiation, such as without limitation a laser, and absorb energy from the captured radiation. The calorimeter is simple to manufacture, operate, and maintain and is compact, highly accurate, able to withstand high power beams, and self-calibrating. NIST traceable electrical wires are used for the measurement. No fluids are used during measurements of the input radiation. A simple built in fluid or gaseous cooling system may be used post-measurement to reset the calorimeter temperature back to ambient for repeated measurement capability.

Description:
RELATED APPLICATION  
       [0001]     This patent application is related to a concurrently-filed U.S. patent application entitled “High Performance System and Method For Capturing and Absorbing Radiation” bearing attorney docket number BOEI-1-1190, the contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to measuring energy and, more specifically, to measuring energy of radiation.  
       BACKGROUND OF THE INVENTION  
       [0003]     Many modern devices, such as high-energy lasers and high powered-lamps like solar simulator lamps, are capable of putting out high levels of energy in the form of radiation. In certain circumstances, it is desirable to capture and absorb all or part of the output beam of such devices. For example, capturing a portion of the output beam may be desirable when the full output of the device provides too much energy for a desired application. Whether capturing all or part of a beam, if such a device simply captures and absorbs the output energy it is referred to as a beam dump.  
         [0004]     In other applications, it may be desirable to capture the output energy in order to measure the output level of the device. Such a measurement may be used to verify a manufacturer&#39;s claimed output levels for a device or to verify the performance of new devices and designs. In this case, the radiation capturing, absorbing, and measuring device is used as a calorimeter or power meter.  
         [0005]     The body of the calorimeter captures and absorbs the radiation, and causes the temperature of the body to rise. Precise knowledge of the thermal capacitance of the body allows the user to correlate the temperature rise of the calorimeter body to the energy absorbed. Thus, an accurate measurement of the temperature rise of the calorimeter body yields the energy content of the radiation. Care must be taken that the heat loss of the body due to conductive, convective, and radiative cooling is minimized and/or well characterized. In order to make an accurate measurement of the energy in the input radiation, the calorimeter must be capable of surviving the radiation (which may be high power) and must absorb substantially all of the input energy.  
         [0006]     Most currently available calorimeters must be cooled to survive high power radiation. Cooling prevents damage to the calorimeter. Cooling also resets the calorimeter to a condition in which the calorimeter is ready to make further measurements.  
         [0007]     In most currently known calorimeters that are designed for high energy beam measurements, cooling and measuring are both effected by water that is pumped through channels in the body of the calorimeter. Energy, in the form of heat, is transferred from the body of the calorimeter to the water, thereby heating the water and subsequently cooling the body of the calorimeter. A precision thermometer of some type measures the temperature rise of the water and a flowmeter with substantial accuracy measures the flow rate of the water flowing through the channels of the calorimeter. The temperature rise of the water together with the measured flow rate of the water is used to approximate the energy absorbed by the calorimeter body.  
         [0008]     However, currently known calorimeters that use water to measure the temperature change of the body include drawbacks. For example, in an attempt to accurately measure substantially all of the energy, the surface area available for heat transfer between the water and the calorimeter body is desired to be large. In order to accomplish this, numerous intricate water channels are machined into the body of the calorimeter. This increases surface area for heat transfer from the body of the calorimeter to the water, but this also introduces a pressure drop in the water flow because of constriction of the channels. Therefore, a high pressure pump is used to pump water through the numerous intricate channels. The high pressure pump itself is expensive.  
         [0009]     Because the water channels are usually small and intricate, it is desirable to keep the channels free of corrosion and contamination. Corrosion and contamination within the channels can reduce the amount of heat transferred to the water or prevent water flow by blocking the channels. However, maintaining water chemistry within desirable limits to reduce contamination and corrosion introduces further costs because water chemistry maintenance is extremely labor-intensive. Moreover, deionized (DI) water is used as the cooling liquid and is treated with fungicide to further reduce corrosion. Use of DI water and fungicide increases costs even further. Further, if channel blockages become severe enough, there may be areas of the calorimeter that experience restricted water flow, thereby causing inaccurate measurement and/or elevated local temperatures at which the equipment may fail.  
         [0010]     Inaccuracies in measurements are also introduced by pumping high-pressure water through intricate channels. For example, water is subject to self-heating due to the friction of the water being pumped at high pressure through the intricate channels. This unintended self-heating of the water results in a temperature rise in the water that is not caused by the input radiation and therefore is a source of inaccuracy in the measurement of input radiation.  
         [0011]     Inaccuracies in measurement of water flow rates also cause inaccuracies in measurements of the input radiation. For example, when water is forced under high pressure to flow through the calorimeter body, it can set up turbulence in the flow that will introduce false readings in the flow meter.  
         [0012]     Furthermore, there is some energy left in the calorimeter body that is not transferred to the water and therefore is not measured by the thermometer. This residual energy will then introduce inaccuracies in the measurement of the total energy of the radiation source.  
         [0013]     As a result, there is an unmet need in the art for a high energy calorimeter that is able to withstand high power radiation, accurately measures substantially all of the energy of the radiation, and is inexpensive to fabricate, operate, and maintain.  
       SUMMARY OF THE INVENTION  
       [0014]     The present invention provides a high energy calorimeter that is able to withstand high power radiation, accurately measures substantially all of the energy of the radiation, and is inexpensive to fabricate, operate, and maintain. Advantageously, embodiments of the present invention directly sense temperature of a body of the calorimeter over a substantial portion of the body of the calorimeter. Embodiments of the calorimeter include a thermal isolation system to isolate the body from the surrounding environment. As a result, accuracy of measurements is improved over currently known calorimeters. Further, embodiments of the present invention do not require cooling during measurements, do not use water to make the measurement, and yet are able to survive high power radiation. Therefore, the present invention avoids the inherent inaccuracies of a water-based system and the costs associated with such systems. If post-measurement cooling is desired, a simple liquid or gaseous system may be used to cool embodiments of the present invention.  
         [0015]     According to an embodiment of the present invention, an exemplary calorimeter includes a body configured to capture radiation generated by a source of the radiation, such as without limitation a laser, and absorb substantially all the energy of the captured radiation. An accurately-measured value of thermal capacitance is determined for the body. A temperature sensor system is attached in thermal communication with the body, and the temperature sensor system is configured to detect temperature changes of a substantial portion of the body. The absorption of the captured radiation by the body causes the temperature changes. Accordingly, the absorbed energy of the captured radiation can be readily calculated using the measured temperature rise and the measured thermal capacitance of the body.  
         [0016]     According to an aspect of the present invention, the temperature sensor includes wire with electrical resistance that varies with temperature, and the wire is attached in thermal communication with the body. The thermal characteristic of the wire is traceable to the National Institute for Standards and Technology (NIST).  
         [0017]     According to another aspect of the present invention, the calorimeter body includes a post-measurement cooling system. A plurality of relatively large, simple channels is defined in thermal communication within the interior of the body, and the plurality of channels is connectable to a source of coolant. The coolant may include gaseous nitrogen or other readily available and inexpensive gases, or readily available liquids such as water. If desired, the cooling system may be used post-measurement to lower the calorimeter body&#39;s temperature (which was elevated by the absorbed radiation).  
         [0018]     According to another aspect of the present invention, the calorimeter is equipped with electrical heaters that are used for calibration of the device. Using the electrical heaters, a known amount of energy is deposited into the body of the calorimeter and the resultant temperature rise is then measured using the resistance wires. Thereby, the actual thermal capacitance can then be determined. In this way, the device has a built-in and rapid calibration system.  
         [0019]     According to another aspect of the present invention, the calorimeter thermal isolation system substantially isolates the calorimeter body from the surrounding environment by using low thermal conductivity materials to mount and to insulate the body. Advantageously, these materials limit absorption of ambient environmental thermal energy by the body and leakage by the body of the captured energy, thereby helping to ensure that the measurement of the temperature rise is substantially affected only by the desired input radiation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0021]      FIG. 1  is a schematic diagram of a calorimeter and interconnections according to an embodiment of the present invention;  
         [0022]      FIG. 2  is a plan view with a partial cutaway of an exemplary calorimeter according to an embodiment of the present invention;  
         [0023]      FIG. 2A  is a cut-away view of an exemplary calorimeter assembly according to an embodiment of the present invention;  
         [0024]      FIG. 2B  is a perspective view of an exterior of a body of an exemplary calorimeter;  
         [0025]      FIG. 3  is a perspective view of a forward portion of the calorimeter of  FIG. 2 ;  
         [0026]      FIG. 4  is a section view of a detail of an exemplary temperature sensor;  
         [0027]      FIG. 5  is another perspective view showing an aft view of the calorimeter of  FIG. 2 ; and  
         [0028]      FIG. 6  is a block diagram of a calorimeter and data analysis system according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]     By way of overview and referring to  FIG. 1 , an exemplary high energy calorimeter  10  is able to withstand high power radiation  22 , accurately measures substantially all of the energy of the radiation  22 , and is inexpensive to fabricate, operate, and maintain. According to an exemplary embodiment of the present invention, the calorimeter  10  includes a body  12  configured to capture radiation  22  generated by a source  14  of the radiation  22 , such as without limitation a laser, and absorb energy from the captured radiation  22 . A temperature sensor system  16  is attached in thermal communication with the body  12 , and the temperature sensor system  16  is configured to detect temperature changes of a substantial portion of the body  12 . The absorption of the captured radiation  22  by the body  12  causes the temperature changes. A liquid or gaseous cooling system  18  is configured to provide post-measurement cooling of the body  12  from temperatures elevated due to absorption of the captured radiation  22 . A secondary temperature sensor system  24  is configured to provide thermal equilibrium state status of a substantial portion of the body  12 . Further, a heater circuit  26  is configured to provide a precision source of electrical calibration for the high energy calorimeter  10 . Details of embodiments of the present invention will now be set forth below.  
         [0030]     Referring now to  FIGS. 1 and 2 , the body  12  is designed to admit and absorb the radiation  22  from the source  14 . The radiation  22  may be any acceptable form of radiation, such as without limitation a laser beam. In one exemplary embodiment given by way without limitation, the radiation  22  may be a laser beam with a power range from around 10 KW to around 40 KW, a wavelength of around 1.315 microns and an intensity profile of around 0.5 to around 2.0 peak-to-average. Laser beam mode size can range from around 4 cm by around 4 cm to around 4 cm by around 12 cm. Duration of the laser beam may be around 2 seconds minimum, around 5 seconds nominal, and around 10 seconds maximum.  
         [0031]     The body  12 , sometimes referred to as a beam dump, may be any acceptable beam dump configured to capture and absorb substantially all of the incoming high energy radiation  22 . Beam dumps are well known in the art and, as a result, details of the geometry internal to the body  12  are not necessary for an understanding of the present invention. However, details of an exemplary beam dump for which the present invention is well suited are set forth in concurrently-filed U.S. patent application entitled “High Performance System and Method For Capturing and Absorbing Radiation” bearing attorney docket number BOEI-1-1190, the contents of which are hereby incorporated by reference.  
         [0032]     In one presently preferred embodiment, the body  12  is a copper body. However, it will be appreciated that the body  12  may be made of other high thermal conductivity materials, as desired, such as without limitation aluminum.  
         [0033]     In an exemplary embodiment, the calorimeter  10  includes a thermal isolation system. In one presently preferred embodiment, a plurality of fasteners  50  extends through a low thermal conductivity clamp  20  of the body  12  and attaches the body  12  to a flange  57 . Given by way of nonlimiting example, in one presently preferred embodiment the clamp  20  is constructed of glass filled PEEK (PolyEtherEtherKetone). It will be appreciated that any material with sufficiently low thermal conductivity and sufficient mechanical strength may be used. In one presently preferred embodiment, a thermal isolator plate  55  is placed between the body  12  and the flange  57 . Without limitation the isolator plate may be constructed of glass reinforced epoxy resin. In one exemplary embodiment, the flange  57  allows mounting support of the calorimeter  10 . Further, a presently preferred embodiment includes low thermal conductivity tubes  61  for transfer of coolant used during post-measurement cooling of the calorimeter body  12 . In one embodiment, the coolant tube suitably is constructed of glass filled epoxy resin. Advantageously, the low thermal conductivity clamp  20 , the thermal isolator plate  55 , and the low thermal conductivity coolant tubes  61  provide conductive thermal isolation of the calorimeter  12 .  
         [0034]     Further, the thermal isolation system includes material to insulate the calorimeter body  12  from the surrounding environment. Given by way of nonlimiting example, in one presently preferred embodiment, the insulation  59  is fabricated from Polyimide foam with an outer covering that reflects radiation. The insulation  59  suitably is designed with structural rigidity such that at installation an airgap  63  is provided between the calorimeter body  12  and the insulation  59 . Advantageously, the insulation  59  and airgap  63  provide radiative and convective isolation of the calorimeter body  12 .  
         [0035]     Referring now to  FIGS. 1 and 2 A, one exemplary embodiment of the calorimeter  10  includes an integral calibration system including a built-in electrical heating system  26 . A plurality of electrical heaters  100  are used as part of the calibration process. By supplying a known amount of electrical power to the heaters  100  for a known period of time a known amount of energy is deposited in the calorimeter body  12 . Subsequently, a measurement of the change in the temperature of the body  12  is performed by the temperature sensor system  16  and the heat capacitance may be calculated. The calorimeter system thus allows direct determination and verification of its own thermal capacitance. Given by way of nonlimiting example, in one preferred embodiment the heaters  100  are Chromalox CIR-20252-120 cartridge heaters.  
         [0036]     In one presently preferred embodiment, the body  12  can absorb radiation within a dynamic range of between around 20 Kilojoules (KJ) and around 400 KJ and does not require any cooling of the body  12  during the measurement process. Because the body  12  of the calorimeter  10  will capture and absorb substantially all of the energy of the radiation  22 , the body  12  may be cooled post-measurement to allow for subsequent measurements. If the input energy is high enough, successive runs without either active cooling or sufficient time for passive cooling may drive the bulk temperature of the device  10  above safe operating temperatures of the materials or pose safety hazards to personnel.  
         [0037]     Advantageously, according to a preferred embodiment of the present invention, the cooling system  18  uses a gaseous coolant. However, it will be appreciated that the cooling system  18  may use other cooling mediums, as desired, such as without limitation deionized water. The gaseous coolant suitably may be used only to cool the body  12  post-measurement, that is, after capture and measurement of the radiation  22 , and is not used as either part of or during the measurement process. Advantageously, embodiments of the present invention use the mass of the body  12 , and in one presently preferred embodiment the copper mass, as the thermal mass to store the captured energy of the radiation  22  for subsequent measurement. Advantageously and as a result, the present invention avoids the inaccuracies and costs inherent in most currently known calorimeters that use high-pressure water or other liquids pumped through numerous constrictive channels as part of the measurement system and for cooling.  
         [0038]     In one exemplary embodiment, the gaseous coolant includes gaseous Nitrogen (GN 2 ). Given by way of nonlimiting example, the GN2 may be provided at around 66 psig at a mass flow rate of around 5.2 lbm/min. This configuration resets the body  12  in about 45 minutes or less to a cooled temperature sufficient for the calorimeter  10  to begin another measurement. However, it will be appreciated that other gaseous coolants, such as inert gases like helium, may be used as desired for a particular application. It will be further appreciated that liquids may also be used to cool the calorimeter body post-measurement. However, care should be taken to ensure that all residual moisture is removed from coolant channels  56  and headers  54  prior to performing new measurements (see  FIG. 2 ).  
         [0039]     In one exemplary embodiment, the inlet ports  52  at a first end  53  of the body  12  are arranged to be coupled to receive an acceptable coolant gas, such as GN 2 , from a supply (not shown) of the coolant gas. An inlet header  54  extends a finite distance, such as about half-way, from the first end  53  into the body  12 . It will be appreciated that the inlet header  54  may extend any distance into the body  12  for a desired application. A longer length of the inlet header  54  may provide for more surface area of the body  12  in thermal communication with the coolant gas. However, the length of the inlet header  54  may depend upon the selected attenuation geometry within the body  12 .  
         [0040]     The inlet header  54  supplies the coolant gas to a plurality of coolant channels  56  that extend throughout the body  12 . In one exemplary embodiment, the coolant channels  56  extend substantially normally from the inlet header  54  across substantially the width of the body  12 . It will be appreciated, again, that longer lengths and/or higher quantities of the coolant channels  56  may provide for more surface area of the body  12  in thermal communication with the coolant gas.  
         [0041]     The coolant channels  56  connect to an outlet header (not shown) that is similar to the inlet header  54 . The outlet header terminates at an outlet port  58 . The outlet port  58  is arranged to be connected to a reservoir (not shown) for dumping expended coolant gas received from the calorimeter  10 .  
         [0042]     Similarly, referring now to  FIG. 5 , a second set of coolant headers  54  and coolant channels  56 , an outlet header (not shown), an inlet port  52 , and an outlet port  58  are configured for the lower half of the body  12 .  
         [0043]     Advantageously, embodiments of the present invention directly sense temperature of the body  12  of the calorimeter  10  over a substantial portion of the body  12 . Referring now to  FIGS. 3 and 4 , the exterior of the body  12  contains a continuous helical groove  60  that extends substantially the length of the body  12  between the first end  53  and a second end  62  of the body  12 . As shown in  FIG. 4 , the helical groove  60  has a depth that is deep enough to implant the temperature sensor system  16  (discussed in detail below) in the interior of the body  12  to improve heat transfer from the body  12  to wires  64 ,  68 , and  70  (discussed in detail below) and to protect the wires. On the other hand, the depth of the helical groove  60  is not so deep as to endanger integrity of the temperature sensor system  16  due to local heating. As such, depth of the helical groove  60  may be selected as desired for a particular application. Because the helical groove  60  extends substantially the length of the body  12 , the temperature sensor system  16  (implanted within the helical groove  60 ) advantageously may directly sense temperature of the body  12  along a substantial portion of the body  12 .  
         [0044]     Referring to  FIG. 4 , details will be set forth regarding the temperature sensor system  16 . In one exemplary embodiment, the temperature sensor system  16  is a continuous wire  64  implanted within the helical groove  60 . Advantageously, resistance of the wire  64  varies proportionally with temperature of the wire  64 . That is, as temperature of the wire  64  increases, resistance of the wire  64  decreases. In one exemplary, nonlimiting embodiment, the wire  64  suitably is polyamide coated copper resistance wire, has a gauge of 30 AWG. Advantageously, the polyamide coating provides electrical isolation of the wire  64  from the body  12 . Therefore the resistance measurement is isolated to the length of the wire  64 . The wire is around 468 inches long in order to extend throughout the length of the helical groove  60 . However, the wire  64  may have any length as desired for a particular application. It will be appreciated that the wire  64  extends substantially the length of body  12  and length of wire wrapped around the body  12  is a substantial portion of total wire length. The temperature dependence of the resistance of wires  64 ,  68 , and  70  are NIST traceable.  
         [0045]     The wire  64  is encapsulated within the helical groove  60  with a potting compound  66 , such as without limitation aluminum filled epoxy or the like. Advantageously, the potting compound  66  has a high coefficient of thermal conductivity. As a result, the wire  64  is in thermal communication with the body  12  along a substantial portion of the body  12 . Given by way of nonlimiting example, this gives rise to a response time from beginning of irradiation to registering a change in temperature on the wire  64  of around 2 seconds. Further, temperature measurements can achieve equilibrium in less than around 5 minutes. For redundancy purposes, if desired, wires  68  and  70  may also be provided along with the wire  64  in the helical groove  60 . If provided, the wires  68  and  70  suitably may be made of the same material as the wire  64  and are also encapsulated by the potting compound  66  in the helical groove  60 .  
         [0046]     Referring now to  FIG. 5 , in one embodiment a plurality of ports  72  are provided for heaters (not shown), such as without limitation 5 KW electrical heating elements like NiChrome wire. The electrical heating elements raise the temperature of the body  12  to a predetermined temperature as desired. Advantageously, the temperature sensor system  16  may be calibrated by comparing the temperature of the body  12  as determined by the temperature sensor system  16  against the expected temperature rise due to the electrical heaters.  
         [0047]     Referring briefly back to  FIG. 3 , thermocouples  73  may be provided throughout the body  12  as another component of the temperature sensor system  16 . The thermocouples  73  generate an output signal proportional to temperature in a known manner. The thermocouples  73  measure local temperatures, thereby allowing determination of whether or not the calorimeter body  12  has reached thermal equilibrium. The thermocouples  73 , along with the temperature sensing system  16 , also permit an operator to determine if the body  12  has cooled sufficiently after use to irradiate the body  12  again. In one exemplary embodiment, the thermocouples  73  have a temperature range from about 10 degrees Celsius to about 50 degrees Celsius.  
         [0048]     Referring now to  FIG. 6 , a system  76  determines energy output of the source radiation  22 . An ohmmeter  74 , such as a digital multimeter, is coupled to the wire  64  and, if provided, the wires  68  and  70 . The ohmmeter  74  has a resolution sufficient to detect changes in resistance of the wire  64 . In one embodiment, the digital multimeter  74  suitably is a 5½ digit digital multimeter with a resolution of around 1 milli ohm and preferably is a 6½ digit digital multimeter with a resolution of around 100 micro ohm. The digital multimeter  74  measures the resistance of the wire  64  and, if provided, the wires  68  and  70 , in a known manner and generates an output signal  78 .  
         [0049]     A data acquisition computer  80  processes the output signal  78 . The data acquisition computer  80  suitably includes any computer that is well known in the art. The output signal  78  is received and conditioned by an input interface  82 , such as without limitation an RS-232 interface. A system bus  92  interconnects a processor  84 , a user interface  86 , magnetic or optical storage media  88 , and a display device  90 . The processor  84  may be any acceptable processor, such as without limitation a Pentium® &amp; or Celeron® processor available from the Intel Corporation or a processor for a personal data assistant (PDA) operating on a Palm® operating system or the like. The user interface  86  may be a keyboard, mouse, trackball, PDA-type stylus, or the like. The magnetic or optical storage  88  includes any acceptable memory or storage device, such as without limitation any type of random access memory (RAM) or read-only memory (ROM), flash memory, a compact disc or digital video disc, or the like. Advantageously, storage  88  includes a mapping of changes in temperature (AT) of the body  12  with the energy absorbed . The display device  90  may be any suitable monitor or screen. Components of the computer  80  are well known and a detailed explanation of their construction and operation is not necessary for an understanding of the present invention.  
         [0050]     The system  76  operates as follows. As discussed above, the calorimeter  10  receives the radiation  22  from the source  14  ( FIG. 1 ) and absorbs the radiation  22 . As also discussed above, energy from the radiation is transferred to the body  12  in the form of heat. As a result, temperature of the body  12  rises.  
         [0051]     As temperature of the body  12  rises, heat is conducted through the potting compound  66  to the wire  64  and, if provided, the wires  68  and  70 . As a result, temperature of the wire  64  and, if provided the wires  68  and  70 , rises and wire resistance lowers  
         [0052]     The digital multimeter  74  determines wire resistance and provides resistance readings to the data acquisition computer  80  via the signal  78 . The processor  84  converts resistance readings provided by the signal  78  to temperature. The processor  84  determines ΔT by subtracting the initial temperature (at the beginning of or before irradiating the body  12 ) from the temperature indicated during irradiation of the body  12 . The processor  84  retrieves from storage  88  the mapping of energy versus ΔT. The energy that correlates to the determined ΔT is divided by the time of irradiation to determine power of the radiation in units as desired, such as without limitation Kilo watts.  
         [0053]     After irradiation of the body  12  and the completion of measurements, cooling gas may be supplied by the cooling system  18 . Energy in the form of heat is transferred from the body  12  to the cooling gas as the cooling gas passes through the inlet header  54 , the coolant channels  56 , and the outlet port  58 . Coolant gas that has exited the outlet port  58  is dumped as desired in any known manner.  
         [0054]     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.