Patent Publication Number: US-11389899-B2

Title: Laser processing system, jet observation apparatus , laser processing method, and jet observation method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a new U.S. Patent Application that claims benefit of Japanese Patent Application No. 2018-157783, dated Aug. 24, 2018, the disclosure of this application is being incorporated herein by reference in its entirety for all purposes. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a laser processing system and a laser processing method. 
     2. Description of the Related Art 
     A laser processing system has been known that includes a nozzle that, when processing a workpiece with a laser beam, emits an assist gas for blowing out a material of a workpiece that is melted by the laser beam (e.g., JP 2017-051965 A). 
     There has been a need for a laser processing system that can effectively blow out a material of a workpiece that is melted by a laser beam by effectively utilizing an assist gas emitted from a nozzle. 
     SUMMARY OF THE INVENTION 
     In an aspect of the present disclosure, a laser processing system comprises a nozzle including an emission opening configured to emit a jet of an assist gas along an optical axis of a laser beam, the nozzle being configured to form a maximum point of velocity of the jet at a position away from the emission opening; a measuring instrument configured to measure a supply flow rate of the assist gas to the nozzle; and a position acquisition section configured to acquire the position of the maximum point from a measurement value of the measuring instrument by predetermined calculation. 
     In another aspect of the present disclosure, a jet observation apparatus comprises a measuring instrument configured to measure a supply flow rate of a gas supplied to a nozzle; and a position acquisition section configured to acquire, from a measurement value of the measuring instrument by predetermined calculation, a position of a maximum point of velocity of a jet of the gas emitted from an emission opening of the nozzle, the maximum point being formed at the position away from the emission opening. 
     In still another aspect of the present disclosure, a method of laser process on workpiece, using the above laser processing system, comprises emitting the jet from the emission opening of the nozzle and processing the workpiece with the laser beam, while the nozzle is disposed with respect to a process portion of the workpiece at a target position determined based on the position of the maximum point. 
     In still another aspect of the present disclosure, a method of observing a jet, comprises measuring a supply flow rate of a gas supplied to a nozzle; and acquiring, from a measurement value of the supply flow rate by predetermined calculation, a position of a maximum point of velocity of a jet of the gas emitted from an emission opening of the nozzle, the maximum point being formed at the position away from the emission opening. 
     According to the present disclosure, since the assist gas emitted from the nozzle during the process on the workpiece can be blown to the workpiece at a velocity which is sufficiently large, it is possible to effectively utilize the assist gas so as to effectively blow out a material of the workpiece melted by the laser beam. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a laser processing system. 
         FIG. 2  is an image obtained by capturing a jet of an assist gas emitted from a nozzle with a high-speed camera. 
         FIG. 3  is a diagram for illustrating a maximum point of velocity of a jet, an upper graph schematically shows a relationship between the velocity of the jet and a position x from an emission opening, and the image of  FIG. 2  is shown below the graph. 
         FIG. 4  is a diagram of a jet observation apparatus. 
         FIG. 5  is a block diagram of the jet observation apparatus illustrated in  FIG. 4 . 
         FIG. 6  is a diagram of another jet observation apparatus. 
         FIG. 7  is a block diagram of the jet observation apparatus illustrated in  FIG. 6 . 
         FIG. 8  is a diagram of yet another jet observation apparatus. 
         FIG. 9  is a block diagram of the jet observation apparatus illustrated in  FIG. 8 . 
         FIG. 10  is a diagram of another laser processing system. 
         FIG. 11  is a block diagram of the laser processing system illustrated in  FIG. 10 . 
         FIG. 12  is a diagram of yet another laser processing system. 
         FIG. 13  is a block diagram of the laser processing system illustrated in  FIG. 12 . 
         FIG. 14  is a flowchart illustrating an example of an operation flow of the laser processing system illustrated in  FIG. 12 . 
         FIG. 15  is a flowchart illustrating an example of the flow of Step S 14  in  FIG. 14 . 
         FIG. 16  is a graph schematically showing a relationship between output data of a measuring instrument illustrated in  FIG. 12  and a distance from an emission opening. 
         FIG. 17  is a diagram of yet another laser processing system. 
         FIG. 18  is a block diagram of the laser processing system illustrated in  FIG. 17 . 
         FIG. 19  is a diagram of yet another laser processing system. 
         FIG. 20  is a block diagram of the laser processing system illustrated in  FIG. 19 . 
         FIG. 21  is a flowchart illustrating an example of an operation flow of the laser processing system illustrated in  FIG. 19 . 
         FIG. 22  is a diagram of a jet adjustment device. 
         FIG. 23  illustrates an example of a mechanism section illustrated in  FIG. 22 . 
         FIG. 24  illustrates another example of the mechanism section illustrated in  FIG. 22 . 
         FIG. 25  is a diagram of yet another laser processing system. 
         FIG. 26  is a block diagram of the laser processing system illustrated in  FIG. 25 . 
         FIG. 27  is a flowchart illustrating an example of an operation flow of the laser processing system illustrated in  FIG. 25 . 
         FIG. 28  illustrates an example of a measuring instrument. 
         FIG. 29  illustrates another example of the measuring instrument. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described in detail below with reference to the drawings. Note that, in the various embodiments described below, the same reference numerals will be given to similar elements, and redundant descriptions thereof will be omitted. First, a laser processing system  10  will be described with reference to  FIG. 1 . 
     The laser processing system  10  includes a laser oscillator  12 , a laser processing head  14 , an assist gas supply device  16 , and a positioning device  18 . The laser oscillator  12  oscillates a laser inside thereof, and emits a laser beam to the outside. The laser oscillator  12  may be of any type, such as a CO 2  laser oscillator, a solid-state laser (YAG laser) oscillator, or a fiber laser oscillator. 
     The laser processing head  14  includes a head main body  20 , optical lenses  22 , a lens driver  23 , and a nozzle  24 . The head main body  20  is hollow, and an optical fiber  26  is connected to a proximal end of the head main body  20 . The laser beam emitted from the laser oscillator  12  propagates inside the optical fiber  26  and enters inside of the head main body  20 . 
     The optical lenses  22  include e.g. a collimating lens and a focusing lens, collimate and focus the laser beam entering into the head main body  20 , and irradiate the laser beam onto a workpiece W. The optical lenses  22  are housed in the head main body  20  so as to be movable in a direction of an optical axis O. 
     The lens driver  23  moves each optical lens  22  in the direction of the optical axis O. The lens driver  23  adjusts a position of each optical lens  22  in the direction of the optical axis O, whereby it is possible to control a position of a focal point of the laser beam emitted from the nozzle  24  in the direction of the optical axis O. 
     The nozzle  24  is hollow and provided at a distal of the head main body  20 . The nozzle  24  has a truncated conical outer shape in which a cross-sectional area orthogonal to the optical axis O decreases as it extends from the proximal end toward the distal end, and has a circular emission opening  28  at the distal end thereof. A hollow chamber  29  is formed inside the nozzle  24  and the head main body  20 . The laser beam propagating from the optical lens  22  is emitted from the emission opening  28 . 
     The assist gas supply device  16  supplies an assist gas to the chamber  29  formed in the nozzle  24  and the head main body  20  via a gas supply tube  30 . The assist gas is e.g. nitrogen or air. The assist gas supplied to the chamber  29  is emitted, as a jet, from the emission opening  28  together with the laser beam along the optical axis O of the laser beam. The nozzle  24  forms a maximum point of velocity of the jet at a position away from the emission opening  28 . 
     The jet of the assist gas emitted from the nozzle  24  will be described below with reference to  FIG. 2  and  FIG. 3 .  FIG. 2  is an image captured by a high-speed camera imaging the jet emitted from the emission opening  28  of the nozzle  24 .  FIG. 3  shows the image of the jet shown in  FIG. 2  and a graph schematically showing a relationship between velocity V of the jet and position x in a direction away from the emission opening  28  along the optical axis O. 
     In this disclosure, the “velocity” of the jet is defined as a parameter including a flow velocity (unit: m/sec) and a flow rate (unit: m 3 /sec) of the assist gas. The jet shown in  FIG. 2  and  FIG. 3  is formed under a condition in which the supply pressure to the chamber  29  is 1 MPa, and the opening dimension (diameter) of the emission opening  28  is 2 mm. 
     As shown in  FIG. 2  and  FIG. 3 , in the jet of the assist gas emitted from the nozzle  24 , a first Mach disk region  33  and a second Mach disk region  35 , where the velocity V of the jet becomes local maximum, are formed at positions away from the emission opening  28  in the direction of the optical axis O. The position x 1  of the first maximum point  32  of the velocity V is included in the first Mach disk region  33 , while the position x 2  of the second maximum point  34  of the velocity V is included in the second Mach disk region  35 . 
     More specifically, as shown in the graph of  FIG. 3 , the velocity V of the jet gradually increases as a distance from the position of the emission opening  28  (i.e., x=0) along the optical axis O increases, and reaches the first maximum point  32  at the position x 1 . Note that, in the jet in the image shown in  FIG. 3 , x 1 ≈4 mm. In the first Mach disk region  33  including the position x 1 , a so-called Mach disk, where reflected waves of the assist gas reflected at a boundary between the jet and an atmosphere outside the jet interfere and strengthen with each other, is formed. 
     The velocity V rapidly decreases as the distance in the direction away from the emission opening  28  from the position x 1  along the optical axis O further increases, then turns to increase, and reaches the second maximum point  34  at the position x 2 . In the second Mach disk region  35  including the position x 2 , a second Mach disk is formed. 
     Thus, in the jet emitted from the nozzle  24 , a plurality of Mach disks are formed in the direction of the optical axis O, whereby the velocity V of the jet has a plurality of maximum points  32  and  34  in the direction of the optical axis O. The number of the Mach disks (i.e., maximum points) to be formed increases depending on the velocity V of the emitted jet. 
     In the present disclosure, when the laser processing system  10  carries out laser process on the workpiece W, the nozzle  24  is disposed with respect to a process portion S of the workpiece W at a target position determined based on the position x 1  or x 2  of the maximum point  32  or  34 , such that the workpiece W (specifically, the process portion S of the workpiece W) is disposed in one of the Mach disk regions  33  and  35 . 
     In the prior art, it has been considered preferable that the pressure of the assist gas blown to the workpiece W during the laser process on the workpiece W is as large as possible. The pressure of the assist gas is maximized at the position of the emission opening  28 . Accordingly, in the prior art, when laser-processing the workpiece W, the workpiece W has been arranged as close as possible to the emission opening  28  at which the pressure is maximized. Specifically, in the prior art, the workpiece W has been disposed in a proximity region  36  in  FIG. 3 . This proximity region  36  is a region closer to the emission opening  28  than the first maximum point  32 , wherein the pressure of the assist gas is almost a maximum value in this proximity region  36 . 
     When the nozzle  24  and the workpiece W are thus arranged closer to each other and the laser process is carried out while the nozzle  24  is moved at high speed with respect to the workpiece W, plasma may be generated easier between the nozzle  24  and the workpiece W. When such plasma is generated, a finished surface of the workpiece W may become rough. Furthermore, when the nozzle  24  and the workpiece W are arranged closer to each other, particles of the workpiece W, that are melted and scattered by the laser process, may enter into the nozzle  24  through the emission opening  28 , whereby a component (e.g., a protective glass) of the laser processing head  14  is more likely to be contaminated. 
     After diligent researching, the inventor obtained a knowledge that, as the velocity V of the assist gas blown to the workpiece W during the laser process on the workpiece W increases, the material of the workpiece W melted by the laser beam can be more effectively blown out by the assist gas. 
     Based on this knowledge, the present inventor focused on a fact that the above-described maximum points  32  and  34  are formed when the jet of the assist gas is emitted from the emission opening  28  of the nozzle  24 , and found that, if the workpiece W is disposed in one of the Mach disk regions  33  and  35  during the laser process on the workpiece W, the assist gas can be blown to the workpiece W at the velocity V greater than that in the proximity region  36  to the emission opening  28 . 
     Referring again to  FIG. 1 , the positioning device  18  disposes the nozzle  24  with respect to the process portion S at the target position determined based on the position x 1  of the maximum point  32  or the position x 2  of the maximum point  34 , in order to dispose the workpiece W (e.g., process portion S) in the Mach disk region  33  or  35 . Specifically, the positioning device  18  includes a work table  38 , a y-axis movement mechanism  40 , an x-axis movement mechanism  42 , and a z-axis movement mechanism  44 . 
     The work table  38  is fixed on a floor of a work cell. For example, the work table  38  has a plurality of needles extending in the z-axis direction in  FIG. 1 , and the workpiece W is installed on an installation surface defined by tips of the plurality of needles. The z-axis direction is substantially parallel to a vertical direction, for example. 
     The y-axis movement mechanism  40  includes a pair of rail mechanisms  46  and  48 , and a pair of columns  50  and  52 . Each of the rail mechanisms  46  and  48  includes e.g. a servo motor and a ball screw mechanism (both not illustrated) therein, and extends in the y-axis direction. The rail mechanisms  46  and  48  move the columns  50  and  52  in the y-axis direction, respectively. 
     The x-axis movement mechanism  42  includes e.g. a servo motor and ball screw mechanism (both not illustrated) therein, and is fixed to the columns  50  and  52  so as to extend between the columns  50  and  52 . The x-axis movement mechanism  42  moves the z-axis movement mechanism  44  in the x-axis direction. The z-axis movement mechanism  44  includes e.g. a servo motor and a ball screw mechanism (both not illustrated) therein, and moves the laser processing head  14  in the z-axis direction. The laser processing head  14  is provided at the z-axis movement mechanism  44  such that the optical axis O of the laser beam to be emitted is parallel to the z-axis. 
     When the workpiece W is processed, the positioning device  18  disposes the nozzle  24  at the target position with respect to the process portion S. For example, a below-described controller (not illustrated) provided in the laser processing system  10  controls the positioning device  18  so as to automatically dispose the nozzle  24  with respect to the workpiece W at the target position. Alternatively, an operator may manually operate the positioning device  18  so as to dispose the nozzle  24  with respect to the process portion S at the target position. 
     Then, the assist gas supply device  16  supplies the assist gas to the chamber  29 , and emits the jet of the assist gas, which has the Mach disk regions  33  and  35 , from the emission opening  28 . Then, the laser oscillator  12  emits the laser beam to the laser processing head  14 , and the laser processing head  14  emits the laser beam from the emission opening  28  so as to irradiate the workpiece W. At this time, the lens driver  23  adjusts the position of each optical lens  22  in the direction of the optical axis O, such that the focal point of the laser beam emitted from the emission opening  28  is located at the process portion S. 
     In this way, the workpiece W is laser-processed in a stated where the workpiece W is disposed in the Mach disk region  33  or  35  of the jet. According to this configuration, since the assist gas emitted from the nozzle  24  can be blown to the workpiece W at the velocity V greater than that in the proximity region  36  of the emission opening  28  during the process on the workpiece W, it is possible to effectively make use of the assist gas so as to effectively blow out the material of the workpiece W melted by the laser beam. 
     Furthermore, it is possible to prevent the above-described plasma from being generated when compared to a case where the workpiece W is disposed in the proximity region  36  to the emission opening  28 , as a result of which, the finishing quality of the workpiece W can be improved. In addition, it is possible to prevent the scattered particles of the workpiece W generated during the laser process from entering into the nozzle  24  when compared to the case where the workpiece W is disposed in the proximity region  36 , as a result of which, the contamination of the component of the laser processing head  14  can be prevented. 
     Next, a jet observation apparatus  60  will be described with reference to  FIG. 4  and  FIG. 5 . The jet observation apparatus  60  acquires information representing the position x 1 , x 2  of the above-described maximum point  32 ,  34 . The jet observation apparatus  60  includes a controller  62 , a dummy workpiece  64 , a measuring instrument  66 , and the above-described positioning device  18 . The controller  62  includes a processor (CPU, GPU, etc.) and a storage (ROM, RAM, etc.), and controls the measuring instrument  66  and the positioning device  18 . 
     The dummy workpiece  64  is installed on the installation surface of the work table  38 . The dummy workpiece  64  has an outer shape (dimensions) the same as the workpiece W, and includes a dummy process portion  64   a  corresponding to the process portion S. In an example illustrated in  FIG. 4 , the dummy workpiece  64  is disposed at a position different from the installation position of the workpiece W during the laser process. 
     The measuring instrument  66  measures the velocity V of the jet emitted from the emission opening  28 , at a position of the dummy process portion  64   a  (or a position slightly displaced from the dummy process portion  64   a  in a direction toward the emission opening  28 ). For example, the measuring instrument  66  includes a hot-wire anemometer configured to measure the velocity V in a contact manner, wherein the hot-wire anemometer includes a hot-wire which is disposed in the jet and the resistance value of which varies in response to the velocity V. Alternatively, the measuring instrument  66  includes a laser anemometer configured to measure the velocity V in a non-contact manner, wherein the laser anemometer includes an optical sensor configured to irradiate the jet with light and measure the velocity V. 
     The measuring instrument  66  measures the velocity V of the jet, and outputs it to the controller  62  as output data (measured values) α. The measuring instrument  66  may be disposed on the dummy workpiece  64 , or may be disposed separate away from the dummy workpiece  64 . The dummy workpiece  64  is disposed at forward (i.e., downstream) in the flow direction of the jet, and the measuring instrument  66  measures the velocity V at a position between the emission opening  28  and the dummy workpiece  64 . 
     The positioning device  18  includes the work table  38 , the y-axis movement mechanism  40 , the x-axis movement mechanism  42 , and the z-axis movement mechanism  44 , and moves the laser processing head  14  in the x-axis, y-axis, and z-axis directions so as to move the laser processing head  14  with respect to the dummy workpiece  64  and the measuring instrument  66 . 
     Next, a method of acquiring the position x 1 , x 2  of the maximum point  32 ,  34  using the jet observation apparatus  60  will be described. First, the controller  62  operates the positioning device  18  so as to dispose the laser processing head  14  at an initial measuring position. When the laser processing head  14  is disposed at the initial measuring position, the laser processing head  14  is positioned with respect to the dummy workpiece  64  and the measuring instrument  66 , such that the optical axis O of the laser processing head  14  intersects the dummy process portion  64   a  of the dummy workpiece  64 , as illustrated in  FIG. 4 . 
     Also, a distance d a  between the emission opening  28  and a measuring position of the measuring instrument  66  (i.e., the position of the dummy process portion  64   a ) is an initial value d a0 . As an example, the initial value d a0  is set such that the measuring position of the measuring instrument  66  is disposed at a position close to the emission opening  28 , such as the proximity region  36  in  FIG. 3 . 
     As another example, the initial value d a0  is set such that the measuring position of the measuring instrument  66  is disposed at a position sufficiently separate to downstream of the jet from a position where a maximum point farthest from the emission opening  28  (in the example of  FIG. 3 , the second maximum point  34 ) is estimated to be located. The initial value d a0  is predetermined by the operator. 
     Then, the controller  62  sends a command to the assist gas supply device  16 , and in response to the command, the assist gas supply device  16  supplies the assist gas to the chamber  29  at a supply pressure P S . The nozzle  24  emits the jet of the assist gas having the maximum points  32  and  34  of the velocity V, as shown in  FIG. 2  and  FIG. 3 . 
     Then, the controller  62  operates the positioning device  18  so as to move the laser processing head  14  in the z-axis direction so as to change the distance d a  between the emission opening  28  and the measuring position of the measuring instrument  66 . As an example, if the above-described initial value d a0  is set to dispose the measuring position of the measuring instrument  66  at the close position to the emission opening  28 , the controller  62  operates the positioning device  18  so as to move the laser processing head  14  in the z-axis positive direction to increase the distance d a . 
     As another example, if the above-described initial value d a0  is set to dispose the measuring position of the measuring instrument  66  at downstream side of the maximum point farthest from the emission opening  28 , the controller  62  operates the positioning device  18  so as to move the laser processing head  14  in the z-axis negative direction to decrease the distance d a . 
     While the positioning device  18  moves the laser processing head  14  in the z-axis direction, the controller  62  sends a command to the measuring instrument  66  and cause the measuring instrument  66  consecutively measure the velocity V. For example, the measuring instrument  66  consecutively measures the velocity V at a predetermined period (e.g., 0.5 seconds) while the positioning device  18  moves the laser processing head  14 . In this way, the measuring position of the measuring instrument  66  is moved relatively along the jet, and the measuring instrument  66  consecutively measures the velocity V along the jet. 
     The measuring instrument  66  outputs the measured velocity V as the output data α (=V) to the controller  62 . A relationship between the output data α, which is outputted by the measuring instrument  66  in this way, and the distance d a  corresponds to the relationship between the velocity V and the position x shown in  FIG. 3 . That is, the output data α acquired by the measuring instrument  66  changes in response to the distance d a , and has a first peak value α max1  at a position corresponding to the first maximum point  32  and a second peak value α max2  at a position corresponding to the second maximum point  34 . 
     The controller  62  acquires the first peak value α max1  of the consecutive output data α outputted by the measuring instrument  66  as information representing the position x 1  of the first maximum point  32 , and acquires the second peak value α max2  as information representing the position x 2  of the second maximum point  34 . In this way, the controller  62  functions as a position acquisition section  68  configured to acquire the information representing the position of the maximum point  32 ,  34  based on the output data α. 
     Then, the controller  62  acquires, as a target distance d T , the distance d a  between the measuring position of the measuring instrument  66  (i.e., the position of the dummy process portion  64   a ) and the emission opening  28  when the first peak value α max1  is measured. This target distance d T  represents the position of the first maximum point  32  with respect to the emission opening  28 , and can be acquired by e.g. a known gap-sensor, a displacement measuring instrument, or the like. 
     Then, the controller  62  resisters the target distance d T  in a data base in association with the opening dimension ϕ of the emission opening  28  and the supply pressure P S  when measuring the velocity V, and stores it in the storage. The operator changes the opening dimension ϕ of the nozzle  24  and the supply pressure P S  in various ways, and the controller  62  acquires the target distance d T  and registers it in the database of the above-described method, each time the opening dimension ϕ and the supply pressure P S  are changed. Note that, if the emission opening  28  is circular, the opening dimension is a diameter. 
     Table 1 below shows an example of the database of the opening dimension ϕ, the supply pressure P S , and the target distance d T . 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Supply Pressure P s   
               
            
           
           
               
               
               
               
            
               
                   
                 0.8 MPa 
                 . . . 
                 2.0 MPa 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Opening dimension ϕ 
                 ϕ1.0 
                 d T  = 4 mm 
                 . . . 
                 d T  = 6 mm 
               
               
                   
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                   
                 ϕ4.0 
                 d T  = 6 mm 
                 . . . 
                 d T  = 10 mm 
               
               
                   
               
            
           
         
       
     
     In the database shown in Table 1, a plurality of the target distances d T  are set in association with the opening dimension ϕ of the nozzle  24  and the supply pressure P S . Note that, the controller  62  may acquire, as a second target distance d T_2 , a distance d a_2  between the emission opening  28  and the measuring position of the measuring instrument  66  when the second peak value α max2  is measured, and may similarly create a database of the second target distance d T_2 . Further, different databases may also be created for different kinds of the assist gas (nitrogen, air, etc.). 
     The database of the target distance d T  created in this manner is used to determine a target position at which the nozzle  24  is to be disposed when the laser process is carried out onto the workpiece W in the laser processing system as described below. For example, if the opening dimension ϕ of the emission opening  28  of the nozzle  24  used during the laser process is 4 mm and the supply pressure P S  to the chamber  29  is 2.0 MPa, the data of d T =10 mm is used to determine the target position. 
     In this way, by measuring the velocity V of the jet of the assist gas, the information representing the position of the maximum point  32 ,  34  can be acquired. According to this configuration, it is possible to obtain the position of the maximum point  32 ,  34  with high accuracy by measurement. 
     In addition, the jet observation apparatus  60  includes the dummy workpiece  64 . In this regard, when the laser process is actually carried out, the assist gas is blown onto the workpiece W. In the jet observation apparatus  60 , the assist gas is blown onto the dummy workpiece  64  instead of the workpiece W, and the positions of the maximum points  32  and  34  are measured from the velocity V measured at the position of the dummy process portion  64   a  of the dummy workpiece  64 . According to this configuration, since the position of the maximum point  32 ,  34  can be measured in a state analogous to actual the laser process, it is possible to measure the position of the maximum point  32 ,  34  with higher accuracy. 
     Further, the dummy workpiece  64  has an outer shape (dimensions) the same as the workpiece W. According to this configuration, since the position of the maximum point  32 ,  34  can be measured in a state significantly analogous to actual laser processing, it is possible to measure the position of the maximum point  32 ,  34  with further higher accuracy. Note that, the dummy workpiece  64  may have an outer shape (dimensions) different from the workpiece W. In this case, the dummy workpiece  64  may have a thickness in the z-axis direction the same as that of the workpiece W, and include the portion  64   a corresponding to the process portion S. Also, the position of the maximum point  32 ,  34  can be acquired without the dummy workpiece  64 . 
     Then, a jet observation apparatus  70  will be described with reference to  FIG. 6  and  FIG. 7 . The jet observation apparatus  70  acquires the above-described information representing the position x 1 , x 2  of the maximum point  32 ,  34 . The jet observation apparatus  70  includes a controller  72 , a measuring instrument  76 , and the positioning device  18 . The controller  72  includes a processor and a storage (not illustrated), and controls the measuring instrument  76  and the positioning device  18 . 
     The positioning device  18  includes the work table  38 , the y-axis movement mechanism  40 , the x-axis movement mechanism  42 , and the z-axis movement mechanism  44 , wherein an object  74  is installed on the work table  38 . The positioning device  18  moves the laser processing head  14  in the x-axis, y-axis, and z-axis directions, thereby moving the nozzle  24  with respect to the object  74 . 
     A circular through hole  74   a  is formed in the object  74 . An opening dimension of the through hole  74   a  is set to be substantially the same as an opening dimension of a through hole that is estimated to be formed when the workpiece W is perforated by a laser beam emitted from the nozzle  24 . The object  74  may have an outer shape (dimensions) the same as the workpiece W, or have a different outer shape (dimensions) from the workpiece W. Furthermore, the object  74  may have a thickness in the z-axis direction the same as the workpiece W, and include a portion corresponding to the process portion S. 
     The measuring instrument  76  is disposed adjacent to the through hole  74   a , and measures a sound pressure SP or a frequency f of a sound generated by the jet emitted from the emission opening  28  of the nozzle  24  impinging on the object  74  when passing through the through hole  74   a . Note that, in the present disclosure, the “sound pressure” of the sound includes not only a sound pressure (unit: Pa), but also a sound pressure level (unit: dB), sound intensity (unit: W/m 2 ), etc. 
     Further, the “frequency” of the sound includes not only the frequency of the sound, but also frequency characteristic of the sound (i.e., a frequency spectrum). The frequency characteristic includes information such as a sound pressure level of at least one frequency component (e.g., 1 Hz), an average sound pressure level in a predetermined frequency band (e.g., 1 kHz to 10 kHz), etc. The measuring instrument  76  includes a microphone  76   a  configured to convert a sound into an electrical signal, and a frequency acquisition section  76   b  configured to acquire a frequency characteristic of the sound from the electrical signal. 
     Next, a method of acquiring the position x 1 , x 2  of the maximum point  32 ,  34  using the jet observation apparatus  70  will be described. First, the controller  72  operates the positioning device  18  so as to dispose the laser processing head  14  at an initial measuring position. When the laser processing head  14  is disposed at the initial measuring position, the laser processing head  14  is positioned with respect to the object  74  such that the optical axis O of the laser processing head  14  passes through the through hole  74   a , as illustrated in  FIG. 6 . Additionally, a distance d b  between the emission opening  28  and the object  74  is an initial value d b0 . 
     As an example, the initial value d b0  is set such that the object  74  is disposed at a position close to the emission opening  28 , such as the proximity region  36  in  FIG. 3 . As another example, the initial value d b0  is set such that the object  74  is disposed at a position sufficiently separate to downstream side of the jet from a position where a maximum point farthest from the emission opening  28  (in the example of  FIG. 3 , the second maximum point  34 ) is estimated to be located. 
     Then, the controller  72  sends a command to the assist gas supply device  16 , and in response to the command, the assist gas supply device  16  supplies the assist gas to the chamber  29  at the supply pressure P S . The nozzle  24  emits the jet of the assist gas having the maximum points  32  and  34 . Then, the controller  72  operates the positioning device  18  so as to move the laser processing head  14  in the z-axis direction to change the distance d b  between the object  74  and the emission opening  28 . 
     As an example, if the above-described initial value d b0  is set so as to dispose the object  74  at the position close to the emission opening  28 , the controller  72  operates the positioning device  18  so as to move the laser processing head  14  in the z-axis positive direction to increase the distance d b . 
     As another example, if the above-described initial value d b0  is set so as to dispose the object  74  at downstream side of a maximum point farthest from the emission opening  28 , the controller  72  operates the positioning device  18  so as to move the laser processing head  14  in the z-axis negative direction to decrease the distance d b . 
     While the positioning device  18  moves the laser processing head  14  in the z-axis direction to bring the nozzle  24  closer to or away from the object  74 , the controller  72  sends a command to the measuring instrument  76  so as to cause the measuring instrument  76  to consecutively measure the sound pressure SP or the frequency f. For example, the measuring instrument  76  consecutively measures the sound pressure SP or the frequency f at a predetermined period (e.g., 0.5 seconds) while the positioning device  18  moves the laser processing head  14 . The measuring instrument  66  sequentially outputs the measured sound pressure SP or the frequency f to the controller  72  as output data β (=SP or f). 
     The sound pressure SP and the frequency f of the sound generated by the jet impinging on the object  74  during passing through the through hole  74   a  are highly correlated with a flow velocity V S  of the assist gas. Specifically, the sound pressure (peak value, effective value, etc.) of the sound generated by the jet impinging on the object  74  and frequency characteristic of the sound (e.g., a sound pressure level of at least one frequency component) are highly correlated with the flow velocity V S  of the assist gas. 
     Therefore, a relationship between the acquired output data β and the distance d b  corresponds to the graph illustrated in  FIG. 3 . That is, the output data β from the measuring instrument  76  changes in response to the distance d b , and has a first peak value β max1  at a position corresponding to the first maximum point  32  and a second peak value β max2  at a position corresponding to the second maximum point  34 . 
     The controller  72  acquires the first peak value β max1  of the consecutive output data β outputted by the measuring instrument  76  as information representing the position x 1  of the first maximum point  32 , and acquires the second peak value β max2  as information representing the position x 2  of the second maximum point  34 . In this way, the controller  72  functions as a position acquisition section  78  configured to acquire the information representing the position of the maximum point  32 ,  34  based on the output data β. 
     Then, the controller  72  acquires, as the target distance d T , the distance d b  between the object  74  and the emission opening  28  when the first peak value β max1  is measured. This target distance d T  represents the position of the first maximum point  32  with respect to the emission opening  28 , and can be acquired using e.g. a known gap-sensor or the like. 
     Then, the controller  72  registers the target distance d T  and the first peak value β max1  in a database in association with the opening dimension ϕ of the emission opening  28  and the supply pressure P S  when the sound pressure SP and the frequency f are measured. Table 2 below shows an example of the database of the opening dimension ϕ, the supply pressure P S , the first peak value β max1 , and the target distance d T . 
     
       
         
           
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Supply Pressure P s   
               
            
           
           
               
               
               
               
            
               
                   
                 0.8 MPa 
                   
                 2.0 MPa 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Peak 
                   
                 . . . 
                 Peak 
                   
               
               
                   
                 Value 
                 Distance 
                 . . . 
                 Value 
                 Distance 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Opening 
                 ϕ1.0 
                 114 dB 
                 d = 4 mm 
                 . . . 
                 120 dB 
                 d = 6 mm 
               
               
                 dimension ϕ 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
                 . . . 
               
               
                   
                 ϕ4.0 
                 110 dB 
                 d = 6 mm 
                 . . . 
                 117 dB 
                 d = 10 mm 
               
               
                   
               
            
           
         
       
     
     In the database shown in Table 2, the first peak value β max1  (sound pressure level) and the target distance d T  are registered in association with the opening dimension ϕ of the nozzle  24  and the supply pressure P S . Note that, the controller  72  may acquire, as a second target distance d T_2 , a distance d b_2  between the emission opening  28  and the object  74  when the second peak value β max2  is measured, and may similarly create a database of the second target distance d T_2 . 
     Further, a plurality of databases may also be created for respective types of assist gas (nitrogen, air, etc.). The database of the target distance d T  created in this manner is used to determine a target position at which the nozzle  24  is to be disposed when the laser process is carried out onto the workpiece W in a laser processing system, as described below. 
     As described above, according to the jet observation apparatus  70 , the information representing the position x 1 , x 2  of the maximum point  32 ,  34  can be acquired, based on the sound generated when the jet of the assist gas impinges on the object  74 . According to this configuration, it is possible to obtain the position x 1 , x 2  of the maximum point  32 ,  34  with high accuracy, by measurement. Further, the jet observation apparatus  70  can acquire the information representing the position x 1 , x 2  of the maximum point  32 ,  34  during the laser process, as described below. 
     Then, a jet observation apparatus  80  will be described with reference to  FIG. 8  and  FIG. 9 . The jet observation apparatus  80  acquires the above-described position x 1  of the first maximum point  32 , by a predetermined calculation. The jet observation apparatus  80  includes a controller  82  and a measuring instrument  84 . The controller  82  includes a processor and a storage (not illustrated), and controls the measuring instrument  84 . 
     The measuring instrument  84  measures a supply flow rate V V  of the assist gas supplied from the assist gas supply device  16  to the chamber  29 . The measuring instrument  84  is installed in the gas supply tube  30 , and measures the flow rate V V  of the assist gas flowing through the gas supply tube  30  from the assist gas supply device  16  toward the chamber  29 . 
     Next, a method of acquiring the position x 1  of the first maximum point  32  by the jet observation apparatus  80  will be described. First, the controller  82  sends a command to the assist gas supply device  16 , and in response to the command, the assist gas supply device  16  supplies the assist gas to the chamber  29 . The nozzle  24  emits the jet of the assist gas having the maximum points  32  and  34 . 
     Then, the controller  82  sends a command to the measuring instrument  84 , and in response to the command, the measuring instrument  84  measures the supply flow rate V V  from the assist gas supply device  16  to the chamber  29 . The measuring instrument  84  outputs output data (measurement value) of the supply flow rate V V  to the controller  82 . Then, the controller  82  calculates a distance d c  from the emission opening  28  to the first maximum point  32 , as information of the position x 1  of the first maximum point  32 , using the output data V V  from the measuring instrument  84  and Equation 1 indicated below.
 
 d   c −0.67×ϕ×(ρ V   S   2 /2) 1/2   (Equation 1)
 
     In the Equation 1, ϕ indicates the opening dimension of the emission opening  28 , ρ indicates a viscosity coefficient of the assist gas, and V S  indicates the flow velocity V S  of the assist gas obtained from the output data V V  and the opening dimension ϕ. In this way, the controller  82  functions as a position acquisition section  86  configured to acquire the position x 1  of the first maximum point  32  by calculation, from the output data V V  of the measuring instrument  84 . 
     According to the jet observation apparatus  80 , it is possible to quickly obtain the position x 1  of the first maximum point  32  with high accuracy, by calculation. Also, the jet observation apparatus  80  can acquire the position x 1  of the first maximum point  32  in real-time during the laser process, as described below. 
     Next, a laser processing system  100  will be described with reference to  FIG. 10  and  FIG. 11 . The laser processing system  100  includes the laser oscillator  12 , the laser processing head  14 , the assist gas supply device  16 , the positioning device  18 , and a controller  102 . The controller  102  includes a processor (not illustrated) and a storage  104 , and controls the laser oscillator  12 , the laser processing head  14 , the assist gas supply device  16 , and the positioning device  18 . The storage  104  stores a database  106 . The database  106  is one as shown in e.g. Table 1 or Table 2 described above. 
     Next, operation of the laser processing system  100  will be described. First, the controller  102  acquires setting values of the opening dimension ϕ of the emission opening  28  of the nozzle  24  to be used and the supply pressure P S  of the assist gas from the assist gas supply device  16  to the chamber  29 , and from the setting values of the opening dimension ϕ and the supply pressure P S , reads out from the storage  104  and acquires the corresponding target distance d T  in the database  106 . 
     Then, the controller  102  operates the positioning device  18  so as to move the laser processing head  14  with respect to the workpiece W to disposes the nozzle  24  at a target position where a distance d between the emission opening  28  and the process portion S coincides with the target distance d T . In this way, the target position is determined using the database  106 , and the controller  102  functions as a movement controller  108  configured to control the positioning device  18  (i.e., the movement mechanisms  40 ,  42  and  44 ) so as to dispose the nozzle  24  at the target position. 
     The controller  102  then operates the assist gas supply device  16  so as to supply the assist gas to the chamber  29  at the supply pressure P S , and the nozzle  24  emits the jet of the assist gas having the maximum points  32  and  34  of the velocity V. Then, the controller  102  operates the laser oscillator  12  so as to emit the laser beam from the emission opening  28 , and operates the lens driver  23  so as to adjust the position of each optical lens  22  in the direction of the optical axis O such that the focal point of the emitted laser beam is positioned at the process portion S. 
     As a result, a through hole is formed at the process portion S of the workpiece W by the laser beam, and the controller  102  operates the positioning device  18  in accordance with a processing program stored in the storage  104  so as to perform the laser process (specifically, laser cutting) on the workpiece W while moving the nozzle  24  with respect to the workpiece W. At this time, the process portion S of the workpiece W is disposed in the first Mach disk region  33  (specifically, the position of the first maximum point  32 ) of the jet of the assist gas. 
     According to the laser processing system  100 , it is possible to effectively make use of the assist gas so as to effectively blow out the material of the workpiece W melted by the laser beam. In addition, since the above-described generation of the plasma can be prevented, it is possible to improve finishing quality of the process portion S of the workpiece W, and prevent contamination of the component of the laser processing head  14 . 
     Further, in the laser processing system  100 , the target position where the nozzle  24  is to be disposed during the process on the workpiece W is determined using the database  106  of the position of the first maximum point  32 . According to this configuration, it is possible to quickly and easily position the nozzle  24  and the workpiece W at the target position to start the laser process. 
     Note that, in the laser processing system  100 , the storage  104  may be provided as a separate element from the controller  102 . In this case, the storage  104  may be built in an external device (a server, etc.) communicatively connected to the controller  102 , or may be a storage medium (hard disk, flash memory, etc.) that can be externally attached to the controller  102 . Furthermore, the controller  102  may fix the distance between the emission opening  28  and the workpiece W when laser-processing the workpiece W. 
     Next, a laser processing system  110  will be described with reference to  FIG. 12  and  FIG. 13 . The laser processing system  110  includes the laser oscillator  12 , the laser processing head  14 , the assist gas supply device  16 , the positioning device  18 , the measuring instrument  76 , and a controller  112 . 
     The controller  112  includes a processor and the storage  104 , and controls the laser oscillator  12 , the laser processing head  14 , the assist gas supply device  16 , the positioning device  18 , and the measuring instrument  76 . The database  106  as shown in above Table 2 is stored in the storage  104 . The controller  112  functions as the above-described position acquisition section  78 . Accordingly, in the laser processing system  110 , the positioning device  18 , the measuring instrument  76 , and the controller  112  constitute the jet observation apparatus  70  described above. 
     Then, operation of the laser processing system  110  will be described with reference to  FIG. 14 . A flow illustrated in  FIG. 14  is started when the controller  112  receives a processing start command from an operator, a host controller, or a processing program. 
     In step S 1 , the controller  112  disposes the nozzle  24  at an initial target position with respect to the process portion S. Specifically, the controller  112  acquires the setting values of the opening dimension ϕ of the emission opening  28  of the nozzle  24  to be used and the supply pressure P S  of the assist gas to the chamber  29 , and from the setting values of the opening dimension ϕ and the supply pressure P S , reads out and acquires the corresponding target distance d T  in the database  106 . Then, the controller  112  functions as the movement controller  108  and operates the positioning device  18  so as to move the laser processing head  14  with respect to the workpiece W to dispose the nozzle  24  at the initial target position where a distance d between the emission opening  28  and the process portion S coincides with the target distance d T . 
     In step S 2 , the controller  112  supplies the assist gas from the assist gas supply device  16  to the chamber  29  at the supply pressure P S , so as to emit the jet of the assist gas from the emission opening  28 . Then, the controller  112  operates the laser oscillator  12  so as to emit the laser beam from the emission opening  28 , and operates the lens driver  23  so as to adjust the position of each optical lens  22  in the direction of the optical axis O such that the focal point of the emitted laser beam is positioned at the process portion S. As a result, a through hole H ( FIG. 12 ) is formed in the workpiece W, and the jet passes through the through hole H. This through hole H corresponds to the above-described through hole  74   a.    
     In step S 3 , the controller  112  starts measurement by the measuring instrument  76 . Specifically, the controller  112  sends a command to the measuring instrument  76 , and in response to the command, the measuring instrument  76  consecutively (e.g., at a predetermined period) measures the sound pressure SP or the frequency f of a sound generated by the jet emitted from the emission opening  28  of the nozzle  24  impinging on the workpiece W when passing through the through hole H. 
     The controller  112  functions as the position acquisition section  78  to sequentially acquires the output data β of the sound pressure SP or the frequency f from the measuring instrument  76 , as the information representing the position x 1  of the first maximum point  32 , and stores the output data β in the storage  104 . As described in connection with the above jet observation apparatus  70 , the output data β including the first peak value β max1  corresponds to the information representing the position x 1  of the first maximum point  32 . 
     In step S 4 , the controller  112  starts the laser process. Specifically, the controller  112  operates the positioning device  18  in accordance with a processing program so as to move the nozzle  24  with respect to the workpiece W, along with which, the controller  112  performs the laser process (laser cutting) on the workpiece W by the laser beam emitted from the emission opening  28 . 
     In step S 5 , the controller  112  determines whether or not the output data β most-recently acquired by the measuring instrument  66  is smaller than a predetermined lower limit value β min . This lower limit value β min  defines a boundary for determining whether or not the velocity V of the jet emitted from the nozzle  24  is abnormally small, and is predetermined by an operator. 
     In this respect, if clogging of the emission opening  28  or abnormality in operation (e.g., out of gas) of the assist gas supply device  16  occurs, the velocity V of the jet may be significantly reduced below a reference value. In this case, the output data β acquired by the measuring instrument  66  differs from (specifically, is smaller than) reference data measured by the measuring instrument  66  when the jet is normally emitted from the nozzle  24 . 
     The controller  112  determines whether or not the output data β is smaller than the lower limit value β min , whereby determining whether or not the output data β is different from the reference data. As described above, the controller  112  functions as an abnormality determination section  113  configured to determine whether or not the output data β is different from the reference data. 
     When the controller  112  determines that the output data β is smaller than the lower limit value β min  (i.e., determines YES), it proceeds to step S 6 . On the other hand, when the controller  112  determines that the output data β is equal to or greater than the lower limit value β min  (i.e., determines NO), it proceeds to step S 8 . In step S 6 , the controller  112  sends a command to the laser oscillator  12  so as to stop a laser oscillation operation, whereby stopping the laser process on the workpiece W. 
     In step S 7 , the controller  112  outputs an alert. For example, the controller  112  generates an alert signal in the form of sound or image, which indicates “There is abnormality in emission of assist gas. Check opening dimension of nozzle or supply pressure of assist gas”. Then, the controller  112  outputs the alert via a speaker or a display (not illustrated). Thus, the controller  112  functions as an alert generation section  118  configured to generate the alert. 
     The speaker or the display may be provided at the controller  112 , or may be provided outside the controller  112 . The operator can intuitively recognize from the alert that there is abnormality in the nozzle  24  or the assist gas supply, and can replace the nozzle  24  or take measures for the operating abnormality (e.g., out of gas) of the assist gas supply device  16 . After carrying out this step S 7 , the controller  112  ends the flow illustrated in  FIG. 14 . 
     In step S 8 , the controller  112  determines whether or not the output data β acquired most-recently by the measuring instrument  76  is smaller than a predetermined threshold value β th . This threshold value β th  is greater than the above-described lower limit value β min . As an example, the threshold value β th  may be set as a value obtained by multiplying the first peak value β max1  stored in the database  106  by a predetermined coefficient a (0&lt;a&lt;1). 
     For example, if the database  106  shown in above Table 2 is used, the opening dimension ϕ is set as ϕ=1.0 mm, the supply pressure P S  is set as P S =2.0 MPa, and the coefficient a is set as a=0.95, the threshold value β th =120[dB]×0.95=114[dB] is obtained. A relationship between the output data β and position x of the process portion S of the workpiece W with respect to the emission opening  28  is schematically shown in  FIG. 16 . The relationship between the output data β and the position x corresponds to the graph illustrated in  FIG. 3 . 
     A range  116  from the threshold value β th  to the first peak value β max1  corresponds to a position range  114  between a position x 3  and a position x 4 . The position x 1  of the first maximum point  32  is within the position range  114 . In this respect, in above-described step S 1 , the nozzle  24  is disposed at the initial target position where the distance d between the emission opening  28  and the process portion S coincides with the target distance d T . Accordingly, just after step S 1 , the process portion S appears to be disposed at or near the position x 1  of the first maximum point  32 . 
     However, while the laser process on the workpiece W is carried out, the distance d between the emission opening  28  and the process portion S may change due to some factor. As such factor, there is a case where a stepped portion is formed at the process portion of the workpiece W, whereby the distance d changes, for example. If the distance d changes in this way, the output data β of the measuring instrument  76  may be below the threshold value β th . 
     When the controller  112  determines that the output data β is smaller than the threshold value β th  (i.e., determines YES) in this step S 8 , it proceeds to step S 9 . On the other hand, when the controller  112  determines that the output data β is equal to or greater than the threshold value β th  (i.e., determines NO), it proceeds to step S 15 . 
     In step S 9 , the controller  112  changes a target position of the nozzle  24 . Specifically, the controller  112  changes the target position of the nozzle  24  set at the start of this step S 9  to a new target position moved in the z-axis negative direction or the z-axis positive direction. Then, the controller  112  functions as the movement controller  108 , and operates the positioning device  18  so as to move the nozzle  24  in the z-axis negative direction or the z-axis positive direction in order to dispose the nozzle  24  at the new target position. As a result, the nozzle  24  moves closer to or away from the workpiece W. 
     In step S 10 , the controller  112  determines whether or not the output data β acquired by the measuring instrument  66  after step S 9  increases from the output data β acquired by the measuring instrument  66  immediately before step S 9 . In this respect, if the output data β decreases as a result of the movement of the nozzle  24  in step S 9 , the position of the workpiece W (specifically, the process portion S) is separated away from the position range  114  in the graph shown in  FIG. 16 . In this case, in order to bring the position of the workpiece W within the position range  114 , it is necessary to reverse the direction in which the nozzle  24  is to be moved in step S 9 . 
     On the other hand, if the output data β increases as a result of the movement of the nozzle  24  in step S 9 , the position of the workpiece W approaches the position x 1  in the graph shown in  FIG. 16 . In this case, it is not necessary to change the direction in which the nozzle  24  is to be moved in step S 9 . 
     In this step S 10 , when the controller  112  determines that the output data β acquired by the measuring instrument  66  just after step S 9  increases from the output data β acquired by the measuring instrument  66  immediately before step S 9  (i.e., determines YES), it proceeds to step S 12 . On the other hand, when the controller  112  determines that the output data β acquired by the measuring instrument  66  just after step S 9  decreases from the output data β acquired by the measuring instrument  66  immediately before step S 9  (i.e., determines NO), it proceeds to step S 11 . 
     In step S 11 , the controller  112  reverses the direction in which the nozzle  24  is to be moved. For example, if the nozzle  24  has been moved in the z-axis negative direction in most-recently executed step S 9 , the controller  112  reverses the direction in which the nozzle  24  is to be moved in next step S 9  to the z-axis positive direction. Then, the controller  112  returns to step S 9 . 
     In step S 12 , the controller  112  determines whether or not the output data β most-recently acquired by the measuring instrument  76  is equal to or greater than the threshold value α th . When the controller  112  determines that the output data β is equal to or greater than the threshold value β th  (i.e., determines YES), it proceeds to step S 15 . On the other hand, when the controller  112  determines that the output data β is still smaller than the threshold value β th  (i.e., determines NO), it proceeds to step S 13 . 
     In step S 13 , the controller  112  determines whether or not the number of times n, for which the controller  112  determines NO in step S 12 , exceeds a predetermined maximum number of times n max . This maximum number of times n max  is predetermined by the operator as an integer of 2 or greater (e.g., n max =10). When the controller  112  determines that the number of times n exceeds the maximum number of times n max  (i.e., determines YES), it proceeds to step S 14 . On the other hand, when the controller  112  determines that the number of times n does not exceed the maximum number of times n max  (i.e., determines NO), it returns to step S 9 . 
     In this way, by carrying out a loop of steps S 9  to S 13  in  FIG. 14 , the controller  112  changes the target position of the nozzle  24  such that the output data β of the measuring instrument  76  is within the range  116  of output data which represents the position range  114 , during the process on the workpiece W, and performs feedback control for the positioning device  18  in accordance with the changed target position so as to move the nozzle  24 . 
     That is, the target position of the nozzle  24  is determined as a predetermined range, based on the first peak value β max1  of the output data β that represents the position x 1  of the first maximum point  32 . By this feedback control, the process portion S can be continuously disposed in the first Mach disk region  33  during the process on the workpiece W. Thus, the first Mach disk region  33  in the laser processing system  110  can be defined as a region of the position range  114  defined by the threshold value P th . 
     On the other hand, when the controller  112  determines YES in step S 13 , in step S 14 , the controller  112  carries out an abnormality handling process. If the output data β does not satisfy β≥β th  even though the feedback control in steps S 9  to S 13  is repeatedly carried out for the number of times n max , the above-described abnormality such as the clogging or the out of gas may possibly occur. 
     In this respect, the characteristic shown in  FIG. 16  is reference data measured by the measuring instrument  76  when the jet is normally emitted from the emission opening  28  without occurrence of the abnormality, wherein the first peak value β max1  constitutes the reference data, and the threshold value β th  is set for the reference data. Therefore, the range  116  in  FIG. 16  is a range of output data which represents the position range  114  and which is determined based on the reference data. 
     If the controller  112  determines YES in step S 13  by functioning as the abnormality determination section  113 , the controller  112  determines that the output data β of the measuring instrument  76  is different from the reference data, and executes the abnormality handling process in step S 14 . This step S 14  will be described with reference to  FIG. 15 . Note that, in the flow illustrated in  FIG. 15 , processes similar to those of the flow illustrated in  FIG. 14  are assigned the same step numbers, and redundant descriptions thereof will be omitted. 
     In step S 21 , the controller  112  sends a command to the assist gas supply device  16  so as to change the supply pressure P S  of the assist gas to the chamber  29 . The controller  112  increases the supply pressure P S  by a predetermined pressure (e.g., 0.2 MPa) in a stepwise manner, each time the controller  112  carries out this step S 21 . In this way, the controller  112  functions as a pressure adjustment section  115  configured to change the supply pressure P S . 
     Then, the controller  112  carries out the above-described step S 12  to determine whether the output data β most-recently acquired by the measuring instrument  76  is equal to or greater than the threshold value β th . The controller  112  proceeds to step S 15  in  FIG. 14  when determining YES, while it proceeds to step S 22  when determining NO. 
     In step S 22 , the controller  112  determines whether or not the number of times m, for which the controller  112  determines NO in step S 12  in  FIG. 15 , exceeds a predetermined maximum number of times m max . This maximum number of times m max  is predetermined by the operator as an integer of 2 or greater (e.g., m max =5). When the controller  112  determines that the number of times m exceeds the maximum number of times m max  (i.e., determines YES), it proceeds to step S 6  in  FIG. 14 . On the other hand, when the controller  112  determines that the number of times m does not exceed the maximum number of times m max  (i.e., determines NO), it returns to step S 21 . 
     Referring again to  FIG. 14 , in step S 15 , the controller  112  determines whether or not the laser process on the workpiece W has been completed, based on e.g. the processing program. When the controller  112  determines that the laser process has been completed (i.e., determines YES), the controller  112  sends a command to the laser oscillator  12  so as to stop the laser oscillation operation, and ends the flow illustrated in  FIG. 14 . On the other hand, when the controller  112  determines that the laser process has not been completed (i.e., determines NO), it returns to step S 8 . 
     As described above, the controller  112  performs the feedback control of the position of the nozzle  24  such that the process portion S is continuously disposed in the first Mach disk region  33 , based on the output data β acquired by the measuring instrument  76  during the process on the workpiece W. According to this configuration, even when the distance d between the emission opening  28  and the process portion S changes due to some factor, it is possible to perform the laser process on the workpiece W in a state where the process portion S is disposed in the first Mach disk region  33 . Thus, the assist gas can be effectively utilized. 
     Furthermore, the controller  112  determines abnormality of the emitted jet by performing step S 14 . If abnormality such as clogging or out of gas occurs, the velocity V of the jet emitted from the emission opening  28  hardly changes even when the supply pressure P s  from the assist gas supply device  16  to the chamber  29  is changed. 
     In this case, the output data β of the measuring instrument  76  also hardly changes, and accordingly, the output data β does not satisfy β≥β th  (i.e., it is not determined YES in step S 12  in  FIG. 15 ) even when a loop of steps S 21  to S 22  in  FIG. 15  is repeatedly carried out. 
     If the output data β does not satisfy β≥βth even after the controller  112  repeatedly performs the loop of steps S 21  to S 22  for the predetermined number of times m max  by functioning as the abnormality determination section  113 , the controller  112  determines that the output data β is different from the reference data, and outputs the alert message in step S 7  in  FIG. 14 . According to this configuration, the operator can intuitively recognize that there is abnormality in the nozzle  24  or the assist gas supply, and replace the nozzle  24  or take measures for the abnormality (out of gas) in operation of the assist gas supply device  16 . 
     On the other hand, a case may occur in which the velocity V of the jet emitted from the emission opening  28  decreases slightly below the reference value due to minor abnormality, such as an error of the opening dimension or a length in the z-axis direction of the emission opening  28 , inclination of the emission opening  28  with respect to the z-axis, or a design dimension error of an interior space of the nozzle  24  (chamber  29 ). In the case of such minor abnormality, the velocity of the jet varies in response to change in the supply pressure P s , but the output data β may not satisfy β≥β th , and it may be determined YES in step S 13  even when the feedback control of steps S 9  to S 13  is repeatedly carried out. 
     According to the laser processing system  110 , in step S 14 , the controller  112  continues the process on the workpiece W if the output data β satisfies β≥β th  by performing step S 21 . According to this configuration, even when the velocity V of the jet decreases below the reference value due to the minor abnormality such as a dimensional error, it is possible to continue the process on the workpiece W in a state in which the jet is blown onto the process portion S at the sufficiently large velocity V, by changing (specifically increasing) the supply pressure P s . 
     Note that, each time the controller  112  determines YES in step S 12  in a first laser process, the controller  112  may sequentially store the distance d between the emission opening  28  and the process portion S at this point of time. Then, the controller  112  may set the initial target position in step S 1  of a second laser process to be carried out next to the first laser process, based on the distance d stored in the first laser process. For example, the controller  112  may set, as the initial target position of the second laser process, an average value of the distance d or the last stored distance d, which has been stored in the first laser process. 
     Note that, as modification of the laser processing system  110 , the above-described measuring instrument  66  may be applied, instead of the measuring instrument  76 . In this case, the measuring instrument  66  is configured to measure the velocity V of the jet in a non-contact manner, at the position of the process portion S (or a position slightly displaced from the process portion S toward the emission opening  28 ). The measuring instrument  66  thus applied to the laser processing system  110 , the positioning device  18 , and the controller  112  constitute the jet observation apparatus  60  described above. 
     In this modification, the controller  112  can carry out the flows illustrated in  FIG. 14  and  FIG. 15  based on the output data α of the measuring instrument  66  instead of the output data β, and perform the laser process on the workpiece W in a state where the workpiece W is disposed in the first Mach disk region  33 . 
     Next, a laser processing system  120  will be described with reference to  FIG. 17  and  FIG. 18 . The laser processing system  120  includes the laser oscillator  12 , the laser processing head  14 , the assist gas supply device  16 , the positioning device  18 , the dummy workpiece  64 , the measuring instrument  66 , and a controller  122 . 
     The controller  122  includes a processor and a storage (not illustrated), and controls the laser oscillator  12 , the laser processing head  14 , the assist gas supply device  16 , the positioning device  18 , and the measuring instrument  66 . The controller  122  functions as the position acquisition section  68  described above. Thus, the positioning device  18 , the dummy workpiece  64 , the measuring instrument  66 , and the controller  122  constitute the above-described jet observation apparatus  60 . 
     Next, operation of the laser processing system  120  will be described. First, the controller  122  acquires the information representing the position x 1  of the first maximum point  32 . Specifically, the controller  122  functions as the position acquisition section  68  to acquire the target distance d T  from the first peak value α max1  of the output data α of the measuring instrument  66 , using the method described in connection with the above jet observation apparatus  60 . 
     Then, the controller  122  disposes the nozzle  24  at the target position. Specifically, the controller  122  functions as the movement controller  108  and operates the positioning device  18  so as to move the laser processing head  14  with respect to the workpiece W to dispose the nozzle  24  at the target position where the distance d between the emission opening  28  and the process portion S coincides with the target distance d T . 
     Then, the controller  122  operates the assist gas supply device  16  so as to supply the assist gas to the chamber  29  at the supply pressure P S  to emit the jet of the assist gas from the emission opening  28 . Further, the controller  122  operates the laser oscillator  12  so as to emit the laser beam from the emission opening  28 , and operates the lens driver  23  so as to adjust the position of the optical lens  22  in the direction of the optical axis O such that the focal point of the emitted laser beam is positioned at the process portion S. 
     In this state, the controller  122  carries out the laser process (laser cutting) on the workpiece W while operating the positioning device  18  in accordance with the processing program so as to move the nozzle  24  with respect to the workpiece W. At this time, the process portion S of the workpiece W is disposed in the first Mach disk region  33  of the jet of the assist gas. 
     As described above, the controller  122  acquires the position x 1  of the first maximum point  32  by the jet observation apparatus  60  before processing the workpiece W, and carries out the laser process on the workpiece W along with disposing the nozzle  24  at the target position determined based on the acquired position x 1  of the first maximum point  32 . According to this configuration, since it is possible to dispose the process portion S in the first Mach disk region  33  during the process on the workpiece W, the assist gas can be effectively utilized. 
     In addition, according to the laser processing system  120 , even when the opening dimension ϕ of the emission opening  28  and the supply pressure P S  of the assist gas are unknown, it is possible to acquire the position x 1  of the first maximum point  32  by the jet observation apparatus  60  before processing the workpiece W, and determine the target position of the nozzle  24  based on the position x 1  of the first maximum point  32 . 
     Next, a laser processing system  130  will be described with reference to  FIG. 19  and  FIG. 20 . The laser processing system  130  includes the laser oscillator  12 , the laser processing head  14 , the assist gas supply device  16 , the positioning device  18 , the measuring instrument  84 , and a controller  132 . 
     The controller  132  includes a processor and the storage  104 , and controls the laser oscillator  12 , the laser processing head  14 , the assist gas supply device  16 , the positioning device  18 , and the measuring instrument  84 . The database  106  as shown in above Table 1 is stored in the storage  104 . The controller  132  functions as the above-described position acquisition section  86 . Thus, the measuring instrument  84  and the controller  132  constitute the jet observation apparatus  80  described above. 
     Next, operation of the laser processing system  130  will be described with reference to  FIG. 21 . A flow illustrated in  FIG. 21  is started when the controller  132  receives a processing start command from an operator, a host controller, or a processing program. Note that, in the flow illustrated in  FIG. 21 , processes similar to those of the flow illustrated in  FIG. 14  are assigned the same step numbers, and redundant descriptions thereof will be omitted. 
     After performing steps S 1  and S 2 , in step S 31 , the controller  132  starts measurement of the supply flow rate V V  of the assist gas supplied from the assist gas supply device  16  to the chamber  29 . Specifically, the controller  132  sends a command to the measuring instrument  84  so as to cause the measuring instrument  84  to measure the supply flow rate V V , consecutively (e.g., at a predetermined period). In addition, the controller  132  starts measurement of the distance d between the emission opening  28  and the process portion S. As described above, the distance d can be acquired using a known gap sensor or the like. 
     After step S 4 , in step S 32 , the controller  132  acquires the position x 1  of the first maximum point  32 . Specifically, the controller  132  functions as the position acquisition section  86  to calculate the distance d c  from the emission opening  28  to the first maximum point  32 , as the information of the position x 1  of the first maximum point  32 , using the output data V V  most-recently acquired by the measuring instrument  84  and above Equation 1. 
     In step S 33 , the controller  132  determines whether or not the difference δ between the distance d and the distance d c  is greater than a predetermined threshold value δ th . Specifically, the controller  132  calculates the difference δ between the most-recently measured distance d between the emission opening  28  and the process portion S and the distance d c  acquired in the most-recent step S 32  (i.e., δ=d-d c ). 
     When the controller  132  determines that an absolute value of the difference δ (i.e., |d-d c |) is greater than the threshold value δ th  (i.e., determines YES), it proceeds to step S 34 . On the other hand, when the controller  132  determines that the absolute value of the difference δ is equal to or smaller than the threshold value δ th  (i.e., determines NO), it proceeds to step S 15 . The threshold value δ th  is predetermined by the operator. 
     In step S 34 , the controller  132  changes the target position of the nozzle  24 . Specifically, if the difference δ calculated in most-recent step S 33  is a positive value, the controller  132  changes the target position of the nozzle  24  set at the start of this step S 34  to a new target position moved from the original target position in the z-axis negative direction. 
     Then, the controller  132  functions as the movement controller  108  to operate the positioning device  18  so as to move the nozzle  24  in the z-axis negative direction in order to dispose the nozzle  24  at the new target position. As a result, the nozzle  24  approaches the workpiece W, and whereby the distance d between the emission opening  28  and the process portion S decreases. 
     On the other hand, if the difference δ calculated in most-recent step S 33  is a negative value, the controller  132  changes the target position of the nozzle  24  set at the start of this step S 34  to a new target position moved from the original target position in the z-axis positive direction. Then, the controller  132  operates the positioning device  18  so as to move the nozzle  24  in the z-axis positive direction in order to dispose the nozzle  24  at the new target position. As a result, the nozzle  24  moves away from the workpiece W, and whereby the distance d between the emission opening  28  and the process portion S increases. After performing step S 34 , the controller  132  returns to step S 32 . 
     On the other hand, when determining NO in step S 33 , the controller  132  performs above-described step S 15 , in which, when determining YES, the controller  132  sends the command to the laser oscillator  12  so as to stop the laser oscillation operation to end the flow illustrated in  FIG. 21 , while it returns to step S 32  when determining NO. 
     Thus, in the laser processing system  130 , the controller  132  changes the target position of the nozzle  24  based on the position x 1  of the first maximum point  32  acquired by the jet observation apparatus  80  during the laser process, and performs feedback control for the positioning device  18  according to the changed target position to move the nozzle  24 . 
     That is, the target position of the nozzle  24  is determined as a predetermined range (the range in which 0≤δ≤δ th  is satisfied) based on the position x 1  of the first maximum point  32 . By this feedback control, the process portion S can be continuously disposed in the first Mach disk region  33  during the process on the workpiece W. That is, the first Mach disk region  33  in the laser processing system  130  can be defined as a region of the range in which the above-mentioned difference δ satisfies 
     According to the laser processing system  130 , it is possible to carry out the laser process on the workpiece W in a state where the process portion S is disposed in the first Mach disk region  33 , even when the distance d between the emission opening  28  and the process portion S changes due to some factor. Accordingly, the assist gas can be effectively utilized. 
     Next, a jet adjustment device  140  will be described with reference to  FIG. 22 . The jet adjustment device  140  is configured to adjust the position x 1 , x 2  of the maximum point  32 ,  34  of the jet emitted from the emission opening  28  of the nozzle  24 , and includes an enclosure  142  and an enclosure driver  144 . The enclosure  142  is a tubular member having a radial inner dimension. 
     The enclosure  142  is comprised of a flexible cylindrical member having a radius R as the radial inner dimension. The cylindrical member is made of e.g. a bristle material, a resin material, or a rubber material. The enclosure  142  is disposed substantially concentric with the emission opening  28  with respect to the optical axis O, and includes an end  142   a  in the z-axis negative direction and an end  142   b  opposite the end  142   a.    
     The end  142   a  is placed on the installation surface of the work table  38 . The end  142   b  is disposed at a position separate away from the emission opening  28  in the z-axis positive direction. In other words, the enclosure  142  has a length in the z-axis direction sufficient to dispose the end  142   b  to separate away from the emission opening  28  in the z-axis positive direction during the process on the workpiece. 
     The enclosure driver  144  includes a mechanism section  146  configured to deform the enclosure  142  so as to change the radius R of the enclosure  142 , and a power section  148  configured to generate power for driving the mechanism section  146 . There are various embodiments as the enclosure  142  and the mechanism section  146  that can change the radius R. Below, examples of the enclosure  142  and the mechanism section  146  will be described with reference to  FIG. 23  and  FIG. 24 . Note that, in  FIG. 23  and  FIG. 24 , the enclosure  142  is illustrated by a dotted line, for the sake of easy understanding. 
     A mechanism section  146 A illustrated in  FIG. 23  is a so-called iris diaphragm mechanism used in e.g. a camera. Specifically, the mechanism section  146 A includes a plurality of blades  150  which are driven to move radially inward while rotating in a circumferential direction. The enclosure  142  is coupled to an inner edge of the plurality of blades  150 , and is deformed to decrease or increase its radius R along with the operation of the blades  150 . The power section  148  include e.g. a servo motor, and drives the mechanism section  146 A so as to decrease and increase the radius R of the enclosure  142 . 
     On the other hand, a mechanism section  146 B illustrated in  FIG. 24  includes an arm  152  extending in a circumferential direction around the optical axis O, and a gear  156  provided in an overlapping region  154  of the arm  152 . Teeth are formed on respective circumferential surfaces of the arm  152  opposing to each other in the overlap region  154 , wherein the gear  156  engages the teeth. The enclosure  142  is coupled to an inner circumference of the arm  152  other than the overlap region  154 . 
     In a state illustrated in Section (a) in  FIG. 24 , the length in the circumferential direction of the overlapping region  154  of the arm  152  is large, and a slack  158  is formed in the enclosure  142 . As the gear  156  is rotated in one direction from the state illustrated in Section (a) in  FIG. 24 , the overlapping region  154  of the arm  152  is reduced, along with which, the slack  158  of the enclosure  142  is gradually diminished in the circumferential direction, whereby the radius R of the enclosure  142  is increased as in a state illustrated in Section (b) in  FIG. 24 . 
     Conversely, as the gear  156  is rotated in the other direction from the state illustrated in Section (b) in  FIG. 24 , the overlapping region  154  of the arm  152  is enlarged, along with which, the slack  158  of the enclosure  142  is gradually formed greater, and whereby the radius R of the enclosure  142  is decreased as in the state illustrated in Section (a) in  FIG. 24 . The power section  148  includes e.g. a servo motor, and rotates the gear  156  so as to decrease and increase the radius R of the enclosure  142 . 
     Referring again to  FIG. 22 , the jet adjustment device  140  can adjust the position x 1  of the first maximum point  32  and the position x 2  of the second maximum point  34 , by changing the radius R of the enclosure  142 . A principle for making it possible to adjust the position x 1 , x 2  of the maximum point  32 ,  34  in this manner will be described below. 
     As described above, the assist gas emitted from the emission opening  28  is reflected at a boundary with outer atmosphere, and whereby the Mach disk is formed in the jet. When the enclosure  142  is installed, an atmospheric layer present between the jet and the enclosure  142  is pressed by the jet, whereby increasing particle density in the atmospheric layer. 
     If the assist gas is reflected at the boundary with the atmospheric layer pressed in this manner, the reflection angle and the reflection position of the assist gas is changed, as a result of which, the position of the Mach disk (i.e., the position x 1 , x 2  of the maximum point  32 ,  34 ) formed in the jet is changed when compared to a case without the enclosure  142 . 
     When the inner dimension of the enclosure  142  is changed, the volume and the particle density of the atmospheric layer present between the jet and the enclosure  142  is changed, and whereby the position of the Mach disk formed in the jet can be changed. By making use of such a principle, the jet adjustment device  140  adjusts the position x 1 , x 2  of the maximum point  32 ,  34  in the direction of the optical axis O. 
     Specifically, the jet adjustment device  140  displaces the position x 1 , x 2  of the maximum point  32 ,  34  to downstream side of the jet (i.e., a direction away from the emission opening  28  along the optical axis O), by decreasing the radius R of the enclosure  142 . On the other hand, the jet adjustment device  140  displaces the position x 1 , x 2  of the maximum point  32 ,  34  to upstream side of the jet, by increasing the radius R of the enclosure  142 . 
     According to the jet adjustment device  140 , by changing the inner dimension (radius R) of the enclosure  142 , it is possible to adjust the position x 1 , x 2  of the maximum point  32 ,  34  so as to dispose the process portion S in the first Mach disk region  33 , in response to variation in the distance d between the emission opening  28  and the process portion S during the process on the workpiece. 
     Further, when the radius R of the enclosure  142  is decreased in a state where the supply pressure P s  to the chamber  29  formed inside the nozzle  24  having the predetermined opening dimension ϕ is constant, the position x 1 , x 2  of the maximum point  32 ,  34  is displaced to downstream side of the jet, along with the velocity V of the jet at the position x 1 , x 2  increasing. Thus, the velocity V of the jet in the Mach disk region  33 ,  35  where the workpiece W is to be disposed can be increased without changing the supply pressure P s . 
     In other words, even when the supply pressure P s  is reduced, the velocity V of the jet in the Mach disk region  33 ,  35  can be maintained by decreasing the diameter of the enclosure  142 . According to this configuration, since a consumption amount of the assist gas can be reduced, it is possible to reduce the cost. Note that, the enclosure driver  144  may be omitted, and the inner dimension of the enclosure  142  may be changed manually. 
     Next, a laser processing system  160  will be described with reference to  FIG. 25  and  FIG. 26 . The laser processing system  160  includes the laser oscillator  12 , the laser processing head  14 , the assist gas supply device  16 , the positioning device  18 , the measuring instrument  84 , the jet adjustment device  140 , and a controller  162 . 
     The controller  162  includes a processor and the storage  104 , and controls the laser oscillator  12 , the laser processing head  14 , the assist gas supply device  16 , the positioning device  18 , the measuring instrument  84 , and the jet adjustment device  140  (specifically, the power section  148 ). The database  106  as shown in above Table 1 is stored in the storage  104 . The controller  162  functions as the above-described position acquisition section  86 . Thus, the measuring instrument  84  and the controller  162  constitute the jet observation apparatus  80  described above. 
     Next, operation of the laser processing system  160  will be described with reference to  FIG. 27 . A flow illustrated in  FIG. 27  is started when the controller  162  receives a processing start command from an operator, a host controller, or a processing program. Note that, in the flow illustrated in  FIG. 27 , processes similar to those of the flow illustrated in  FIG. 21  are assigned the same step numbers, and redundant descriptions thereof will be omitted. 
     After starting the flow illustrated in  FIG. 27 , the controller  162  carries out steps S 1  to S 33  similar to the flow illustrated in  FIG. 21 . When determining YES in step S 33 , in step S 41 , the controller  162  controls the position x 1  of the first maximum point  32 . Specifically, when the difference δ (=d-d c ) calculated in most-recent step S 33  is a positive value, the controller  162  sends a command to the power section  148  of the enclosure driver  144  so as to decrease the inner dimension (radius R) of the enclosure  142 . Due to this, the position x 1  of the first maximum point  32  is displaced to downstream side of the jet. 
     On the other hand, when the difference δ calculated in most-recent step S 33  is a negative value, the controller  162  operates the enclosure driver  144  so as to increase the inner dimension (radius R) of the enclosure  142 . Due to this, the position x 1  of the first maximum point  32  is displaced to upstream side of the jet. 
     In this way, the controller  162  functions as a maximum point controller  164  configured to control the position x 1  of the first maximum point  32  by changing the inner dimension of the enclosure  142  based on information of the position x 1  of the first maximum point  32  acquired by the position acquisition section  86  in step S 32 . After executing step S 41 , the controller  162  returns to step S 32 . 
     According to the laser processing system  160 , it is possible to continuously dispose the process portion S in the first Mach disk region  33  during the process on the workpiece W, by changing the inner dimension of the enclosure  142 , without moving the nozzle  24 . According to this configuration, even when the distance d between the emission opening  28  and the process portion S changes due to some factor, it is possible to carry out the laser process on the workpiece W along with disposing the process portion S in the first Mach disk region  33 . Thus, the assist gas can be effectively utilized. 
     Note that, in the laser processing system  160 , the storage  104  may store a database in which a plurality of the target distances d T  are recorded in association with the opening dimensions ϕ of the nozzle  24 , the supply pressures P S , and the inner dimensions (radii R) of the enclosure  142 . In this case, in above-described step S 41 , the controller  162  can determine a target inner dimension of the enclosure  142  by applying the opening dimension ϕ, the supply pressure P S , and the distance d c  calculated in step S 32  as the target position distance d T  to the database. 
     Then, in step S 41 , the controller  162  operates the enclosure driver  144  so as to change the inner dimension of the enclosure  142  to the target inner dimension acquired from the database. As a result, it is possible to accurately dispose the process portion S in the first Mach disk region  33 . 
     Further, the above-described laser processing system  130  can also perform the flow illustrated in  FIG. 27 . In this case, in step S 41 , the controller  132  may control the position x 1 , x 2  of the maximum point  32 ,  34  by changing the supply pressure P s  to the chamber  29 . In this respect, if the supply pressure P s  to the chamber  29  is increased, the position x 1 , x 2  of the maximum point  32 ,  34  is displaced to downstream side of the jet (i.e., the direction away from the emission opening  28  along the optical axis O). 
     On the other hand, if the supply pressure P s  to the chamber  29  is decreased, the position x 1 , x 2  of the maximum point  32 ,  34  is displaced to upstream side of the jet (i.e., the direction approaching the emission opening  28  along the optical axis O). In step S 41 , when the difference δ (=d-d c ) calculated in most-recent step S 33  is a positive value, the controller  132  sends a command to the assist gas supply device  16  so as to increase the supply pressure P s . Due to this, the position x 1  of the first maximum point  32  is displaced to downstream side of the jet. 
     On the other hand, when the difference δ calculated in most-recent step S 33  is a negative value, the controller  132  sends a command to the assist gas supply device  16  so as to decrease the supply pressure P. Due to this, the position x 1  of the first maximum point  32  is displaced to upstream side of the jet. In this way, the controller  132  functions as a maximum point controller configured to control the position x 1  of the first maximum point  32  by changing the supply pressure P s  based on the information of the position x 1  of the first maximum point  32  acquired by the position acquisition section  86  in step S 32 . 
     Note that, the positioning device  18  is not limited to the above-described structure, but may include e.g. a work table movable along the x-y plane, and a z-axis movement mechanism configured to move the nozzle  24  along the z-axis. Alternatively, the positioning device may be configured to simply fix the nozzle  24  at a position with respect to the workpiece W, manually, without any movement mechanism. 
     Further, there are various embodiments of the dummy workpiece  64  and the measuring instrument  66  of the jet observation apparatus  60  illustrated in  FIG. 4 . Below, examples of the dummy workpiece  64  and the measuring instrument  66  will be described with reference to  FIG. 28  and  FIG. 29 . In an example illustrated in  FIG. 28 , the dummy workpiece  64  has a circular through hole  64   b  formed at a position corresponding to the above-described dummy process portion  64   b . The opening dimension of the through hole  64   b  is set to be substantially the same as the opening dimension of the through hole that is estimated to be formed when the workpiece W is perforated by the laser beam emitted from the nozzle  24 . 
     The measuring instrument  66  includes a pair of columns  170  and a hot-wire  172 . The pair of columns  170  extends from a surface  64   c  of the dummy workpiece  64  in the z-axis positive direction, so as to be opposite to each other. The hot-wire  172  is linearly strained between the pair of columns  170 , and the resistance value thereof varies in response to the velocity V of the jet emitted from the emission opening  28 . 
     For example, the length L of the hot-wire  172  extending between the pair of columns  170  (i.e., the distance between the pair of columns  170 ) may be set to be equal to or smaller than the opening dimension ϕ of the emission opening  28 , or equal to or smaller than the opening dimension of the through hole  64   b . Alternatively, the hot-wire  172  may be comprised of a material having high stiffness. By setting the length L to be small or making the hot-wire  172  from the material with high stiffness in this manner, it is possible to prevent the hot-wire  172  from bending when the hot-wire  172  is disposed in the jet. 
     Further, the distance from the surface  64   c  of the dummy workpiece  64  to the hot-wire  172  may be set to e.g. 0.5 mm or less. By setting the distance from the surface  64   c  to the hot-wire  172  to be smaller in this way, the velocity V of the jet can be measured at a position closer to the process portion S of the workpiece W during the laser process. 
     In an example illustrated in  FIG. 29 , the measuring instrument  66  includes a hot-wire  174  strained in the through hole  64   b . The length L of the hot-wire  174  coincides with the opening dimension of the through hole  64   b . The hot-wire  174  is disposed at the position of the surface  64   c  of the dummy workpiece  64 . According to such an arrangement of the hot-wire  174 , the velocity V of the jet can be measured at a position closer to the process portion S of the workpiece W during the laser process. As described above, the measuring instrument  66  in  FIG. 28  and  FIG. 29  constitutes a hot-wire anemometer. 
     Note that, in the embodiments described above, when the workpiece W is processed, the workpiece W may be disposed in the second Mach disk region  35  (second maximum point  34 ) or in an n-th Mach disk region (n is an integer of 3 or greater). Further, instead of the above-described jet observation apparatuses  60 ,  70  or  80 , a high-speed camera may be employed to capture an image as shown in  FIG. 2 , and the position x 1 , x 2  of the maximum point  32 ,  34  may be measured based on the image, for example. 
     In addition, the shape of the emission opening  28  is not limited to a circular shape, but may have any shape such as a polygonal shape or an elliptical shape. Further, the features of the various embodiments described above may be combined with each other. For example, the jet observation apparatus  80  may be combined with the laser processing system  110  or  120 , or the jet adjustment device  140  may be combined with the laser processing system  110  or  120 . 
     While the present disclosure has been described through specific embodiments, the above-described embodiments do not limit the invention as defined by the appended claims.