Patent Publication Number: US-5628664-A

Title: System for manufacturing x-ray tubes

Description:
RELATED APPLICATIONS 
     This application is related to commonly assigned U.S. patent application Ser. No. 538,144. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to equipment for diagnostic and therapeutic radiology and methods of making the same and, more particularly, to methods for exhausting x-ray tubes during the manufacturing process. 
     Recently, it has been found that the internal vacuum obtained in the x-ray tube envelope has been only been about 1×10 -5  torrs. This internal vacuum has allowed &#34;spitting&#34; which occurs when the electrical path of the electron beam is diverted to some other point in the vacuum space rather than the focal track of the x-ray tube target. Spitting occurs because there are more particles left in the vacuum space that can attract the electrons being generated. Additionally, the manufacturing process called &#34;exhaust&#34; presently requires up to thirty hours to complete, which is entirely too long in the manufacturing process. 
     Current manufacturing &#34;exhaust&#34; practice utilizes a small about 1/2&#34; to about 3/4&#34; inside diameter tubulation connected to a turbomolecular pump having a pumping speed of approximately 1 liter per second as measured at the target. As is known, pumping speed or conductance is directly related to the inside diameter of the pumping port or tubulation. While length of the tube which does have an effect, it is much less than the effect of the diameter. 
     During x-ray tube manufacturing, the exhaust port of the envelope/tubulation connection of the x-ray tube is sealed off after evacuation by standard glass blowing technique of thermal collapse, fusion and separation of the small diameter (1 to 2 cm inside diameter) exhaust tubulation. The lowest pressure that can be achieved with the current configuration is limited by the conductance of the exhaust tubulation. The conductance (c) of this tube is proportional to the diameter (d) and to the length (I): 
     
         c˜d.sup.3 /I                                         [1] 
    
     To achieve lower pressures, the conductance must be increased. To increase the conductance, a larger diameter exhaust tubulation must be used. 
     Post &#34;exhaust&#34; process inspection has revealed that the current method may be insufficient to provide effective removal of the gases evolved during the exhaust process and thereby leave the x-ray tube enclosure with a high pressure condition which in turn has been related to early failure of the assembly in the field. The &#34;exhaust&#34; process method has not been changed to a larger diameter pumping port or tubulation because of the past inability to effectively seal the envelope/tubulation connection after the completion of the &#34;exhaust&#34; process step. 
     The seal-off configuration currently used does not work with larger diameter tubulations, The &#34;thermal collapse&#34; phase becomes extremely unstable and the tubulation buckles in an uncontrollable fashion. Effective &#34;fusion&#34; of the buckled tubulation is not possible with this prior configuration. 
     Due to unacceptable failures after seasoning and prior to being shipped, the need for an improved x-ray tube having an envelope evacuated to about 1×10 -5  torr that would reduce or possibly eliminate the spitting while shortening the manufacturing cycle became apparent. Such an x-ray tube would have the exhaust process or a combination exhaust and seasoning process of the manufacturing process effective to evacuate the x-ray tube envelope to greater than about 1×10 -5  torr, reducing the particles left in the vacuum space that could attract the electrons being generated such that failure due to &#34;spitting&#34; , which occurs when the electrical path of the electron beam is diverted to some other point in the vacuum space rather than the focal track of the target, should be significantly reduced, if not eliminated and reduce the about thirty (30) hours presently required to complete the exhaust process step. 
     SUMMARY OF THE INVENTION 
     In carrying out the present invention in preferred forms thereof, we provide improved systems for the manufacture of x-ray tubes, such as those incorporated in diagnostic and therapeutic radiology machines, for example, computer tomography scanners. Illustrated systems of the invention disclosed herein, are in the form of systems for exhausting and for exhausting and seasoning an x-ray tube envelope for use in x-ray systems. 
     One specific system of the present invention includes a system for exhausting an x-ray tube envelope utilizing a large diameter tubulation comprising: a tubulation having a diameter greater than 20 mm, operatively connected to the x-ray tube envelope; a disk, operatively positioned inside the tubulation, having a smaller outside diameter than the tubulation inside diameter; a vacuum operatively connected to the tubulation; means for heating the anode of the x-ray tube to a temperature inside the x-ray tube envelope to about 1500° C.; means, operatively positioned inside the tubulation, for positioning the disk inside the tubulation; heating means, operatively positioned on the outside of the tubulation, for heating the tubulation proximate the disk to a temperature sufficient to collapse the tubulation onto the disk; means for checking for sealing contact between the tubulation and the disk; and means for cooling the tubulation/disk interface until the temperature is sufficient to seal the tubulation to the disk. 
     Another aspect of the present invention includes a system for exhausting and seasoning an x-ray tube envelope utilizing a large diameter tubulation comprising: a glass tubulation having a diameter greater than 20 mm operatively connected to the x-ray tube envelope; pump means for providing a vacuum to the envelope through the tubulation; a disk, operatively positioned inside the tubulation, the disk having a smaller diameter than the tubulation; means, operatively connected to the x-ray tube, for generating x-rays such that the temperature inside the x-ray tube envelope is about 1500° C.; means, operative connected to the disk, for positioning the disk inside the tubulation; heating means, operatively positioned on the outside of the tubulation for heating the tubulation proximate the disk sufficient to cause the tubulation to collapse into the disk;; and means for cooling the tubulation/disk interface sufficient to seal the tubulation to the disk. 
     One other aspect of the present invention includes a system for sealing off a large diameter tube under vacuum comprising: a tube; a disk operatively positioned inside the tube, the disk having a smaller diameter than the tube; a vacuum operatively connected to the tube; heating means, operatively positioned on the outside of the tube proximate the position of the disk the inside of the glass tube, for heating the tube proximate the disk to about 700° C. in about two (2) minutes and for further heating the tube proximate the disk to about 870° C.; means for holding the temperature of the tube proximate the disk at about 870° C.; heating means for heating the tube proximate the disk to about 1200° C. and then to about 1300° C. such that the tube collapses into the disk; means for checking for sealing contact between the tube and the disk; and means for cooling the tube/disk interlace at about 100° C. per minute until the temperature of the tube/disk interface is below about 300° C. 
     Accordingly, an object of the present invention is to provide an improved exhausting system for the manufacture of x-ray tubes. 
     Another object of the present invention is to provide an improved system for the combined exhausting and seasoning of an x-ray tube during the manufacturing process. 
     A further object of the present invention is to provide a system for the exhausting step in the manufacture of an x-ray tube requiring less time to complete. 
     Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1a is a plan view of a representative x-ray system having an x-ray tube positioned therein; 
     FIG. 1b is a sectional view with parts removed of the x-ray system of FIG. 1a; 
     FIG. 2 is a schematic representation of another representative x-ray system; 
     FIG. 3 is a partial sectional view of an x-ray tube illustrating representative thermal paths; 
     FIG. 4 is a partial perspective view of a representative x-ray tube with parts removed, parts in section, and parts broken away; and 
     FIG. 5 is a sectional view of a representative large diameter tubulation of the tubes that would be used for the exhausting and/or the seasoning of an x-ray tube during the manufacturing process. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Typical x-ray tubes are normally enclosed in an oil-filled protective casing. An envelope, typically glass contains a cathode plate, a rotating disk target and a rotor that is part of a motor assembly that spins the target. A stator is provided outside the tube proximate to the rotor and overlapping therewith about two-thirds of the rotor length. The glass envelope is enclosed in an oil-filled lead casing having a window for the x-rays that are generated to escape the tube. The casing in some x-ray tubes may include an expansion vessel, such as a bellows. 
     X-rays are produced when, in a vacuum, electrons are released, accelerated and then abruptly stopped. This takes place in the x-ray tube. To release electrons, the filament in the tube is heated to incandescence (white heat) by passing an electric current through it. The electrons are accelerated by a high voltage (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative) and impinge on the anode, whereby they are abruptly slowed down. The anode, usually referred to as the target, is often of the rotating disc type, so that the electron beam is constantly striking a different point on the anode perimeter. The x-ray tube is enclosed in a protective casing that is filled with oil to absorb the heat produced. High voltages for operating the tube are supplied by a transformer (not shown). The alternating current is rectified by means of rectifier tubes (or &#34;valves&#34;) in some cases by means of barrier-layered rectifiers. 
     For therapeutic purposes--e.g., the treatment of tumors, etc.--the x-rays employed are in some cases generated at much higher voltages (over 4,000,000 volts). Also, the rays emitted by radium and artificial radiotropics, as well as electrons, neutrons and other high speed particles (for instance produced by a betatron), are used in radio therapy. 
     A typical x-ray system is illustrated as generally designated by the numeral 20 in FIGS. 1a, 1b and 2. As can be seen, the system 20 comprises an oil pump 22, an anode end 24, a cathode end 26, a center section 28 positioned between the anode end and the cathode end, which contains the x-ray tube 30. A radiator 32 for cooling the oil is positioned to one side of the center section and may have fans 34 and 36 operatively connected to the radiator 32 for providing cooling air flow over the radiator as the hot oil circulates therethrough. The oil pump 22 is provided for circulating the hot oil through the system 20 and through the radiator 32, etc. As shown in FIG. 1b, electrical connections are provided in the anode receptacle 42 and the cathode receptacle 44. 
     As shown in FIG. 2, the x-ray system 20 comprises a casing 52 preferably made of aluminum and lined with lead and a cathode plate 54, a rotating target disc 56 and a rotor 58 enclosed in a glass envelope 60. A stator 43 is positioned outside the glass envelope 60 inside the lead lined casing 52 relative to the rotor 58. The casing 52 is filled with oil for cooling and high voltage insulation purposes as was explained above. A window 64 for emitting x-rays is operatively formed in the casing 52 and relative to the target disc 56 for allowing generated x-rays to exit the x-ray system 20. 
     As stated above, very high voltages and currents are utilized in the specific x-ray tube and range from an approximate voltage maximum 160 KV to an approximate minimum of 80 KV and from an approximate current maximum of 400 ma to an approximate minimum of 250 ma. 
     As shown in FIGS. 3 and 4, the cathode 54 is positioned inside the glass envelope 60. As is well known, inside the glass envelope 60 there is suppose to be a vacuum of about 10 -5  to about 10 -9  torr at room temperature. The electricity generates x-rays that are aimed from the cathode filament 68 to the anode target or the top of the target disc 56. The target disc is operatively connected to a rotating shaft 61 at one end by a Belleville nut 62 and by another nut at the other end 64. A front bearing 66 and a rear bearing 68 are operatively positioned on the shaft 61 and are held in position in a conventional manner. The bearings 66 and 68 are usually silver lubricated and are susceptible to failure at high operating temperatures. 
     A preload spring 70 is positioned about the shaft 60 between the bearings 66, 68 for maintaining load on the bearings during expansion and contraction of the anode assembly. A rotor stud 72 is utilized to space the end of the rotor most proximate the target 56 from the rotor hub 74. The bearings, both front 66 and rear 68, are held in place by bearing retainers 78 and 80. The rotor assembly also includes a stem ring and a stem all of which help to provide for the rotation of the rotor 58 with the target 56. 
     As stated above, the current manufacturing exhaust process practice for exhausting or evacuating the gases from the interior of the envelope utilizes a small (about 1/2&#34; inch to about 3/4&#34;) inside diameter tubulation connected to a turbomolecular pump having a pumping speed of approximately one liter per second at the target. As is also discussed above, the current manufacturing process does not work with a larger diameter tubulation because the &#34;thermal collapse&#34; phase becomes extremely unstable and the tubulation buckles in an uncontrollable fashion. 
     As mentioned above, during the prior manufacturing &#34;exhaust&#34; processes, the x-ray tube envelope had not apparently been fully exhausted, resulting in x-ray tube failures. Thus, it is important to attain a lower internal vacuum in the x-ray tube envelope during the manufacturing process and specifically during the exhaust process. Specifically, a vacuum of about 1×10 -6  to about 1×10 -8  torr is believed to be adequate. 
     It is believed that such an internal vacuum would provide more room within the envelope for the outgassing of components when the x-ray unit is in service before a high pressure condition in the envelope is reached that would shut the x-ray system off. 
     Presently, one &#34;exhaust&#34; process is being performed utilizing an about 12.5 mm vacuum tubulation connected to an x-ray tube envelope. As is known, &#34;Spitting&#34; can occur when there are relatively more particles left in the vacuum space inside the envelope that can attract the electrons being generated. Envelopes evacuated or exhausted using the 12.5 mm vacuum tubulation connected to an x-ray tube envelope and a one (1) liter /sec pumping speed have experienced failures due to Spitting. In other words, the vacuum inside the envelope was less than desired. 
     The amount of time needed to complete the exhaust portion or step of the x-ray tube manufacturing process is an important consideration. Presently, if the x-ray tube is to pass inspection on the first try after the &#34;exhaust&#34; process or step, up to thirty (30) hours has been needed to complete the &#34;exhaust&#34; process or step. If the first try is unacceptable, several additional attempts may be needed before a decision relative to having attained an acceptable vacuum inside the envelope is reached. 
     It has been found that, by utilizing a large diameter vacuum tubulation, exhaust process time has been reduced to about ten (10) hours from the about thirty (30) hours previously required. Additionally, an increased potential for passing final test on the first attempt because of the lower starting pressure in the envelope has been realized. 
     One method of the present invention includes an improved connection to a high performance x-ray tube envelope in order to improve the part of the manufacturing process known as &#34;exhaust&#34;. During the &#34;exhaust&#34; process, the anode portion of the x-ray tube is typically placed in an envelope, presently preferably made of Pyrex, glass, and evacuated to about 1×10 -6  to about 1×10 -9  torr. The x-ray tube anode is then heated, for example by induction heating, in order to remove gases from the envelope that are evolved when any material is heated. Compositions of CO, CO 2 , H 2  O, N 2 , O 2 , etc. are driven out of the anode materials into the envelope and then evacuated from the envelope by a vacuum pump, as discussed above. This basic approach could be used with some necessary modification if other materials such as for example, metal/ceramic materials, are found to be acceptable for use as x-ray tube envelopes. 
     As illustrated in FIG. 5, a bulkhead or disk 100, presently preferably made of glass, positioned inside the larger diameter tubulation 102 is used to stabilize the &#34;thermal collapse&#34; phase during the seal-off of the tubulation/envelope connection. Initially, during the &#34;exhaust&#34; process, the bulkhead 100 is positioned so that it does not interfere with the evacuation of the gases from inside the envelope. For the seal-off of the envelope/tubulation connection, the bulkhead 100 is moved to a location selected for the seal-off. During the &#34;thermal collapse&#34; phase of the seal-off, the heated portion 104 of the large diameter tubulation 102 shrinks down or collapses until it contacts the bulk head 100. The small displacement (about 1/16 inch to about 1/8 inch) required between the inner surface of the tubulation 102 and the outer surface 106 of the bulkhead 100 can be achieved without the tubulation buckling. The &#34;fusion&#34; phase then takes place between the tubulation 102 and the bulkhead 100 to complete the seal-off of the tubulation/envelope connection thereby retaining the vacuum inside the envelope. 
     The following method describes how a seal off can be accomplished utilizing larger diameter tubulations such as about 20 mm to as large a diameter as practicable. 
     FIG. 5 illustrates an induction coil 108 and a graphite ring 110 as utilized in one method of the present invention. The graphite ring length may be varied to suit the particular x-ray tube envelope seal-off application, or any envelope requiring a faster more complete evacuation/lower vacuum inside thereof. 
     Power is supplied to the heating means, as illustrated an induction coil 108, which in turn heats the graphite ring 110 to a temperature sufficient to cause the illustrated Pyrex, glass tubulation wall to collapse while under vacuum. This collapse phase is stabilized by the sealing bulkhead or disk 100 which is positioned within the tubulation 102 at the location of the desired seal between the envelope and the tubulation. The temperature of the graphite 110 may be monitored by an optical pyrometer or other known means for monitoring temperature and at least one heating and cooling schedule has been defined which has been successful in providing for a controlled collapse and anneal of the tubulation to the disk. 
     It is believed that resistance heating, with proper element design, could be utilized to accomplish the same type of seal off. One example of such a device is a split furnace made up of two half cylinders, with air diameters presently available from about 100 mm to about 400 min. Such split furnaces can be adapted for use up to 1600° C. for continuous operations, for creep testing, bilatometers and most other standard tests. 
     In one implementation of the present invention, a large diameter tubulation with high conductance pumping is utilized in the &#34;exhaust&#34; process or step with the bulk target anode temperature being increased from the current about 1150° C. to about 1500° C. 
     EXAMPLE 1 
     An x-ray tube envelope is fitted with a large diameter vacuum tubulation 45 mm to about as large a diameter as practicable with about 59 mm presently being preferred. A resistance type tube furnace is fitted over the tubulation to perform the seal-off after the process step of &#34;Exhaust&#34; is completed. A split furnace could also be used. Vacuum connections are made to a turbomolecular vacuum pump. Vacuum system conductance of about 25 liter/sec or greater is preferred, as calculated at the target. The envelope is processed through the &#34;exhaust&#34; process, which includes a resistance bakeout at about 450° C. and induction heating of the anode to about 1500° C. 
     With the x-ray tube envelope still being evacuated, a sealing bulkhead or disk 100 is positioned inside the tubulation 102 at the desired sealing location. The resistance furnace is then centered on the disk. A preprogrammed heating ramp is then started. The vacuum pump is on throughout the entire &#34;Exhaust&#34; process in order to remove outgas products, believed to be primarily water vapor, developed, by heating the glass envelope. A very localized region 104 (about 1/8 inch to about 1/4 inch in length) of the tubulation wall is heated to a temperature just above the softening point of the illustrated Pyrex, glass or other material used as the envelope and tubulation connecting the pump 112 to the envelope. 
     As the temperature of the localized region of the tubulation wall rises, the forces applied by the vacuum collapse the tubulation&#39;s walls onto the sealing disk. This temperature is held for about two (2) to about five (5) minutes to provide for good fusion of the tubulation wall to the sealing disk. The temperature at the collapse point is then lowered per a defined annealing schedule. 
     One heating and cooling schedule which produced an acceptable envelope/tubulation seal follows: Heat the graphite ring to about 700° C. in about 2 minutes; Heat the graphite ring to about 870° C. in about 2:45 minutes; Hold the temperature of the graphite ring at 870° C. for about 1 minute; Heat the graphite ring to about 1200° C. in about 5:30 minutes; Heat the graphite ring to about 1300° C. in about 7:00 minutes; Hold the graphite ring temperature at 1300° C. for about 2:00 minutes; Visually check for sealing between the tubulation and the disk; Cool the graphite ring at about 100° C. per minute until below about 300° C. in order to reduce the stresses developed in the sealing disk and the tubulation wall. 
     At this point in the exhaust process, a rudimentary test can be performed to assure that the tubulation connected to the envelope is adequately sealed. The test requires that the target be heated briefly to a temperature above the highest temperature used during the &#34;exhaust&#34; process step, which could be as little as about 10° C. above that highest temperature. This addition of heat should cause a rise in total pressure within the envelope because additional outgassing of the anode will occur. If a leak is present in the envelope/tubulation seal at the collapse point, the vacuum system pressure would also rise on the pump side of the seal. No pressure rise, no leak. 
     At this point the vacuum pump is disconnected from the envelope and any excess tubulation protruding from the envelope will be cut or ground away. A stress check should also be part of the seal-off inspection. 
     It has also been proposed that the final seal-off step be performed after the x-ray tube manufacture process step known as &#34;Seasoning&#34;. &#34;Seasoning&#34; is usually performed after &#34;exhaust&#34; and uses the electron beam source to actually generate X-Rays and heat the target in a rotating, dynamic manner. This manufacturing process step accomplishes what is called &#34;seasoning&#34; of the focal track and verifies the spot size of the electron beam. This step is believed to increase the overall life of the x-ray tuber assembly and also is the final process check for an envelope prior to field installation into an x-ray system. 
     Using the prior manufacturing process protocol, additional outgassing occurs in the &#34;Seasoning&#34; process step. This is because the prior &#34;Exhaust&#34; process step heats the target bulk temperature to only about 1150° C. Actual operating temperatures of about 1475° C. are reached when the electron beam is in operation. As is known, when a higher temperature is reached, additional outgassing inside the envelope takes place. 
     If the final seal-off of the tubulation connecting the envelope to the vacuum pump is performed after &#34;Exhaust&#34; and &#34;Seasoning&#34;, then any additional outgassing that occurs in the &#34;Seasoning&#34; step is also pumped away or evacuated from the envelope. In the prior manufacturing process, the envelope was sealed prior to &#34;seasoning&#34; and only a very small ion appendage pump, which was attached via a different tubulation, was used to remove gases during the seasoning step. It should be understood that the vacuum generated for exhaust is via a turbomolecular pump and the additional evacuation after the exhaust tubulation was sealed was conducted via the small ion appendage pump. 
     EXAMPLE 2 
     Experiments have shown that the amount of gases evolved by heating from about 1150° C. in the prior &#34;Exhaust&#34; process or step to the full operating temperature of about 1475° C. in the &#34;Seasoning&#34; process or step is enormous and that the small appendage pump previously used was incapable of removing the amount of evolved gases generated during &#34;Seasoning&#34; in a reasonable time period. While it is believed that the methods of the present invention would reduce the initial outgassing generated during the &#34;Seasoning&#34; process, continued high conductance pumping during the &#34;Seasoning&#34; process would then remove any additional outgassing which occurs when the anode is rotated and x-rays are generated. 
     In one additional new method, the seal-off of the tubulation envelope connection in the above &#34;Exhaust&#34; process or step would be delayed until after the &#34;Seasoning&#34; step is completed. During the &#34;Seasoning&#34; step, the fully operational large diameter tubulation and envelope connection to the vacuum pump would remain in place and in operation. It is believed that by delaying the seal-off until after the &#34;Seasoning&#34; step has been completed, considerable processing time over the old method would be saved and the maximum envelope vacuum would be achieved. 
     Several experiments were conducted which verified the feasibility of the utilization of a larger diameter tubulation for the exhaust process step. Unfortunately, as with all new approaches, initial results were not successful, as indicated by Example 3 and Example 4 below. 
     EXAMPLE 3 
     Seal-Off Run #1 
     Setup parameters: 
     A graphite tube was placed around a Pyrex, sample envelope. This was about 4&#34; long×about 1/8&#34; wall. A pyrometer was setup to read the graphite tube just above an induction coil. Sample tube vacuum measured about 5&#39; from sample was about 200 microns. Sample tube was suspended above a firebrick 1/4&#34;. It is believed that as tub walls soften, the tube will stretch and draw around the sealing disk. 
     Results: 
     The sealing disk support rod melted off due to heat concentration being too high up the sample envelope. Lack of support from sealing disk allowed for uncontrolled collapse of tube wall. 
     EXAMPLE 4 
     Seal-Off Run #2 
     The length of the graphite tube was reduced to about 1/4&#34; in order to reduce the hot zone. The temperature was increased very slowly to limit thermal shock and held steady at about 870° C., the softening point for Pyrex. The temperature was increased to about 1300° C. per pyrometer reading and held. This allowed for the slow collapse of the tube wall onto the sealing disk. Waited until the tubulation had collapsed on the sealing disk was observed visually. Then the temperature was reduced rapidly in order to limit the deformation of tube wall. 
     Results: 
     Two small scallop fractures in the sealing disk were observed most likely due to thermal shock from rapid cooling. The tube wall collapsed slightly above the sealing disk, but did bond to the sealing disk. Vacuum compromised due to cracks in sealing disk, otherwise considered a success. 
     EXAMPLE 5 
     Seal-off runs 3 and 3a. 
     The sample tube design was revised such that the support rod for the sealing disk was now fixed. In order to prevent thermal shock, a preliminary heating/cooling schedule was devised as follows: 
     Heat to 700° C. in 2 minutes. 
     Heat to about 870° C. in about 2:45. 
     Hold for 1 minute. 
     Heat to about 1200° C. in about 5:30 minutes. 
     Heat to about 1300° C. in about 7.00 minutes. 
     Hold for about 2:00 minutes. Visually check for sealing of the wall with the disk. 
     Cool at about 100° C. per minute until below about 300° C. 
     Results showed no cracking was visible in the sealed disk or the tube wall. The sealed portion of the tube remained under vacuum. The sealed was He gas leak checked to about 1.0×10 -8  torr. At this point, the new sealing disk and the new exhausting method had been proven. 
     While the systems contained herein constitute preferred embodiments of the Invention, it is to be understood that the invention is not limited to these precise systems, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.