Abstract:
A beam position detector for measuring the properties of a charged particle beam, including the beam&#39;s position, size, shape, and intensity. One or more absorbers are constructed of thermo-resistive material and positioned to intercept and absorb a portion of the incoming beam power, thereby causing local heating of each absorber. The local temperature increase distribution across the absorber, or the distribution between different absorbers, will depend on the intensity, size, and position of the beam. The absorbers are constructed of a material having a strong dependence of electrical resistivity on temperature. The beam position detector has no moving parts in the vicinity of the beam and is especially suited to beam areas having high ionizing radiation dose rates or poor beam quality, including beams dispersed in the transverse direction and in their time radio frequency structure.

Description:
The United States of America may have certain rights to this invention under Management and Operating contract No. DE-AC05-84ER 40150 from the Department of Energy. 

   FIELD OF THE INVENTION 
   This invention relates to beam position detectors and specifically to an apparatus and method for measuring the position, size, shape, and intensity of a particle beam in a particle accelerator or light-generating device. 
   BACKGROUND OF THE INVENTION 
   When studying the behavior of charged particles at relativistic speeds, such as in particle accelerators, it is necessary and advantageous to measure the properties of the charged particle beam, including beam position, size, shape, and intensity. In a particle accelerator, this task becomes extremely challenging in those areas that have poor beam quality, such as in the vicinity of power beam dumps, which absorb the beam after it has been utilized in experimental targets or material treatment facilities. In these areas, as a result of the particle beam being dispersed by the targets or the treated materials, the quality of the beam is typically very poor. The beam is typically degraded in both the transverse direction and in its time radio frequency (RF) structure. In addition, the areas close to the beam dumps typically experience very high ionizing radiation dose rates from the dumps, and any equipment positioned there must be extremely resistant to radiation damage. 
   Several U.S. patents disclose apparatus and methods for measuring various properties of particle beams. However, each of these prior art patents either need good RF quality of the beam and small aperture, or need to implement moving parts and respective control systems that are difficult to maintain in working condition in the high radiation environment. 
   Therefore, what is needed is an apparatus and method for measuring the properties of a charged particle beam in high radiation areas and in areas in which the beam quality is poor. 
   SUMMARY OF THE INVENTION 
   The invention is a beam position detector for measuring the properties of a charged particle beam, including the beam&#39;s position, size, shape, and intensity. The beam position detector includes one or more absorbers constructed of thermo-resistive material and positioned to intercept and absorb a portion of the incoming beam power. Absorbing a portion of the incoming beam power causes local heating of each absorber. The local temperature increase distribution across the absorber, or the distribution between different absorbers, will depend on the intensity, size, and position of the beam. By constructing the absorbers of a material having a strong dependence of electrical resistivity on temperature and measuring the electrical resistance distribution across the absorber or between different absorbers, a beam position detector is constructed that is capable of measuring beam properties such as beam position, size, shape, and intensity. The absorbers are preferably in the form of rectangular plates or wires constructed of chemical vapor deposition silicon carbide. 
   OBJECTS AND ADVANTAGES 
   The beam position detector of the present invention provides a method of measuring particle beam properties in areas in which the quality of the beam is very poor, such as in the vicinity of beam dumps. In these areas the beam is dispersed in both the transverse direction and in its time RF structure and present methods of beam property measurement are inadequate to properly monitor the beam. 
   A further advantage of the beam position detector of the present invention is that it is highly resistant to radiation damage, and therefore may be used in areas, which exhibit very high ionizing radiation dose rates. 
   Another advantage of the beam position detector of the present invention is that it does not employ any moving parts, which would be difficult to maintain in an area susceptible to high ionizing radiation dose rates. 
   These and other objects and advantages of the present invention will be better understood by reading the following description along with reference to the drawings. 

   
     DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a preferred embodiment of a Beam Position Detector (BPD) according to the present invention using two resistive absorbers. 
       FIG. 2  is a schematic depicting the wiring connections for a voltage measurement across each of the absorbers of  FIG. 1 . 
       FIG. 3  is a schematic depicting the wiring connections for a current measurement at each end of one of the absorbers of  FIG. 1 . 
       FIG. 4  is a plot of X BPM , the beam x-coordinate extrapolated from a conventional beam position monitor (BPM) and projected to the position of the BPD, versus time during calibration of the beam position detector of  FIG. 1 . 
       FIG. 5  is a plot of X BPD , beam x-coordinate calculated using the balance of currents from the beam position detector (BPD) of  FIG. 1 , versus time during calibration. 
       FIG. 6  is a scatter plot of X BPD  versus X BPM . 
       FIG. 7  is a plot of Y BPM , the beam y-coordinate extrapolated from a conventional beam position monitor and projected to the position of the BPD, versus time during calibration of the beam position detector of  FIG. 1 . 
       FIG. 8  is a plot of Y BPD , beam y-coordinate calculated using the balance of currents from the beam position detector of  FIG. 1 , versus time during calibration. 
       FIG. 9  is a scatter plot of Y BPD  versus Y BPM . 
       FIG. 10  is a perspective view of an alternate embodiment of a beam position detector according to the present invention using four resistive absorbers. 
       FIG. 11  is a perspective view of another alternate embodiment of a beam position detector for providing more detailed information on the beam size and profile by the use of sets of thin parallel metallic conductors deposited on the plates. 
   

   TABLE OF NOMENCLATURE 
   The following is a listing of part numbers used in the drawings along with a brief description: 
   
     
       
             
             
           
         
             
                 
             
             
               Part Number 
               Description 
             
             
                 
             
           
           
             
               20 
               beam position detector 
             
             
               22 
               first absorber 
             
             
               22A 
               first end on first absorber 
             
             
               22B 
               second end on first absorber 
             
             
               24 
               second absorber 
             
             
               24A 
               first end on second absorber 
             
             
               24B 
               second end on second absorber 
             
             
               26 
               beam path 
             
             
               30 
               first current monitor 
             
             
               32 
               second current monitor 
             
             
               34 
               third current monitor 
             
             
               36 
               fourth current monitor 
             
             
               38 
               first voltage monitor 
             
             
               40 
               second voltage monitor 
             
             
               42 
               electrical leads to first absorber 
             
             
               44 
               electrical leads to second absorber 
             
             
               50 
               beam position detector (first alternate embodiment) 
             
             
               52 
               metallic conductor 
             
             
               60 
               beam position detector (second alternate embodiment) 
             
             
               62 
               first vertical absorber 
             
             
               64 
               second vertical absorber 
             
             
               66 
               first horizontal absorber 
             
             
               68 
               second horizontal absorber 
             
             
               A 
               current monitor or ammeter 
             
             
               V 
               voltage monitor or voltmeter 
             
             
                 
             
           
        
       
     
   
   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention comprises a beam position detector for measuring the position, size, shape, and intensity of a charged particle beam. 
   With reference to  FIG. 1 , the beam position detector  20  includes a first absorber  22  and a second absorber  24  positioned to intercept the incoming beam path  26 . The absorbers  22 ,  24  may be in the form of plates, as shown in  FIG. 1 , or wires. The first absorber  22  is positioned vertically and orthogonal with respect to the path  26 . The second absorber  24  is placed farther down the beam path  26  and is positioned horizontally and orthogonal with respect to the path  26 . Each absorber  22 ,  24  has ends including ends  22 A and  22 B on the first absorber  22  and ends  24 A and  24 B on the second absorber  24 . The beam position detector  20  includes a current monitor A at the ends of each of the absorbers  22 ,  24 . A first current monitor  30  measures the current at the first end  22 A of absorber  22  and a second current monitor  32  measures the current at the second end  22 B of absorber  22 . A third current monitor  34  measures the current at the first end  24 A of absorber  24  and a fourth current monitor  36  measures the current at the second end  24 B of absorber  24 . Voltage meters V measure the voltage across each of the absorbers, including a first voltage monitor  38  measuring the voltage across absorber  22  and a second voltage monitor  40  measuring the voltage across absorber  24 . To charge the plates preferably one of the absorbers  22 ,  24  is biased with a positive voltage relative to the ground and the other absorbers is biased with a negative voltage relative to the ground to create a low-voltage difference between the plates. For the purposes of calibration and data gathering, a voltage of +3 volts was applied to the electrical leads  42  to the first absorber  22  and a voltage of −3 volts was applied to the electrical leads  44  to the second absorber  24 . In this preferred embodiment, the plates  22 ,  24  are immersed in the gas atmosphere, preferably an inert gas such as helium. The intensive particle beam crossing the space between the plates  22 ,  24  ionizes the gas, thus producing a conductivity path between the plates. The applied potential difference between the plates causes electric current to flow in the space region between the plates traversed by the beam. Depending on the beam transverse coordinates, the balance of the horizontal and vertical currents read from the plates will correspond to the position of beam center. The beam position detector  20  collects and processes the information from each of the current monitors  30 ,  32 ,  34 ,  36  and each of the voltage monitors  38 ,  40 . In an especially preferred method of viewing the output of the beam position detector  20 , a microprocessor is used to convert the outputs of the current and voltage monitors to a representation of the position of the particle beam path  26  on a two-coordinate grid. In an alternate embodiment, the beam position can be determined even if the plates are immersed in a vacuum. 
   Referring to the schematics of  FIGS. 2 and 3 , the voltage is measured across the absorbers  22 ,  24  and the current is measured at the ends of the absorbers  22 ,  24 . The voltage meters  38 ,  40  measure the electric potential generated in each plate  22 ,  24  in the presence of a temperature gradient caused by the incident particle beam  26 . The voltage readout shows a good correlation with the beam position at each plate  22 ,  24 . The balance of the currents are read at the plate corners to characterize the beam position.  FIG. 3  depicts the second absorber  24  with current monitors  34 ,  36  at the two ends  24 A and  24 B. Electrons are knocked out of the plates at a given rate depending on the beam current. This rate is typically 10 nA per 1 μA beam. The currents are all equal to zero when there is no beam incident upon the plates. The currents are all non-zero and equal if the beam striking the plates is small and symmetrical and hits exactly at the middle of the plates. When the beam moves toward one end of a plate, the current readings increase from that end of the plate and decrease at the opposite end of the plate. By monitoring the balance of currents read at the plate ends, the beam position can be measured. 
   Referring to  FIG. 1 , two currents  13 ,  14 , are read from the ends  22 A,  22 B of the first or vertical absorber  22 , two currents  11 ,  12 , are read from the ends  24 A,  24 B of the second or horizontal absorber  24 , and one voltage across each absorber for a total of 6 readings in all. The vertical  22  and horizontal  24  absorbers are biased plus or minus 3 volts relative to the ground to create a low-voltage difference between the plates. Beam ionization in helium gas creates an electrical “short” between the plates, allowing the coordinate readout. Balances of currents from the ends of the vertical  22  and horizontal  24  plates determine the coordinates. Mathematically, in the first approximation, the x and y coordinates are determined by the following formulas:
 
 X=C   x ( I   1   −I   2 )/( I   1   +I   2 )  (1)
 
 Y=C   y ( I   3   −I   4 )/( I   3   +I   4 )  (2)
         where I 1 , and I 2  are the currents read at the ends of the vertical absorber  22 , I 3  and I 4  are the currents read at the ends of the horizontal absorber  24 , and C x  and C y  are calibration coefficients that convert current balance readings into coordinates. C x  and C y  are determined in reference calibration runs with known beam positions.       

   The balance of the currents from the end of the vertical plate  22  determine the Y coordinate and the balance of the currents from the end of the horizontal plate  24  determine the X coordinate. The voltage readouts V are used to check the consistency of the measured currents or can be used to set an alarm signal or to lock the beam in the center position. 
   The absorbers  22 ,  24  are preferably formed of thermo-resistive material. An especially preferred thermo-resistive material of construction for the absorbers is chemical vapor deposition (CVD) silicon carbide (SiC). CVD SiC is a chemically inert, extremely radiation-hard, thermo-resistive semiconductor capable of withstanding working temperatures up to 2000 degrees Kelvin, with its electrical resistivity very sensitive to temperature. The good thermoconductivity of CVD SiC enables it to be used in high-current particle beams. 
   The beam position detector  20  of  FIG. 1  consists of two CVD SiC plates  22 ,  24  positioned orthogonal to the beam direction or path  26 , one vertically  22  and one horizontally  24 . The width of the plates  22 ,  24  would correspond to the designed area on the dump where the beam must be directed. The correctly positioned high-energy charged particle beam would cause a measurable temperature increase in both plates and a misdirected beam would be indicated by a missing signal in one or both of the plates. 
     FIGS. 4-9  present the calibration data obtained on the prototype Beam Position Detector (BPD) device corresponding to the preferred embodiment shown in  FIG. 1 , set up inside the electron beam line of an electron accelerator at the vicinity of the beam dump. The conventional Beam Position Monitors (BPMs) were installed approximately 30 meters upstream from the BPD and a few meters upstream of the relatively thick experimental target. The BPMs were thus used to measure the position of the beam prior to the thick helium target, where the quality is good and a conventional beam position monitor is adequate. The quality focused electron beam with energies from 1 to 5 GeV and beam currents in the range of 1 to 120 μA, with transverse dimensions of the order of 0.1 mm by 0.1 mm at the target, was dispersed to the transverse size of about 1 cm by 1 cm at the BPM position by scattering in the target. The calibrations were performed at a beam current of 30 μA. The symbols in the plots represent a series of measurements, one measurement every 10 seconds. The resultant BPM data is used to project the beam position to the place where the BPD is installed, assuming there is no non-linear beam deflection by magnetic fields around the beam line. The correlation of BPD and BPM readings is clearly seen. 
     FIG. 4  depicts a plot of the x-coordinate of the conventional beam position monitor (X BPM ) projected to the location of the beam position detector versus time.  FIG. 5  is a plot of the x-coordinate output of the beam position detector (X BPD ) of  FIG. 1  versus time for the same time frame studied in  FIG. 4 . A scatter plot was then made in  FIG. 6  plotting X BPD  versus X BPM . 
     FIG. 7  is a plot of the y-coordinate output of the conventional beam position monitor (Y BPM ) projected to the location of the beam position detector versus time.  FIG. 8  is a plot of the y-coordinate output of the beam position detector (Y BPD ) of  FIG. 1  versus time for the same time frame studied in  FIG. 7 .  FIG. 9  is a scatter plot of Y BPD  versus Y BPM . 
   As previously stated, the BPM is used to measure the position of the beam prior to the target, at a point where the quality of the beam is good. X BPM  and Y BPM  are therefore extrapolated to the BPD position using BPM readings. X BPD  is calculated using the balance of currents from the first absorber plate  22  of the BPD of  FIG. 1 . Y BPD  is calculated using the balance of currents from the second absorber plate  24  of the BPD of  FIG. 1 . The plot of X BPD  versus X BPM  and the plot of Y BPD  versus Y BPM  show clear correlation. 
   With reference to  FIG. 11 , a first alternate embodiment of the beam position monitor  50  includes sets of thin parallel metallic conductors  52  deposited on the plates  22 ,  24  to provide more detailed information on the beam size and profile. The electrical resistance of the sets of thin parallel metallic conductors would be much smaller than the resistance of the plate itself between the two conductors  52 . A voltage is applied between the first and the last conductor on a plate to create a voltage distribution across the plate, which is then measured at the ends of the conductors. Directing a high-energy particle beam on the plates  22 ,  24  between two conductors will cause local heating of the plate thereby changing the resistance, and, correspondingly, the measured voltage between the conductors, thus locating the position of the beam across the plate. The pattern of changed voltage distribution measured at every conductor can be used to evaluate the beam profile across the plate. Using two orthogonal plates, one can measure the detailed beam distribution in horizontal and vertical directions. 
   Referring to  FIG. 10  there is depicted a second alternate embodiment of a particle beam position monitor  60  according to the present invention. The monitor  60  includes four resistive absorbers set across the beam path  26 . Two absorbers  62 ,  64  are positioned orthogonal and vertically with respect to the beam path  26  and two absorbers  66 ,  68  are positioned orthogonal and horizontally with respect to the beam path  26 . Each absorber provides information about a slice of the beam profile. The full beam profile can be obtained using the combined information from all of the absorbers. 
   Another alternate embodiment of the beam position detector, not depicted herein, would include similar absorbers made of a transparent thermo-resistive glass material and be used to monitor the position of a powerful laser beam. A small fraction of the laser beam power dissipated in the glass absorber will cause local heating and thus a measurable resistance change. 
   The preferred embodiment of the beam position monitor  20  shown in  FIG. 1  features a simplified construction, using one vertical  22  and one horizontal  24  plate. The CVD SiC plates are preferably 50 mm width, 200 mm length, and 0.25 mm thick. Using the CVD SiC provides several advantages, including thermoconductivity comparable with copper and beryllium, high stiffness, machinability, stable up to 2000 degrees Kelvin, a resistance of between 200 and 600 kohm measured across the long ends of the rectangular plates, and high resistance response with resistivity falling 100 times in the temperature range of 50 to 500 degrees C. The CVD SiC plates are also resistant to degradation due to plasmas, acids, bases, and radiation. 
   Experimentation with different experimental targets shows that the response from the BPD may depend on the target, and therefore each target must be calibrated separately. Different targets scatter the beam differently, producing different beam spot sizes and generating different numbers of secondary electrons, which go in the line along with the beam, with smaller energies. The magnetic fields along the beamline deviate these lower energy electrons more easily than the main beam, thus producing non-symmetric beam image at the BPD. Effectively, it may shift the BPD readout along the major direction of the deviation. 
   Alternatively, the absorbers could be constructed of wires constructed of CVD SiC. The wires would be strung parallel to one another to form a rectangular shape, with the wires running longitudinally along the rectangular-shaped absorber. Each wire, if hit by a particle beam, would have its temperature elevated, thus allowing it to be detected by measuring its resistance. 
   Having thus described the invention with reference to a preferred embodiment, it is to be understood that the invention is not so limited by the description herein but is defined as follows by the appended claims.