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Beam structure studies of low-energy ion beams
1088
Nuclear Instruments and Methods in Physics Research B56f57 (1991) 1088-1090 North-Holland
Beam structure studies of low-energy ion beams * K. Saadatrnand
* *? J.D. Schneider, C, Ceisik and R.R. Stevens, Jr.
The ion beam structure at various axial positions along the beg-transport line has been monitory and studied utihzing a fluor screen and a video camera. The fluor material is ahnninum oxide that is plasma-jet sprayed onto the surface of an ahnninum or a water-cooled copper substrate. The visual representation of the beam structure is digitized and enhanced through use of faise-cotor coding and displayed on a TV monitor for the on-line viewing by the expe~m~~talist. Digitized video signais are stored for further off-line processing and extracting more information about the beam, such as beam profiles. This inexpensive and effective diagnostic enables the experimentahst to observe the real-time beam response (such as evolution of the beam structure, shifts in the beam intensity at various spatial locations within the beam perimeter, and shifts in the beam center and position) to parameter changes.
1. Introiktio~ A video diagnostic technique has beefi developed to monitor and characterize the two-dimensional beam-intensity profiles and local angular divergence of low-energy (25-35 keV, IO-80 mA) ion beams [l]. A fluor screen is placed in the low-energy beam transport (LEBT) line between two solenoid magnets, where the observed beam can be made converging, diverging, or nearly parallel. When the particle beam strikes the surface, a bluish-white fluorescence results that is directly proportional to the beam intensity at that location. A CCD camera and an image-grabber board (using an IBM AT as a host) is employed to observe and record the beam image. While this technique provides an on-line image for real-time viewing of the beam shape and structure, the recorded data can be further analyzed to give more detailed beam profiles in both transverse directions. Furthermore, the use of an optional, upstream shadowing wire grid allows the dete~nation of local beam divergence (with the knowledge of physical beam size, directly traceable to an area of an ellipse in phase space and to the beam emittance [ 11) and a measure of the convergence (or divergence) of the beam.
scanner (5.5 cm downstream of the ion source). The emittance scanner ]Z] is the standard diagnostic used for profile and emittance measurement of the low-energy ion beams. Fig. 1 shows the digitized image of a single 35keV beam pulse when the LEBT’s first solenoid current was set for converging beam. This beam has a peaked structure as expected. Fig. 2 shows the corresponding horizontal beam profile, along with the beam profile measured by the emittance scanner. The agreement between the beam profiles provided by these two different diagnostics, indicates the accuracy and the effectiveness of this visual technique. Fig. 3 is the beam intensity profile of a 25keV beam pulse, which shows a
2. Beam profile measurements In order to determine the beam profiles and investigate the accuracy of this visual diagnostic, the fluor was placed at the same axial position as the emittance * Work supported and funded by the US Department of Defense, US Army Strategic Defense Command, under the auspices of the Department of Energy. * * Grumman Corporate Research Center. 0168-583X/91/$03.50
Fig. 1. A 35-keV beam intensity profile; darker colors indicate lower intensity.
0 1991 - Eisevier Science Publishers B.V. (North-Holfand)
K. Saadatmond
et al. / Beam structure studies
0.8
-
Ruor l
scanner
1.5 Harfzontal
Dtstmce (cm)
Horizontal
Fig. 2. HorizontaI beam profile of fig. f.
Fig. 4. Horizontal
ring structure in the beam. This beam structure is believed to be the result of the improperly-matched beam perveance in the extractor optics. The extractor optics is designed for the 35keV ion beam, Fig. 4 is the horizontal profiIe of the beam of fig 3; the beam structure is also apparent in the profile measured by the emittance scanner. 3. Beam divergencemeasurements The standard two-dimensional
emittance-scanner phase-space (x-x
Fig. 3. A 25-keV beam intensity
profile;
tower intensity.
diagnostic or f-y’)
provides beam in-
2.5
Distance (cm)
beam profile of fig. 3.
formation taken over many beam pulses 123. However, in the fluor diagnostic technique, placing an upstream shadowing wire grid can provide four-dimensional phase-space (x-x’-y-y’) information in a single beam pulse [l]. This capability ailows the simultaneous determination of local beam divergence in both transverse planes. Furthermore, the comparison of the wire spacings of the grid and the shadow spacing on the flour gives a measure of the convergence (or divergence) of the beam, which quantifies the tilt of the phase-space ellipse. In the experiment reported here, a two-dimen-
darker colors indicate Fig. 5. A 3%keV beam intensity
profile
with the wire shadows.
XIV. ACCELERATOR
TECHNOLOGY
given shadowing wire size, the relative depth of the wire shadows with respect to the beam intensity at that location (if the wire were not present) is related to the local divergence of the beam fl]. Both methods give a la value of 6-8 mrad. The numbers for the local horizontal
beam divergence given above are for the integrated signal over the vertical dimension. However, since the data from the visual diagnostic contain the information about the x-x’-y plane, the local horizontal beam divergence for every vertical position can be determined. In almost ah m~surements, the local horizontal beam divergence in the beam center was 5-10 times larger than the values at the edge of the beam. d 0
100
Horizontal
200 Distance
300 (pixel#)
Fig. 6. Horizontal beam profile of fig. 5 (integrated over the
vertical dimension). sional wire grid (mesh) was not avaiIable and a grid containing onfy vertical wires (alternating wire diameters of 10, 20 and 30 mils) was placed 5 cm upstream of the fluor. The fluor was placed 65 cm downstream of the ion source. This ~aogement permits mapping a ~re~imension~ phase space (x-x/-y). Fig. 5 is a digitized image of a 3%keV beam (first solenoid current was set for parallel beam) along with the associated wire shadows. The beam is 5 cm in diameter, with a peaked structure similar to that observed at the axial position closer to the ion source (fig. 1). In order to make a direct comparison between this new visual technique and the standard emittance scanner, the visual data is integrated over the vertical dimension and the result, the beam intensity vs horizontal dimension, is shown in fig. 6. The one-dimensions, beam-profile intensity is peaked and looks more parabolic than Gaussian, in agreement with the emittance scanner profiles. For a
4. Conclusion This diagnostic has proven its usefulness and effectiveness by providing real-time visual knowledge of the beam structure, shape and size. However, its promising capability for providing simultaneous, quantitative information about the four-dimensional x-x’-y-y’ plane make this diagnostic a primary candidate for any particle beam system.
We would like to thank J.H. Marquardt, N. Nereson and M.T. Smith for their assistance in the experimental setup. We would also like to thank W.B. Ingalls and J.D. Sherman for the operation of the injector.
References [l] K. Saadatmand, J.D. Schneider, C. Geisik and R.R. Stevens, Jr., Proc. Linac Conf., Albuquerque, NM, Z990, p. 465. &?] F.W. Allison, J.D. Sherman and D.B. Hoi&, IEEE Trans. NucI. Sci. NS-30 (4) (1983) 22Q4.
Nuclear Instruments and Methods in Physics Research B56f57 (1991) 1088-1090 North-Holland
Beam structure studies of low-energy ion beams * K. Saadatrnand
* *? J.D. Schneider, C, Ceisik and R.R. Stevens, Jr.
The ion beam structure at various axial positions along the beg-transport line has been monitory and studied utihzing a fluor screen and a video camera. The fluor material is ahnninum oxide that is plasma-jet sprayed onto the surface of an ahnninum or a water-cooled copper substrate. The visual representation of the beam structure is digitized and enhanced through use of faise-cotor coding and displayed on a TV monitor for the on-line viewing by the expe~m~~talist. Digitized video signais are stored for further off-line processing and extracting more information about the beam, such as beam profiles. This inexpensive and effective diagnostic enables the experimentahst to observe the real-time beam response (such as evolution of the beam structure, shifts in the beam intensity at various spatial locations within the beam perimeter, and shifts in the beam center and position) to parameter changes.
1. Introiktio~ A video diagnostic technique has beefi developed to monitor and characterize the two-dimensional beam-intensity profiles and local angular divergence of low-energy (25-35 keV, IO-80 mA) ion beams [l]. A fluor screen is placed in the low-energy beam transport (LEBT) line between two solenoid magnets, where the observed beam can be made converging, diverging, or nearly parallel. When the particle beam strikes the surface, a bluish-white fluorescence results that is directly proportional to the beam intensity at that location. A CCD camera and an image-grabber board (using an IBM AT as a host) is employed to observe and record the beam image. While this technique provides an on-line image for real-time viewing of the beam shape and structure, the recorded data can be further analyzed to give more detailed beam profiles in both transverse directions. Furthermore, the use of an optional, upstream shadowing wire grid allows the dete~nation of local beam divergence (with the knowledge of physical beam size, directly traceable to an area of an ellipse in phase space and to the beam emittance [ 11) and a measure of the convergence (or divergence) of the beam.
scanner (5.5 cm downstream of the ion source). The emittance scanner ]Z] is the standard diagnostic used for profile and emittance measurement of the low-energy ion beams. Fig. 1 shows the digitized image of a single 35keV beam pulse when the LEBT’s first solenoid current was set for converging beam. This beam has a peaked structure as expected. Fig. 2 shows the corresponding horizontal beam profile, along with the beam profile measured by the emittance scanner. The agreement between the beam profiles provided by these two different diagnostics, indicates the accuracy and the effectiveness of this visual technique. Fig. 3 is the beam intensity profile of a 25keV beam pulse, which shows a
2. Beam profile measurements In order to determine the beam profiles and investigate the accuracy of this visual diagnostic, the fluor was placed at the same axial position as the emittance * Work supported and funded by the US Department of Defense, US Army Strategic Defense Command, under the auspices of the Department of Energy. * * Grumman Corporate Research Center. 0168-583X/91/$03.50
Fig. 1. A 35-keV beam intensity profile; darker colors indicate lower intensity.
0 1991 - Eisevier Science Publishers B.V. (North-Holfand)
K. Saadatmond
et al. / Beam structure studies
0.8
-
Ruor l
scanner
1.5 Harfzontal
Dtstmce (cm)
Horizontal
Fig. 2. HorizontaI beam profile of fig. f.
Fig. 4. Horizontal
ring structure in the beam. This beam structure is believed to be the result of the improperly-matched beam perveance in the extractor optics. The extractor optics is designed for the 35keV ion beam, Fig. 4 is the horizontal profiIe of the beam of fig 3; the beam structure is also apparent in the profile measured by the emittance scanner. 3. Beam divergencemeasurements The standard two-dimensional
emittance-scanner phase-space (x-x
Fig. 3. A 25-keV beam intensity
profile;
tower intensity.
diagnostic or f-y’)
provides beam in-
2.5
Distance (cm)
beam profile of fig. 3.
formation taken over many beam pulses 123. However, in the fluor diagnostic technique, placing an upstream shadowing wire grid can provide four-dimensional phase-space (x-x’-y-y’) information in a single beam pulse [l]. This capability ailows the simultaneous determination of local beam divergence in both transverse planes. Furthermore, the comparison of the wire spacings of the grid and the shadow spacing on the flour gives a measure of the convergence (or divergence) of the beam, which quantifies the tilt of the phase-space ellipse. In the experiment reported here, a two-dimen-
darker colors indicate Fig. 5. A 3%keV beam intensity
profile
with the wire shadows.
XIV. ACCELERATOR
TECHNOLOGY
given shadowing wire size, the relative depth of the wire shadows with respect to the beam intensity at that location (if the wire were not present) is related to the local divergence of the beam fl]. Both methods give a la value of 6-8 mrad. The numbers for the local horizontal
beam divergence given above are for the integrated signal over the vertical dimension. However, since the data from the visual diagnostic contain the information about the x-x’-y plane, the local horizontal beam divergence for every vertical position can be determined. In almost ah m~surements, the local horizontal beam divergence in the beam center was 5-10 times larger than the values at the edge of the beam. d 0
100
Horizontal
200 Distance
300 (pixel#)
Fig. 6. Horizontal beam profile of fig. 5 (integrated over the
vertical dimension). sional wire grid (mesh) was not avaiIable and a grid containing onfy vertical wires (alternating wire diameters of 10, 20 and 30 mils) was placed 5 cm upstream of the fluor. The fluor was placed 65 cm downstream of the ion source. This ~aogement permits mapping a ~re~imension~ phase space (x-x/-y). Fig. 5 is a digitized image of a 3%keV beam (first solenoid current was set for parallel beam) along with the associated wire shadows. The beam is 5 cm in diameter, with a peaked structure similar to that observed at the axial position closer to the ion source (fig. 1). In order to make a direct comparison between this new visual technique and the standard emittance scanner, the visual data is integrated over the vertical dimension and the result, the beam intensity vs horizontal dimension, is shown in fig. 6. The one-dimensions, beam-profile intensity is peaked and looks more parabolic than Gaussian, in agreement with the emittance scanner profiles. For a
4. Conclusion This diagnostic has proven its usefulness and effectiveness by providing real-time visual knowledge of the beam structure, shape and size. However, its promising capability for providing simultaneous, quantitative information about the four-dimensional x-x’-y-y’ plane make this diagnostic a primary candidate for any particle beam system.
We would like to thank J.H. Marquardt, N. Nereson and M.T. Smith for their assistance in the experimental setup. We would also like to thank W.B. Ingalls and J.D. Sherman for the operation of the injector.
References [l] K. Saadatmand, J.D. Schneider, C. Geisik and R.R. Stevens, Jr., Proc. Linac Conf., Albuquerque, NM, Z990, p. 465. &?] F.W. Allison, J.D. Sherman and D.B. Hoi&, IEEE Trans. NucI. Sci. NS-30 (4) (1983) 22Q4.