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A novel operation mode of a tandem electrostatic accelerator for producing low energy ion beams
Nuclear Instruments and Methods in Physics Research A 348 (1994) 13-14 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Secbon A
A novel operation mode of a tandem electrostatic accelerator for producing low energy ion beams M.M. Narayanan *, D. Kanjilal, S. Chopra, A.K. Sinha Nuclear Science Centre, Aruna Asaf Ali Marg, New Delhi-67, lndia
Received 23 March 1994
An alternative method for producing low energy ion beams from a large tandem accelerator is described. In this method the negative ions are allowed to decelerate after their acceleration up to the high voltage terminal. They are stripped only at the second stripper located further down in the accelerating column. Results of some preliminary tests are discussed.
In a tandem Van de Graaff accelerator the injected singly charged negative ions are accelerated up to the terminal kept at a high positive potential. The tandem accelerators are provided with a stripper assembly at the terminal for converting negatively charged ions into positive ions which are then further accelerated. The resulting energy of the beams approximately equals ( q + 1 ) × terminal voltage (here q is the selected charge state of the ions after stripping). In some of the accelerators a second stripper assembly is provided after a few acceleration units beyond the terminal which helps in enhancing the beam energy by further stripping to higher charge states. This increases the range of the beam energy available from such type of accelerators. In order to go to lower energies, such machines are operated with a gas stripper so as to get a reasonable intensity of the lower charge states o r / a n d by shorting some units to lower the terminal voltage. To get low energy beams of light ions from such accelerators a large number of units have to be shorted which is a time consuming process and can cause deconditioning of the shorted units. In this paper another approach to deliver beam of lower energy from large tandem machines is presented. In this method the strippers in the terminal are not used. Instead the negative ions pass through the terminal. They are stripped at the second stripper located further down in the accelerating column. As a result the negative ions experience an amount of deceleration before reaching the second stripper and the whole machine acts as a tandem accelerator with a reduced effective terminal potential. This reduction in the effective terminal voltage depends on the location of the second stripper and the number of shorted units. Some preliminary tests of this mode of tandem * Corresponding author.
operation which we call "decelerating m o d e " have been conducted with 15UD Pelletron accelerator at the Nuclear Science Centre (NSC), New Delhi. The NSC Pelletron [1] is a vertical 15 UD tandem accelerator supplied by Electrostatic International Inc., USA. There are 15 units each capable of holding slightly more than 1 MV on both sides of the high voltage terminal. There are two dead sections six units away from the terminal on both low energy and high energy sides. The vacuum pumps and the electron traps are housed in these dead sections. The accelerator is provided with a foil stripper assembly and a gas stripper assembly in the terminal. This is followed by another foil stripper assembly in the dead section (HEDS) located on the high energy side. With no units shorted in the "decelerating m o d e " of operation the effective potential through which the particles get accelerated will be 60% of the actual terminal potential. Proton and carbon beams were accelerated in the decelerating mode and in the normal mode in which only a terminal stripper is used. The terminal was kept at 14 MV potential and in the decelerating mode the effective accelerating potential will be 8.4 MV. The ion optical elements before the accelerator were only slightly adjusted in the "decelerating mode" from their optimised values for the normal mode of operation using terminal stripper. The transmissions through the accelerator in both modes of operation are shown in Table 1 for protons and carbon. It is found that the analysed current for protons after acceleration was smaller by a factor of 3. For carbon the above factor is different for different charge states. While looking at these comparisons the things to be considered are that the energy of the beam incident at the second stripper is reduced by a factor of 0.6 and they are stripped by thicker foils mounted in the second stripper (thickness
0168-9002/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0168 -9002(94)00513 - 7
14
M.M. Narayanan et al. / Nucl. Instr. and Meth. in Phys. Res. A 348 (1994) 13-14
Table 1 The beam currents (enA) of proton and carbon at various stages in the normal mode and the decelerating mode of operation at 14 MV terminal potential
Injected current Current just after the accelerator Analysed current
Normal mode
Deceleration mode
Proton
Proton
62 54 30
Carbon 95 340 16( + 4) 46( + 5) 34( + 6)
62 54 10
Carbon 95 245 8( + 4) 15( + 5) 2.5( + 6)
of the stripper foils used in the HEDS is 10-15 p , g / c m 2 as compared to the thickness of those in the terminal which is 5 txg/cm2). The beam gets flared up by the time it reaches the HEDS in the normal acceleration mode as the beam waist is at the gas stripper canal in the terminal. Since the ions are subjected to some deceleration down to the HEDS the flaring is further increased. However, the measurements
indicate that there is no transmission loss inside the accelerator for protons while for carbon ions approximately a loss of 30% can be seen. From studying the beam profiles outside the tank and correlating those with the currents after the accelerator and after analyser magnet it is felt that the acceptance of the energy analysing magnet also limits the amount of the analysed beams. In another test, we extracted a 7 MeV proton beam from our machine which is lower than the minimum proton energy of 8 MeV deliverable in the normal mode of operation by shorting 11 units. This test was done with an actual terminal potential of 8.5 MV and with only 5 units shorted. Satisfactory performance was obtained. The proposed decelerating mode of Pelletron operation is shown to be a viable method for delivering the low energy light ion beams quickly without shorting of the units of tandem accelerators. Also it will be possible to deliver very low energies by shorting some of the units.
Reference [1] D. Kanjilal et al., Nucl. Instr, and Meth. A 328 (1993) 97.
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Secbon A
A novel operation mode of a tandem electrostatic accelerator for producing low energy ion beams M.M. Narayanan *, D. Kanjilal, S. Chopra, A.K. Sinha Nuclear Science Centre, Aruna Asaf Ali Marg, New Delhi-67, lndia
Received 23 March 1994
An alternative method for producing low energy ion beams from a large tandem accelerator is described. In this method the negative ions are allowed to decelerate after their acceleration up to the high voltage terminal. They are stripped only at the second stripper located further down in the accelerating column. Results of some preliminary tests are discussed.
In a tandem Van de Graaff accelerator the injected singly charged negative ions are accelerated up to the terminal kept at a high positive potential. The tandem accelerators are provided with a stripper assembly at the terminal for converting negatively charged ions into positive ions which are then further accelerated. The resulting energy of the beams approximately equals ( q + 1 ) × terminal voltage (here q is the selected charge state of the ions after stripping). In some of the accelerators a second stripper assembly is provided after a few acceleration units beyond the terminal which helps in enhancing the beam energy by further stripping to higher charge states. This increases the range of the beam energy available from such type of accelerators. In order to go to lower energies, such machines are operated with a gas stripper so as to get a reasonable intensity of the lower charge states o r / a n d by shorting some units to lower the terminal voltage. To get low energy beams of light ions from such accelerators a large number of units have to be shorted which is a time consuming process and can cause deconditioning of the shorted units. In this paper another approach to deliver beam of lower energy from large tandem machines is presented. In this method the strippers in the terminal are not used. Instead the negative ions pass through the terminal. They are stripped at the second stripper located further down in the accelerating column. As a result the negative ions experience an amount of deceleration before reaching the second stripper and the whole machine acts as a tandem accelerator with a reduced effective terminal potential. This reduction in the effective terminal voltage depends on the location of the second stripper and the number of shorted units. Some preliminary tests of this mode of tandem * Corresponding author.
operation which we call "decelerating m o d e " have been conducted with 15UD Pelletron accelerator at the Nuclear Science Centre (NSC), New Delhi. The NSC Pelletron [1] is a vertical 15 UD tandem accelerator supplied by Electrostatic International Inc., USA. There are 15 units each capable of holding slightly more than 1 MV on both sides of the high voltage terminal. There are two dead sections six units away from the terminal on both low energy and high energy sides. The vacuum pumps and the electron traps are housed in these dead sections. The accelerator is provided with a foil stripper assembly and a gas stripper assembly in the terminal. This is followed by another foil stripper assembly in the dead section (HEDS) located on the high energy side. With no units shorted in the "decelerating m o d e " of operation the effective potential through which the particles get accelerated will be 60% of the actual terminal potential. Proton and carbon beams were accelerated in the decelerating mode and in the normal mode in which only a terminal stripper is used. The terminal was kept at 14 MV potential and in the decelerating mode the effective accelerating potential will be 8.4 MV. The ion optical elements before the accelerator were only slightly adjusted in the "decelerating mode" from their optimised values for the normal mode of operation using terminal stripper. The transmissions through the accelerator in both modes of operation are shown in Table 1 for protons and carbon. It is found that the analysed current for protons after acceleration was smaller by a factor of 3. For carbon the above factor is different for different charge states. While looking at these comparisons the things to be considered are that the energy of the beam incident at the second stripper is reduced by a factor of 0.6 and they are stripped by thicker foils mounted in the second stripper (thickness
0168-9002/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSDI 0168 -9002(94)00513 - 7
14
M.M. Narayanan et al. / Nucl. Instr. and Meth. in Phys. Res. A 348 (1994) 13-14
Table 1 The beam currents (enA) of proton and carbon at various stages in the normal mode and the decelerating mode of operation at 14 MV terminal potential
Injected current Current just after the accelerator Analysed current
Normal mode
Deceleration mode
Proton
Proton
62 54 30
Carbon 95 340 16( + 4) 46( + 5) 34( + 6)
62 54 10
Carbon 95 245 8( + 4) 15( + 5) 2.5( + 6)
of the stripper foils used in the HEDS is 10-15 p , g / c m 2 as compared to the thickness of those in the terminal which is 5 txg/cm2). The beam gets flared up by the time it reaches the HEDS in the normal acceleration mode as the beam waist is at the gas stripper canal in the terminal. Since the ions are subjected to some deceleration down to the HEDS the flaring is further increased. However, the measurements
indicate that there is no transmission loss inside the accelerator for protons while for carbon ions approximately a loss of 30% can be seen. From studying the beam profiles outside the tank and correlating those with the currents after the accelerator and after analyser magnet it is felt that the acceptance of the energy analysing magnet also limits the amount of the analysed beams. In another test, we extracted a 7 MeV proton beam from our machine which is lower than the minimum proton energy of 8 MeV deliverable in the normal mode of operation by shorting 11 units. This test was done with an actual terminal potential of 8.5 MV and with only 5 units shorted. Satisfactory performance was obtained. The proposed decelerating mode of Pelletron operation is shown to be a viable method for delivering the low energy light ion beams quickly without shorting of the units of tandem accelerators. Also it will be possible to deliver very low energies by shorting some of the units.
Reference [1] D. Kanjilal et al., Nucl. Instr, and Meth. A 328 (1993) 97.