- Email: [email protected]
Automatic external filling for the ion source gas bottle of a Van de Graaff accelerator
__ __
&
Nuclear Instruments
and Methods in Physics Research B 129 (1997) 548-550
NIOMI B
Beam Interactions with Materials 6 Atoms
@
ELSEVIER
Automatic external filling for the ion source gas bottle of a Van de Graaff accelerator T. Bastin ‘, C. Dehove, P.D. Dumont, H. Garnir, A. Marchal, D. Strivay lnstitut de Physique Nucl&ire
* , G. Weber ’
Exp&-immtale, Unir;ersite’ de Likge. Sart Tilmun (B15/. B-4000 Liege. Belgium
Received
19 February
1997: revised form received 7 May 1997
Abstract We describe a fully automatic system we developed to fill, from an external gas bottle, the ion source terminal gas storage bottle of a 2 MV Van de Graaff accelerator without depressing the 25 bar insulating gas. The system is based on a programmable automate ordering electropneumatical valves. The only manual operation is the connection of the external gas cylinder. The time needed for a gas change is reduced to typically 15 min (depending on the residual pressure wished for the gas removed from the terminal bottle). To check this system we study the ionic composition of the ion beam delivered by our accelerator after different gas changes. The switching magnet of our accelerator was used to analyse the ionic composition of the accelerated beams in order to verify the degree of elimination of the previous gases in the system. 0 1997 Elsevier Science B.V. PACS: 29.27.A: 29.17; 07.77.K. 41.75.A Keywords:
lon source; Van de Graaff; Automatic
filling
1. Introduction
2. Gas changing set-up
The scientific production of our Institute (Beam-Foil Spectroscopy and Ion Beam Analysis) is based on use of two Van de Graaff accelerators of 2 and 2.5 MV. These accelerators are surrounded with tanks containing an insulating gas mixture N,-CO, at 25 bar. Our experiments require frequent changes in the ion beam nature. By depressing the Van de Graaff tank, a gas switching typically takes 24 h. As partial solution our accelerators were equipped with additional internal gas storage bottles located inside the high voltage terminal. A more attractive solution consists in installing an external gas supply system as realized recently by Raybum [ 1I. We improved this concept by developing a new fully automatic system to fill the ion source gas bottle without depressing the insulating gas of the Van de Graaff tank. We will present our gas changing set-up. The composition of the ion beam produced by the source under different filling conditions has also been analyzed. Data acquisition principle and results of this investigation will be presented.
The gas changing set-up is shown schematically on Fig. 1. This set-up is composed of tubings and 7 electropneumatical valves ordered in a given fashion by a micro programmable controller. Tubings inside and outside the Van de Graaff tank are respectively made of polypropylene and inox.
* Corresponding author. Fax: + 32-4-3662884; [email protected] ’ Research Assistant of the Belgian FNRS. ’ Research Associate of the Belgian FNRS.
Van de
Pnmarv
“BC”“rn
email:
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO168-583X(97)00352-2
I.
>
1 bar
Fig.
1.Gas changing set-up.
Graaff
tank
T. Bastin et al./Nucl. Table I Step-by-step -
operations
for changing
the gas nature inside the ion source bottle Operations
(0 = Open, C = Close)
Vacuum pump .
V?
v4
vs v, v, pr
p: P “Do wait until
at Pvoc-6
bar (action on V, or V,)
-
-
OorO
-
o-
-
-
-0 -
-
_
_
ON OFF
P i&3
core
cc-
4. Gas bottle filling with new gas 5. Gas bottle closing 6. Inlet tubing evacuation
0
_ oc
oc-
7. Inlet tubing filling with VDG gas
o_
l%al state
OFF
ooccccc
The pneumating valves are actuated by using a 6 bar pressurized circuitry. For valves V, to V,. this actuation is electrically ordered, whereas the valve V, inside the Van de Graaff tank is actuated by depressing the tubing between V, and V, at a pressure of 6 bar below that existing inside the tank. This is performed by use of a gas cylinder the tank at the required
pressure.
for changing the gas nature inside the ion source bottle are described in Table 1 hereafter, according to notations of Fig. 1. These opera-
5
10
15
20
_
-
P.llrn lo-’ Torr
-c P\U,r;
tions are conditioned by the pressures respectively measured inside the gas cylinder (P,) and at the beginning of the inlet tubing (P,). They are automatically performed by use of the programmable controller. In our gas changing system, only the connection of the external cylinder containing the new gas to put inside the ion source bottle is manual. This connection takes place at valve Vs. The time needed for a gas change is typically 15 min (depending on the residual pressure wished for the gas removed from the terminal bottle).
operations
0
Prtm P>I”, lo-’ Torr 14bar
-c -co
ON OFF
The step-by-step
v,
Pressures
ooccccc
OFF
2. inlet tubing evacuation 3. Gas bottle evacuation
outside
Resulting effects
Electrovalves
v, Initial state Steps I Gas cylinder pressurization
549
Instr. and Meth. in Phys. Rex B 129 (1997) 548-550
25
30
35
40
45
50
55
60
Mass (amu) Fig. 2. Typical mass spectrum of the Van de Graaff accelerator
beam when switching
from neon to helium gas.
5.50
7: Bastin et al./Nucl.
Instr. md Meth. in Phys. Kes. B I29 (1997) 548-550
3. Data acquisition The composition of the ion beam produced by the source under different filling conditions has been analysed by using the bending magnet of our accelerator as a mass analyser. The accelerator voltage has been kept fixed (1.0 MeV) and the ion beam current has been recorded as a function of the bending magnet magnetic field. The ion beam is collected in a large box (acting as a Faraday cup) situated 4 m away from the exit of the magnet. The beam enters the box through a small aperture of 5 mm diameter. The current is measured by a current digitizer (ORTEC 439) giving a pulse each time a charge of IO-’ C is collected. The magnetic field is measured by a nuclear magnetic resonance probe (DRUSCH RMN2) with a relative accuracy of 10 -5. The field is varied smoothly from 0.13 to 1.3 T. During the sweep, a computer program [2] monitors the beam current entering the Faraday box and the magnetic field. This program writes periodically these couples of values into an ASCII file, ready for further analysis by a spreadsheet program. For a fixed accelerator voltage, there is a direct relationship between the mass of the detected particle and the square of the corresponding RMN. It is thus possible to draw mass spectra of the beam composition. A typical example is given on Fig. 2.
4. Results The spectrum presented in Fig. 2 has been recorded just after switching from neon to helium gas. The bottle has been pumped for 15 min and pressurized at 14 bar with helium gas. The Fig. 2 clearly shows that the He+ beam is the strongest and overlaps all other components by at least a factor of 20. The residual Ne+ beam, due to trace of the previous gas filling, is still present but has almost disappeared. Many components appear in the mass spectrum. Beams of Hl, C+, N+, O+ and many molecular ions formed by these elements are always present in our accelerated beam. They are probably due to small leaks allowing the insula-
tion gas mixture (80% N,, 20% CO, and trace of water) to penetrate into the source. Mass 52, 56 and 58 are formed by sputtering on the stainless steel (Cr 18%. Fe 76%. Ni 6%) exit channel of the RF source [3]. Residues of a previous filling with argon gas are also visible in our mass spectrum. When charge exchanges are present in the beam line, spurious beams and continuums appear in the mass spectra. This phenomenon explains the higher background around mass 14 and 28 and the weak peak at mass 7 which is due to Nz’ ions produced, not in the source, but by electron stripping of NC molecules after acceleration and before mass analysis. Let us note that charge exchange processes induced by collision, are proportional to the product of the residual pressure in the beam line time the pipe length [4.5]. We have also carefully checked the possibility to obtain intense helium beam immediately after hydrogen filling of the gas bottle. With I.5 min hydrogen pumping and 14 bar pressurization with helium, more than 10 mA of analyzed helium beam is easily produced. The residual hydrogen Hz + beam being overpowered by a factor of more than 100.
Acknowledgements This work was supported by the Belgian Institut Interuniversitaire des Sciences Nucleaire and by the Fonds de la Recherche of the Liege University.
References [l] L.A. Rayburn, Nucl. Instr. and Meth. B 40/41 (1989) 1069. [2] F.S. Monjoie, H.P. Garnir. Camp. Phys. Commun. 61 (19901 267. [3] P.D. Dumont, H.P. Gamir, Y. Baudinet-Robinet, Nucl. lnstr. and Meth. 164 (1979) 193. [4] W.K. Chu. J.W. Mayer and M.A. Nicolet, Backscattering Spectroscopy (Academic Press, New York, 1978) chap. 6. [5] S. Picraux, J.A. Borders, R.A. Langley. Thin Solid Films 19 (19731371.
&
Nuclear Instruments
and Methods in Physics Research B 129 (1997) 548-550
NIOMI B
Beam Interactions with Materials 6 Atoms
@
ELSEVIER
Automatic external filling for the ion source gas bottle of a Van de Graaff accelerator T. Bastin ‘, C. Dehove, P.D. Dumont, H. Garnir, A. Marchal, D. Strivay lnstitut de Physique Nucl&ire
* , G. Weber ’
Exp&-immtale, Unir;ersite’ de Likge. Sart Tilmun (B15/. B-4000 Liege. Belgium
Received
19 February
1997: revised form received 7 May 1997
Abstract We describe a fully automatic system we developed to fill, from an external gas bottle, the ion source terminal gas storage bottle of a 2 MV Van de Graaff accelerator without depressing the 25 bar insulating gas. The system is based on a programmable automate ordering electropneumatical valves. The only manual operation is the connection of the external gas cylinder. The time needed for a gas change is reduced to typically 15 min (depending on the residual pressure wished for the gas removed from the terminal bottle). To check this system we study the ionic composition of the ion beam delivered by our accelerator after different gas changes. The switching magnet of our accelerator was used to analyse the ionic composition of the accelerated beams in order to verify the degree of elimination of the previous gases in the system. 0 1997 Elsevier Science B.V. PACS: 29.27.A: 29.17; 07.77.K. 41.75.A Keywords:
lon source; Van de Graaff; Automatic
filling
1. Introduction
2. Gas changing set-up
The scientific production of our Institute (Beam-Foil Spectroscopy and Ion Beam Analysis) is based on use of two Van de Graaff accelerators of 2 and 2.5 MV. These accelerators are surrounded with tanks containing an insulating gas mixture N,-CO, at 25 bar. Our experiments require frequent changes in the ion beam nature. By depressing the Van de Graaff tank, a gas switching typically takes 24 h. As partial solution our accelerators were equipped with additional internal gas storage bottles located inside the high voltage terminal. A more attractive solution consists in installing an external gas supply system as realized recently by Raybum [ 1I. We improved this concept by developing a new fully automatic system to fill the ion source gas bottle without depressing the insulating gas of the Van de Graaff tank. We will present our gas changing set-up. The composition of the ion beam produced by the source under different filling conditions has also been analyzed. Data acquisition principle and results of this investigation will be presented.
The gas changing set-up is shown schematically on Fig. 1. This set-up is composed of tubings and 7 electropneumatical valves ordered in a given fashion by a micro programmable controller. Tubings inside and outside the Van de Graaff tank are respectively made of polypropylene and inox.
* Corresponding author. Fax: + 32-4-3662884; [email protected] ’ Research Assistant of the Belgian FNRS. ’ Research Associate of the Belgian FNRS.
Van de
Pnmarv
“BC”“rn
email:
0168-583X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO168-583X(97)00352-2
I.
>
1 bar
Fig.
1.Gas changing set-up.
Graaff
tank
T. Bastin et al./Nucl. Table I Step-by-step -
operations
for changing
the gas nature inside the ion source bottle Operations
(0 = Open, C = Close)
Vacuum pump .
V?
v4
vs v, v, pr
p: P “Do wait until
at Pvoc-6
bar (action on V, or V,)
-
-
OorO
-
o-
-
-
-0 -
-
_
_
ON OFF
P i&3
core
cc-
4. Gas bottle filling with new gas 5. Gas bottle closing 6. Inlet tubing evacuation
0
_ oc
oc-
7. Inlet tubing filling with VDG gas
o_
l%al state
OFF
ooccccc
The pneumating valves are actuated by using a 6 bar pressurized circuitry. For valves V, to V,. this actuation is electrically ordered, whereas the valve V, inside the Van de Graaff tank is actuated by depressing the tubing between V, and V, at a pressure of 6 bar below that existing inside the tank. This is performed by use of a gas cylinder the tank at the required
pressure.
for changing the gas nature inside the ion source bottle are described in Table 1 hereafter, according to notations of Fig. 1. These opera-
5
10
15
20
_
-
P.llrn lo-’ Torr
-c P\U,r;
tions are conditioned by the pressures respectively measured inside the gas cylinder (P,) and at the beginning of the inlet tubing (P,). They are automatically performed by use of the programmable controller. In our gas changing system, only the connection of the external cylinder containing the new gas to put inside the ion source bottle is manual. This connection takes place at valve Vs. The time needed for a gas change is typically 15 min (depending on the residual pressure wished for the gas removed from the terminal bottle).
operations
0
Prtm P>I”, lo-’ Torr 14bar
-c -co
ON OFF
The step-by-step
v,
Pressures
ooccccc
OFF
2. inlet tubing evacuation 3. Gas bottle evacuation
outside
Resulting effects
Electrovalves
v, Initial state Steps I Gas cylinder pressurization
549
Instr. and Meth. in Phys. Rex B 129 (1997) 548-550
25
30
35
40
45
50
55
60
Mass (amu) Fig. 2. Typical mass spectrum of the Van de Graaff accelerator
beam when switching
from neon to helium gas.
5.50
7: Bastin et al./Nucl.
Instr. md Meth. in Phys. Kes. B I29 (1997) 548-550
3. Data acquisition The composition of the ion beam produced by the source under different filling conditions has been analysed by using the bending magnet of our accelerator as a mass analyser. The accelerator voltage has been kept fixed (1.0 MeV) and the ion beam current has been recorded as a function of the bending magnet magnetic field. The ion beam is collected in a large box (acting as a Faraday cup) situated 4 m away from the exit of the magnet. The beam enters the box through a small aperture of 5 mm diameter. The current is measured by a current digitizer (ORTEC 439) giving a pulse each time a charge of IO-’ C is collected. The magnetic field is measured by a nuclear magnetic resonance probe (DRUSCH RMN2) with a relative accuracy of 10 -5. The field is varied smoothly from 0.13 to 1.3 T. During the sweep, a computer program [2] monitors the beam current entering the Faraday box and the magnetic field. This program writes periodically these couples of values into an ASCII file, ready for further analysis by a spreadsheet program. For a fixed accelerator voltage, there is a direct relationship between the mass of the detected particle and the square of the corresponding RMN. It is thus possible to draw mass spectra of the beam composition. A typical example is given on Fig. 2.
4. Results The spectrum presented in Fig. 2 has been recorded just after switching from neon to helium gas. The bottle has been pumped for 15 min and pressurized at 14 bar with helium gas. The Fig. 2 clearly shows that the He+ beam is the strongest and overlaps all other components by at least a factor of 20. The residual Ne+ beam, due to trace of the previous gas filling, is still present but has almost disappeared. Many components appear in the mass spectrum. Beams of Hl, C+, N+, O+ and many molecular ions formed by these elements are always present in our accelerated beam. They are probably due to small leaks allowing the insula-
tion gas mixture (80% N,, 20% CO, and trace of water) to penetrate into the source. Mass 52, 56 and 58 are formed by sputtering on the stainless steel (Cr 18%. Fe 76%. Ni 6%) exit channel of the RF source [3]. Residues of a previous filling with argon gas are also visible in our mass spectrum. When charge exchanges are present in the beam line, spurious beams and continuums appear in the mass spectra. This phenomenon explains the higher background around mass 14 and 28 and the weak peak at mass 7 which is due to Nz’ ions produced, not in the source, but by electron stripping of NC molecules after acceleration and before mass analysis. Let us note that charge exchange processes induced by collision, are proportional to the product of the residual pressure in the beam line time the pipe length [4.5]. We have also carefully checked the possibility to obtain intense helium beam immediately after hydrogen filling of the gas bottle. With I.5 min hydrogen pumping and 14 bar pressurization with helium, more than 10 mA of analyzed helium beam is easily produced. The residual hydrogen Hz + beam being overpowered by a factor of more than 100.
Acknowledgements This work was supported by the Belgian Institut Interuniversitaire des Sciences Nucleaire and by the Fonds de la Recherche of the Liege University.
References [l] L.A. Rayburn, Nucl. Instr. and Meth. B 40/41 (1989) 1069. [2] F.S. Monjoie, H.P. Garnir. Camp. Phys. Commun. 61 (19901 267. [3] P.D. Dumont, H.P. Gamir, Y. Baudinet-Robinet, Nucl. lnstr. and Meth. 164 (1979) 193. [4] W.K. Chu. J.W. Mayer and M.A. Nicolet, Backscattering Spectroscopy (Academic Press, New York, 1978) chap. 6. [5] S. Picraux, J.A. Borders, R.A. Langley. Thin Solid Films 19 (19731371.