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Accelerator tube tests with a 5 MV Van de Graaff
82
ACCELERATOR G. KORSCHINEK
Nuclear Instruments and Methods in Physics Research 220 (1984) 82-85 North-Holland, Amsterdam
TUBE TESTS
WITH
A 5 MV VAN DE GRAAFF
a n d J. H E L D
Fachbereich Physik, Technische Universitat Miinchen, 8046 Garching, IV. Germany A. ISOYA
Dep. of Physics, Faculty of Science, Kyushu University, Japan W. ASSMANN
Sektion Physik der Ludwig Maximilians Universiti~t Miinchen, W. Germany H. MONZER
Beschleunigerlaboratorium der Ludwig Maximilians u. Technischen Universiti~t Miinchen, IV. Germany A test Van de Graaff has been installed. By using the column of a KN-3000 HVEC machine, it was possible to achieve voltages above 5 MV. With this facility, accelerator tube tests have been made. It was found that after treating the tubes by a low voltage arc discharge the achieved gradient of 3 M V / m along the tubes was about 50% higher than in other presently used electrostatic accelerators.
1. Introduction During the past few years, m a n y large t a n d e m accelerators have been upgraded to remarkable voltages.
Also, new machines have been constructed which are now u n d e r voltage. Every machine is limited in voltage for particular reasons, however it is well k n o w n that most large accelerators have reached higher terminal
cpy Gv
~ '---L -435m
Fig. 1. The Munich test Van de Graaff. 0 1 6 7 - 5 0 8 7 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
83
G. Korschinek et al. / Accelerator tube tests with a 5 M V Van de Graaff
voltages without tubes than with tubes. Even if a detailed discussion might show that the limitation in voltage is also due to other effects, an improvement in the voltage capability of the tubes is in most cases beneficial to an accelerator. Recently, a low voltage arc discharge has been shown to be an impressive method of improving the terminal voltage of a tandem accelerator [1]. An important point for applying this method is the use of bakeable tubes. At the Munich tandem accelerator, we are using bakeable N E C tubes. After recently having finished the construction of a test Van de Graaff, it seemed to be worthwhile to use this to study the influence of a gaseous discharge on the voltage performance of N E C tubes.
2. T h e M u n i c h test V a n de Graaff
The main components of this high voltage facility are the column, the pressure tank and the gas handling system (fig. 1). (i) The column is a H V E C belt generator originally designed for a 3 MV machine (N2, CO 2 as insulating gas). The active column has a length of 172 cm. (ii) The pressure tank is somewhat larger than commonly used for machines of this voltage. (iii) The gas handling system consists of an oil free compressor, a vacuum pump with an oil trap and oil filters, a gas dryer and a pressure storage tank. After minor changes to the column and using SF6 as an insulating gas, we achieved a voltage above 5 MV (fig. 2). The voltage was measured w i t h a generating voltmeter. The dots in fig. 2 show the terminal voltage at breakdown. It is remarkable that even with a small machine it is possible to perform tests at rather high voltages and high gradients. As it is limited by sparks along the column, additional improvements might result in an even higher voltage.
IE
3. T h e tube tests
The order to study the influence of a low voltage arc discharge, an arrangement, as shown in fig. 3, was set up. For the tests we used five tubes. They were separated by decoupling units as usual. Only 1 m of the active column length was used. To avoid end effects the special tube termination was set up as shown in fig. 3. The oxide cathode at ground potential was situated about 10 cm away from the first tube. During the discharge treatment we used the needle valve at the termination for pressure control. During the discharge, only the turbo-molecular pump was connected. After stopping the discharge, the gas was pumped off and the vacuum system was switched to the cryopump. We applied a 1 A, 3 A, and 6 A current to the 5 tubes. From tests done on one unit at a test bench, it was clear that a current up to - 15 A ( - 40 V along the tube) does not damage the tube. Above - 1 5 A the electrodes started to warp. Table 1 shows the discharge parameters. As sputtering and the resultant deposition of sputtered material onto insulators is very dangerous with this treatment, we used hydrogen. The low mass results in a low sputter yield. The oxide cathode producing a low voltage drop also helps to minimize sputtering. Even under such circumstances we could not avoid sputtering onto the first insulator of the tube. At a pressure of around 2 × 10- 2 Torr we obtained the lowest discharge voltage of about 220 V. The outside measured temperature on the flanges, electrodes and insulators was always moderate. In the case of 6 A, the temperature was not higher than 120°C. This is different for the temperature of the inner electrodes, especially the diaphragms, which can reach a few hundred degrees. After treatment, we compared these tubes with untreated tubes (table 2.) During the high voltage measurements the vacuum was always in the range of 2 to 4 × 10 -8 Torr. As parameters we measured the X-ray activity, the vacuum, the conditioning time and the final achieved voltage in the test Van de Graaff. After conventionally heating with external electric heaters and heating the diaphragms electrically, we reached a rather high voltage of about 2.5 MV after 2 weeks of strong conditioning. However this voltage was unstable in time and always fell to a value of - 2 MV overnight. The observed X-ray level rising with the voltage was proba-
0~ o
Table 1 Typical parameters for the low voltage arc discharge (5 tubes) I 5
I 6 Pressure, SF6[ bar]
I
7
Fig. 2. The terminal voltage of the test Van de Graaff at different SF6 pressures.
Current Voltage Hydrogen Pressure [AI [Vl [Torr]
Temperature Time outside [°CI [hl
1 3 6
-45 -70 ~120
-240 -210 - 230
~ 2 × 1 0 -2 ~ 2 × 1 0 -2 ~ 2x10 -2
2 3 3
II. PROBLEMS/TUBES
84
Ca. Korschinek et al. / Accelerator tube tests with a 5 M V Van de Graaff
cryopump
172cm insulating length - - - ~ , " 100cm
I1
\
window
!
// .
.
.
.
'cathod
°'YA
rntnat / ~ l
turbom. /
)
---~
needle va~e
~
I: :g/:: ; ; ~
d"
U
w
Fig. 3. The experimental set up for the tube tests.
bly due to a kind of field emission. It went up exponentially and was not accompanied by any change of the vacuum. A rather different situation appeared after the discharge treatment. We did not observe any conditioning. The X-ray intensity was low but variable in time at 1 A, or practically unmeasurable in the case of the 3 A treatment. This shows that a drastic change in the surface conditions had occurred. In the case of the 6 A treatment the X-rays might be due to a fault when
starting the discharge. The vacuum was stable in the range of 2 to 4 x 10 -8 Torr and no changes were visible when raising the gradient. Almost instantly (within the range of one hour) we reached the highest voltages. The only limitation in voltage is the presently highest possible gradient of - 3 M V / m of the column of the test Van de Graaff. Only in the case of the 1 A treatment, which is probably too low a current, were sparks produced at lower gradients followed by strong outgassing.
Table 2 The results of the high voltage tests Electrical heating (up to 200°C)
Low voltage arc discharge
starting at - 1.6 MV, increases exponentially with the voltage
starting at - 1.5 MV, intensity changes with time
Vacuum during conditioning
strong outgassing
no outgassing
no outgassing
no outgassing
Conditioning time
2 weeks
Achieved voltage
max. - 2.5 MV
instantly - 2.3 MV
instantly - 2.7 MV
instantly - 3 MV
deconditioning to - 2 MV overnight
after a spark at 2.3 MV deconditioning of the tubes
limited by the column, no deconditioning with time
Treatment
X-ray intensity
Remarks
1A
3A
6A starting at - 1.6 MV, increases exponentially with time
G. Korschinek et al. / Accelerator tube tests with a 5 M V Van de Graaff
4. Discussion
We have shown that a low voltage arc discharge is an excellent method by which to improve the voltage capability of an NEC tube. These tests were performed in a small Van de Graaff which we have upgraded to a voltage of more than 5 MV. With the intention of treating such tubes within a larger machine, we should keep in mind the following two things. One has to do with the discharge itself. Having dead sections installed as in many large machines we have to avoid a large voltage drop after such a section to minimize sputtering. This might be solved by additional cathodes in the dead sections. The other point concerns the construction of the tubes themselves and could arise when achieving very high voltages at and beyond the rated voltage gradient of 1 MV for three tubes. It is known by calculation that N E C tubes do not allow charged particles, released from the diaphragms, to pass through more than two tube sections [2]. However this is not true for electrons which already at the rated voltage can
85
penetrate the titanium apertures (~< 0.5 mm thick) between adjacent tubes, if they gain the energy of two tubes. During the tests with 5 tubes there was no indication that these electrons or the releasing of secondary electrons from passing through the diaphragms had become a severe problem. But when using more tubes one cannot exclude such an effect. Therefore it might be worthwhile to think of thicker stopping diaphragms when working at very high gradients. We would like to thank Professor H. Morinaga for his encouragement and strong support of this project.
References
[1] A. Isoya, Y. Nakajima, T. Nakashima, N. Kato, K. Kobayashi, T. Sugimitsu, K. Kimura, L. Mitarai, T. Maeda and Y. Miyake, Proc. 3rd Int. Conf. on Electrostatic Accelerator Technology, Oak Ridge (IEEE Press, New York, 1981) p. 98. [2] R.G. Herb, ibid., p. 258.
II. PROBLEMS/TUBES
ACCELERATOR G. KORSCHINEK
Nuclear Instruments and Methods in Physics Research 220 (1984) 82-85 North-Holland, Amsterdam
TUBE TESTS
WITH
A 5 MV VAN DE GRAAFF
a n d J. H E L D
Fachbereich Physik, Technische Universitat Miinchen, 8046 Garching, IV. Germany A. ISOYA
Dep. of Physics, Faculty of Science, Kyushu University, Japan W. ASSMANN
Sektion Physik der Ludwig Maximilians Universiti~t Miinchen, W. Germany H. MONZER
Beschleunigerlaboratorium der Ludwig Maximilians u. Technischen Universiti~t Miinchen, IV. Germany A test Van de Graaff has been installed. By using the column of a KN-3000 HVEC machine, it was possible to achieve voltages above 5 MV. With this facility, accelerator tube tests have been made. It was found that after treating the tubes by a low voltage arc discharge the achieved gradient of 3 M V / m along the tubes was about 50% higher than in other presently used electrostatic accelerators.
1. Introduction During the past few years, m a n y large t a n d e m accelerators have been upgraded to remarkable voltages.
Also, new machines have been constructed which are now u n d e r voltage. Every machine is limited in voltage for particular reasons, however it is well k n o w n that most large accelerators have reached higher terminal
cpy Gv
~ '---L -435m
Fig. 1. The Munich test Van de Graaff. 0 1 6 7 - 5 0 8 7 / 8 4 / $ 0 3 . 0 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
83
G. Korschinek et al. / Accelerator tube tests with a 5 M V Van de Graaff
voltages without tubes than with tubes. Even if a detailed discussion might show that the limitation in voltage is also due to other effects, an improvement in the voltage capability of the tubes is in most cases beneficial to an accelerator. Recently, a low voltage arc discharge has been shown to be an impressive method of improving the terminal voltage of a tandem accelerator [1]. An important point for applying this method is the use of bakeable tubes. At the Munich tandem accelerator, we are using bakeable N E C tubes. After recently having finished the construction of a test Van de Graaff, it seemed to be worthwhile to use this to study the influence of a gaseous discharge on the voltage performance of N E C tubes.
2. T h e M u n i c h test V a n de Graaff
The main components of this high voltage facility are the column, the pressure tank and the gas handling system (fig. 1). (i) The column is a H V E C belt generator originally designed for a 3 MV machine (N2, CO 2 as insulating gas). The active column has a length of 172 cm. (ii) The pressure tank is somewhat larger than commonly used for machines of this voltage. (iii) The gas handling system consists of an oil free compressor, a vacuum pump with an oil trap and oil filters, a gas dryer and a pressure storage tank. After minor changes to the column and using SF6 as an insulating gas, we achieved a voltage above 5 MV (fig. 2). The voltage was measured w i t h a generating voltmeter. The dots in fig. 2 show the terminal voltage at breakdown. It is remarkable that even with a small machine it is possible to perform tests at rather high voltages and high gradients. As it is limited by sparks along the column, additional improvements might result in an even higher voltage.
IE
3. T h e tube tests
The order to study the influence of a low voltage arc discharge, an arrangement, as shown in fig. 3, was set up. For the tests we used five tubes. They were separated by decoupling units as usual. Only 1 m of the active column length was used. To avoid end effects the special tube termination was set up as shown in fig. 3. The oxide cathode at ground potential was situated about 10 cm away from the first tube. During the discharge treatment we used the needle valve at the termination for pressure control. During the discharge, only the turbo-molecular pump was connected. After stopping the discharge, the gas was pumped off and the vacuum system was switched to the cryopump. We applied a 1 A, 3 A, and 6 A current to the 5 tubes. From tests done on one unit at a test bench, it was clear that a current up to - 15 A ( - 40 V along the tube) does not damage the tube. Above - 1 5 A the electrodes started to warp. Table 1 shows the discharge parameters. As sputtering and the resultant deposition of sputtered material onto insulators is very dangerous with this treatment, we used hydrogen. The low mass results in a low sputter yield. The oxide cathode producing a low voltage drop also helps to minimize sputtering. Even under such circumstances we could not avoid sputtering onto the first insulator of the tube. At a pressure of around 2 × 10- 2 Torr we obtained the lowest discharge voltage of about 220 V. The outside measured temperature on the flanges, electrodes and insulators was always moderate. In the case of 6 A, the temperature was not higher than 120°C. This is different for the temperature of the inner electrodes, especially the diaphragms, which can reach a few hundred degrees. After treatment, we compared these tubes with untreated tubes (table 2.) During the high voltage measurements the vacuum was always in the range of 2 to 4 × 10 -8 Torr. As parameters we measured the X-ray activity, the vacuum, the conditioning time and the final achieved voltage in the test Van de Graaff. After conventionally heating with external electric heaters and heating the diaphragms electrically, we reached a rather high voltage of about 2.5 MV after 2 weeks of strong conditioning. However this voltage was unstable in time and always fell to a value of - 2 MV overnight. The observed X-ray level rising with the voltage was proba-
0~ o
Table 1 Typical parameters for the low voltage arc discharge (5 tubes) I 5
I 6 Pressure, SF6[ bar]
I
7
Fig. 2. The terminal voltage of the test Van de Graaff at different SF6 pressures.
Current Voltage Hydrogen Pressure [AI [Vl [Torr]
Temperature Time outside [°CI [hl
1 3 6
-45 -70 ~120
-240 -210 - 230
~ 2 × 1 0 -2 ~ 2 × 1 0 -2 ~ 2x10 -2
2 3 3
II. PROBLEMS/TUBES
84
Ca. Korschinek et al. / Accelerator tube tests with a 5 M V Van de Graaff
cryopump
172cm insulating length - - - ~ , " 100cm
I1
\
window
!
// .
.
.
.
'cathod
°'YA
rntnat / ~ l
turbom. /
)
---~
needle va~e
~
I: :g/:: ; ; ~
d"
U
w
Fig. 3. The experimental set up for the tube tests.
bly due to a kind of field emission. It went up exponentially and was not accompanied by any change of the vacuum. A rather different situation appeared after the discharge treatment. We did not observe any conditioning. The X-ray intensity was low but variable in time at 1 A, or practically unmeasurable in the case of the 3 A treatment. This shows that a drastic change in the surface conditions had occurred. In the case of the 6 A treatment the X-rays might be due to a fault when
starting the discharge. The vacuum was stable in the range of 2 to 4 x 10 -8 Torr and no changes were visible when raising the gradient. Almost instantly (within the range of one hour) we reached the highest voltages. The only limitation in voltage is the presently highest possible gradient of - 3 M V / m of the column of the test Van de Graaff. Only in the case of the 1 A treatment, which is probably too low a current, were sparks produced at lower gradients followed by strong outgassing.
Table 2 The results of the high voltage tests Electrical heating (up to 200°C)
Low voltage arc discharge
starting at - 1.6 MV, increases exponentially with the voltage
starting at - 1.5 MV, intensity changes with time
Vacuum during conditioning
strong outgassing
no outgassing
no outgassing
no outgassing
Conditioning time
2 weeks
Achieved voltage
max. - 2.5 MV
instantly - 2.3 MV
instantly - 2.7 MV
instantly - 3 MV
deconditioning to - 2 MV overnight
after a spark at 2.3 MV deconditioning of the tubes
limited by the column, no deconditioning with time
Treatment
X-ray intensity
Remarks
1A
3A
6A starting at - 1.6 MV, increases exponentially with time
G. Korschinek et al. / Accelerator tube tests with a 5 M V Van de Graaff
4. Discussion
We have shown that a low voltage arc discharge is an excellent method by which to improve the voltage capability of an NEC tube. These tests were performed in a small Van de Graaff which we have upgraded to a voltage of more than 5 MV. With the intention of treating such tubes within a larger machine, we should keep in mind the following two things. One has to do with the discharge itself. Having dead sections installed as in many large machines we have to avoid a large voltage drop after such a section to minimize sputtering. This might be solved by additional cathodes in the dead sections. The other point concerns the construction of the tubes themselves and could arise when achieving very high voltages at and beyond the rated voltage gradient of 1 MV for three tubes. It is known by calculation that N E C tubes do not allow charged particles, released from the diaphragms, to pass through more than two tube sections [2]. However this is not true for electrons which already at the rated voltage can
85
penetrate the titanium apertures (~< 0.5 mm thick) between adjacent tubes, if they gain the energy of two tubes. During the tests with 5 tubes there was no indication that these electrons or the releasing of secondary electrons from passing through the diaphragms had become a severe problem. But when using more tubes one cannot exclude such an effect. Therefore it might be worthwhile to think of thicker stopping diaphragms when working at very high gradients. We would like to thank Professor H. Morinaga for his encouragement and strong support of this project.
References
[1] A. Isoya, Y. Nakajima, T. Nakashima, N. Kato, K. Kobayashi, T. Sugimitsu, K. Kimura, L. Mitarai, T. Maeda and Y. Miyake, Proc. 3rd Int. Conf. on Electrostatic Accelerator Technology, Oak Ridge (IEEE Press, New York, 1981) p. 98. [2] R.G. Herb, ibid., p. 258.
II. PROBLEMS/TUBES