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Status report of the VICKSI CN-injector
50
STATUS P. A R N D T
Nuclear Instruments and Methods in Physics Research A244 (1986) 50-51 North-Holland, Amsterdam
REPORT OF THE V I C K S I C N - I N J E C T O R and the VICKSI-Group,
Hahn - Meaner- lnstitut fftr Kernforschung Berlin GmbH, Bereich Kern - und Strahlenphysik, Glienicker Strasse 100, D-1000 Berlin 39, Germany
In the accelerator facility VICKSI [1,2] at the Hahn-Meitner-lnstitute in Berlin a 6 MV Van de Graaff accelerator (HVEC CN-type) is used as injector for a separated sector isochronous cyclotron. The facility is in operation since 1978. In the following we will report about the operating experience with the CN-injector, which had to undergo a near complete reconstruction before it could be used as a heavy ion injector.
1. Introduction The facility was originally designed to accelerate ions with a maximum mass of 40 ainu to energies of up to 200 MeV. In the meantime, however, the mass range was extended considerably. The heaviest ion we have accelerated so far was'129Xe. Ion currents of 10-20 F A could be accelerated in the injector without any difficulty. A summary of ion beams and intensities behind the analysing magnet of the CN-injector is given in table 1.
2. The VICKSi CN-injector, problems and solutions In the beginning the N E C high vacuum accelerating tube, which was installed in the C N to make it suitable for heavy ion acceleration [3], caused some difficulties in reaching the design voltage of 6 MV without sparks.
Table 1 Electron current of ion beams out of the CN (examples)
4He 6Li JiB ~2C 14N 160
19F 2°Ne 32S
35C1 36Ar S6Kr
129Xe(nat)
i+
2 +
3+
4 +
5+
[~AI >1 1.3 3.5 10 10 10 6 10 6 4.5 10 10 10
[~A]
[~AI
[hAl
[hAl
0.7 0.6 0.6 0.1 10 0.5 0.18 10 6 10
0.012 0.01 0.7 0.06 0.06 1.5 2 0.5
20
150 500 300
20 15 75
0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
But after about 6 - 8 months of operation we were able to reach the 6 MV without problems. In the beginning the tube also had several small vacuum leaks. After these initial difficulties during the first year the injector has been operating at voltages between 5.8 and 6 MV without longer conditioning periods ( < 2 - 8 h). It is particularly important to perform a careful optical alignment of the tube during installation, to avoid any bend or offset in the tube, which could cause the beam to hit internal apertures and to generate sparks. The vacuum in the tube is now routinely - 1 0 -9 hPa. This vacuum is maintained by a differential pumping system in the terminal [3,5,6] and by pumps in the beam line behind the CN-injector. Even after venting the accelerator tube this good vacuum is reached after 3 - 4 d of pumping. However, it is important that the venting is done slowly and with dry nitrogen. Because of the larger dimensions and the higher weight of the new terminal (3 m long, 1.2 m O and - 2000 kp) [3] the mechanical stability of the CN-column was not sufficient. This problem was solved by installing an intermediate plate in the middle of the column. Due to the larger dimensions of the terminal [3] and the necessary shape of the spinning electrical fields of up to 20 M V / m were obtained [4]. This and the fact, that we wanted to get away from the high pressure of 16 bar, necessary with the isolation gas-mixture of N 2 - C O 2 + 105[ SF6, made us switch to pure SF6 at a pressure of 6 bar. Electrostatically this was successful, it caused however some difficulties with the charge transport by the HVEC belt (cf. ref. [7]). Difficulties in the terminal were associated with the lifetime of ion sources and with the Wienfiher for charge state selection [5,6]. In both units short circuits developed after relative short operating times. After a series of modifications and off-line tests ion source service needs to be scheduled every two to three weeks,
P. Arndt et al. / VICKSI-CN injector
51
Table 2 Summary of CN-injector properties and operational data
Specifdcation: Type Tank dimensions Gas Terminal dimensions Column Charge-system Accelerator tubes Vacuum Ion source Terminal charge separator Bunching Beam transport Control
6 MV-CN-type Van de Graaff (HVEC) 8.6m, O 2.5m SF6 (99%), 6 bar 3.05m, O 1.2m HVEC-standard HVEC black type belt, 120/tA, 17 m / s 20 standard NEC-modules turbo- and ion-getter pumps 1 x 10 -9 hPa (base), I x 10 -6 hPa (terminal) (a) cold-PIG-source: p, D, 3He, 12C, 129 Xe (b) field emitting source: 6 : Li ExB-filter 5-20 MHz, 60 ° analysing magnet: 0 = 1.5 m, BPma~= 280 ( M E / Z 2) steerers, quadrupoles computerized control system [9] based on DEC computers and CAMAC interfaces
Performance Start of operation Maximum voltage without tube Maximum voltage with tube Normal operation voltage Operating time Belt lifetime Tube lifetime Operation schedule Future plans
1964 ( H V E C machine), 1976 (modified machine) 9 MV 6.2 M V 5.6 M V - 7000 h / y
3000-14000 h 40000 h (accumulated until now, so far no deterioration) 24 h / d , 7d/week, 25 tank openings/y 8 h routine service every 2 weeks new charging belt, fast capacitive regulation system
only. Similarly the Wienfilter has a lifetime of a b o u t three months, now. All other terminal c o m p o n e n t s used for the ion optics, the power supplies as well as the infrared d a t a transmission [8] link from g r o u n d to terminal level have been r u n n i n g without p r o b l e m s a n d w i t h o u t significant failures. Table 2 summarizes the present C N - i n j e c t o r properties a n d operational data.
3. Future plans F o r the future we plan the installation of a belt, which can be glued together, thus eliminating the need to completely d i s m a n t l e a n d rebuild the c o l u m n for each belt change. F u r t h e r m o r e we are in the process of installing a capacitive fast regulating system for the terminal voltage to further reduce the terminal ripple.
References
[1] K.H. Lindenberger, J. de Physique Coil. C5, suppl, au no. 11 (37) (1976) p. 237. [2] K. Ziegler et al., IEEE Trans. Nucl. Sci. N S - 39 (2) (1983) p. 1374. [3] D. Hilscher et a]., Rev. Physique Appl. 12 (1977) p. 1337. [4] U. Janetzki, HMI-Report B-167 (1975). [5] P. Arndt et al., IEEE Trans. Nucl. Sci. NS-24 (3) (1977) p. 1162. [6] P. Arndt et al., Particle Accelerator Conf., Chicago (1977). [7] P. Arndt, these Proceedings (4th Int. Conf. on Electrostatic Accelerator Technology, Buenos Aires) Nucl. Instr. and Meth. 125. [8] R. Conrad, 9th Int. Symp. on Nuclear Electronics, Varna (1977) p. 160. [9] W. Busse et al., Nucl. Instr. and Meth. 184 (1981) 275.
1. EXISTING FACILITIES
STATUS P. A R N D T
Nuclear Instruments and Methods in Physics Research A244 (1986) 50-51 North-Holland, Amsterdam
REPORT OF THE V I C K S I C N - I N J E C T O R and the VICKSI-Group,
Hahn - Meaner- lnstitut fftr Kernforschung Berlin GmbH, Bereich Kern - und Strahlenphysik, Glienicker Strasse 100, D-1000 Berlin 39, Germany
In the accelerator facility VICKSI [1,2] at the Hahn-Meitner-lnstitute in Berlin a 6 MV Van de Graaff accelerator (HVEC CN-type) is used as injector for a separated sector isochronous cyclotron. The facility is in operation since 1978. In the following we will report about the operating experience with the CN-injector, which had to undergo a near complete reconstruction before it could be used as a heavy ion injector.
1. Introduction The facility was originally designed to accelerate ions with a maximum mass of 40 ainu to energies of up to 200 MeV. In the meantime, however, the mass range was extended considerably. The heaviest ion we have accelerated so far was'129Xe. Ion currents of 10-20 F A could be accelerated in the injector without any difficulty. A summary of ion beams and intensities behind the analysing magnet of the CN-injector is given in table 1.
2. The VICKSi CN-injector, problems and solutions In the beginning the N E C high vacuum accelerating tube, which was installed in the C N to make it suitable for heavy ion acceleration [3], caused some difficulties in reaching the design voltage of 6 MV without sparks.
Table 1 Electron current of ion beams out of the CN (examples)
4He 6Li JiB ~2C 14N 160
19F 2°Ne 32S
35C1 36Ar S6Kr
129Xe(nat)
i+
2 +
3+
4 +
5+
[~AI >1 1.3 3.5 10 10 10 6 10 6 4.5 10 10 10
[~A]
[~AI
[hAl
[hAl
0.7 0.6 0.6 0.1 10 0.5 0.18 10 6 10
0.012 0.01 0.7 0.06 0.06 1.5 2 0.5
20
150 500 300
20 15 75
0168-9002/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
But after about 6 - 8 months of operation we were able to reach the 6 MV without problems. In the beginning the tube also had several small vacuum leaks. After these initial difficulties during the first year the injector has been operating at voltages between 5.8 and 6 MV without longer conditioning periods ( < 2 - 8 h). It is particularly important to perform a careful optical alignment of the tube during installation, to avoid any bend or offset in the tube, which could cause the beam to hit internal apertures and to generate sparks. The vacuum in the tube is now routinely - 1 0 -9 hPa. This vacuum is maintained by a differential pumping system in the terminal [3,5,6] and by pumps in the beam line behind the CN-injector. Even after venting the accelerator tube this good vacuum is reached after 3 - 4 d of pumping. However, it is important that the venting is done slowly and with dry nitrogen. Because of the larger dimensions and the higher weight of the new terminal (3 m long, 1.2 m O and - 2000 kp) [3] the mechanical stability of the CN-column was not sufficient. This problem was solved by installing an intermediate plate in the middle of the column. Due to the larger dimensions of the terminal [3] and the necessary shape of the spinning electrical fields of up to 20 M V / m were obtained [4]. This and the fact, that we wanted to get away from the high pressure of 16 bar, necessary with the isolation gas-mixture of N 2 - C O 2 + 105[ SF6, made us switch to pure SF6 at a pressure of 6 bar. Electrostatically this was successful, it caused however some difficulties with the charge transport by the HVEC belt (cf. ref. [7]). Difficulties in the terminal were associated with the lifetime of ion sources and with the Wienfiher for charge state selection [5,6]. In both units short circuits developed after relative short operating times. After a series of modifications and off-line tests ion source service needs to be scheduled every two to three weeks,
P. Arndt et al. / VICKSI-CN injector
51
Table 2 Summary of CN-injector properties and operational data
Specifdcation: Type Tank dimensions Gas Terminal dimensions Column Charge-system Accelerator tubes Vacuum Ion source Terminal charge separator Bunching Beam transport Control
6 MV-CN-type Van de Graaff (HVEC) 8.6m, O 2.5m SF6 (99%), 6 bar 3.05m, O 1.2m HVEC-standard HVEC black type belt, 120/tA, 17 m / s 20 standard NEC-modules turbo- and ion-getter pumps 1 x 10 -9 hPa (base), I x 10 -6 hPa (terminal) (a) cold-PIG-source: p, D, 3He, 12C, 129 Xe (b) field emitting source: 6 : Li ExB-filter 5-20 MHz, 60 ° analysing magnet: 0 = 1.5 m, BPma~= 280 ( M E / Z 2) steerers, quadrupoles computerized control system [9] based on DEC computers and CAMAC interfaces
Performance Start of operation Maximum voltage without tube Maximum voltage with tube Normal operation voltage Operating time Belt lifetime Tube lifetime Operation schedule Future plans
1964 ( H V E C machine), 1976 (modified machine) 9 MV 6.2 M V 5.6 M V - 7000 h / y
3000-14000 h 40000 h (accumulated until now, so far no deterioration) 24 h / d , 7d/week, 25 tank openings/y 8 h routine service every 2 weeks new charging belt, fast capacitive regulation system
only. Similarly the Wienfilter has a lifetime of a b o u t three months, now. All other terminal c o m p o n e n t s used for the ion optics, the power supplies as well as the infrared d a t a transmission [8] link from g r o u n d to terminal level have been r u n n i n g without p r o b l e m s a n d w i t h o u t significant failures. Table 2 summarizes the present C N - i n j e c t o r properties a n d operational data.
3. Future plans F o r the future we plan the installation of a belt, which can be glued together, thus eliminating the need to completely d i s m a n t l e a n d rebuild the c o l u m n for each belt change. F u r t h e r m o r e we are in the process of installing a capacitive fast regulating system for the terminal voltage to further reduce the terminal ripple.
References
[1] K.H. Lindenberger, J. de Physique Coil. C5, suppl, au no. 11 (37) (1976) p. 237. [2] K. Ziegler et al., IEEE Trans. Nucl. Sci. N S - 39 (2) (1983) p. 1374. [3] D. Hilscher et a]., Rev. Physique Appl. 12 (1977) p. 1337. [4] U. Janetzki, HMI-Report B-167 (1975). [5] P. Arndt et al., IEEE Trans. Nucl. Sci. NS-24 (3) (1977) p. 1162. [6] P. Arndt et al., Particle Accelerator Conf., Chicago (1977). [7] P. Arndt, these Proceedings (4th Int. Conf. on Electrostatic Accelerator Technology, Buenos Aires) Nucl. Instr. and Meth. 125. [8] R. Conrad, 9th Int. Symp. on Nuclear Electronics, Varna (1977) p. 160. [9] W. Busse et al., Nucl. Instr. and Meth. 184 (1981) 275.
1. EXISTING FACILITIES