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A multicharged lithium Penning ion source for the Rossendorf cyclotron U-120
Nuclear Instruments and Methods in Physics Research A241 (1985) 596-597 North-Holland, Amsterdam
596
Letter to the Editor
A MULTICHARGED LITHIUM PENNING ION SOURCE FOR THE ROSSENDORF CYCLOTRON U-120 J. DIETRICH, G. KERBER, H. ODRICH and W. NAUMANN
Academy of Sciences of the GDR, Central Institute for Nuclear Research, Rossendorf, GDR
Received
27
March
1985
and in revised form
30 July 1985
An internal hot cathode Penning ion source using a vaporizer without external heating was developed for the acceleration of lithium ions with the Rossendorf cyclotron U-120 . The Rossendorf fixed frequency cyclotron U-120 can be used to accelerate ions in the range of about 1 .5 A/Z _< 2.7 with a final energy of E = 27Z2 IA . For the acceleration of lithium ions with sufficient energy, an ion source is necessary which can produce triply charged lithium ions. A variety of ion sources for multiply charged ions are reported . Lithium ions can be produced in a Penning ion source by (1) vaporizing the solid in a furnace [1], (2) sputtering the material with ions accelerated from the discharge [2], and (3) by a process that involves ions unable to cross the first acceleration gap between the ion source and dee, which are accelerated back into the ion source where they sputter charge material into the arc [3]. An internal hot cathode Penning ion source was designed using a vaporizer without external heating. This ion source has a better reliability of operation in comparison with internal ion sources using a vaporizer with external heating . The geometrical layout of the ion source is shown in fig. 1 . The vaporizer (9) is directly assembled to the anode (1) and has a good thermal contact with the discharge chamber. Turning slots in the upper (13) and lower (14) parts of the anode cylinder gives a better thermal isolation between discharge chamber and water-cooled anode supports (15,16) . The anode and the vaporizer are made of stainless steel . The anode has a 6 mm bore diameter and its height is 90 mm, corresponding to an arc column volume of 2.5 cm3. The hot cathode (2) and the water-cooled anticathode (5) are made of tantalum. The extraction slit has a height of 10 mm and a width of 2 mm. The vaporizer is filled with about 1 g metallic lithium. For the acceleration of 7Li 3+ ions natural Li and for 6Li3+ ions enriched 6 Li is used . The temperature of the vaporizer is measured by a thermocouple (11) . Measures were taken to provide hermetic sealing of the discharge chamber in order to prevent lithium vapour from escaping in any other way 0168-9002/85/$03 .30 C Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
B
13
3
L0,11 .70 I
il
R
M
7 12
Î
417-
1010 M;
R
0
0.
11 16 8 501ntn
Fig . 1. Multicharged lithium ion source: (1) anode; (2) cathode ; (3) magnetic field; (4) discharge chamber; (5) anticathode ; (6) vapour feed; (7,8) gas inlets; (9) vaporizer ; (10) lithium ; (11) thermocouple; (12) lithium inlet; (13,14) turning slots; (15,16) anode supports ; (17) extraction slit; (18) accelerating electrode . than through the extraction slit. The source has operated under pulsed conditions with a frequency of 50 Hz and a pulse duration from 8 to 18 ms. The pulsed arc voltage is synchronized with the rf dee voltage. At the beginning of the operation of the ion source the discharge chamber was heated by the discharge struck on hydrogen gas. As the vapour of lithium entered into the discharge chamber, the feeding of the hydrogen gas gradually ceased. The lithium vapour gets into the discharge chamber through the vapour feed (6) with a diameter of 3 mm and a length of 5 mm. The vaporizer is mainly heated by the discharge power. Special shielding from the rf dee voltage is not necessary.
J. Dietrich et al. / A multicharged Li Penning ton source
In our case for heating the vaporizer an average electrical power of about 1 kW is needed to feed enough lithium vapour into the discharge chamber. It is known that for efficient production of triply charged lithium ions an arc voltage of about 330 V is needed [4] . Our experiments showed that -an arc current of 5 A is necessary in order to get a high intensity Li 3+ beam . Using these optimum parameters under do arc conditions the heating of the vaporizer will be too high . Therefore, pulsed arc operation is advantageous . By reduction of the pulse duration of the arc voltage the heating power is decreased without significant loss of external Li 3+ intensity (fig. 2). In such a way the lithium evaporation (vapour flow rate) can be controlled by means of the pulse duration . Fig . 3 shows the temperature of the vaporizer as a function of time after ignition of the arc discharge in the case of an arc power of about 800 W . The arrow indicates the moment of switching on the rf dee voltage . An additional increase of the vaporizer temperature occurs . This can be explained by the fact that ions return to the source because they are unable to cross the source-to-dee gap before the polarity of the dee voltage reverses . The rf phase at which ions leave the source also determines whether they return to the source. Rf heating also contributes to an increase of the vaporizer temperature . The lithium consumption is about 0 .1 g/h. The lifetime of the source amounts to about 10 h at the maximum intensity of Li 3 + ions . It is limited by the complete consumption of lithium. The operation of the cyclotron has been carried out at an rf dee voltage of 100 kV . Extracted beam currents up to 100 nA for 6 Li 3+ at 40 .5 MeV and for 7 Li 3+ at 34 .7 MeV have been obtained .
u
5
10
rM
15
597
20
Fig. 2 . Temperature of the vaporizer vs pulse duration. 900
800
700
600
References
400
300 1
5
i
10
I
t Imin]
15
Fig . 3. Temperature of the vaporizer vs time.
I
20
[11 P .I . Vasiliev, N .I . Venikov, D.V . Zevjakin, A .A . Ogloblin, N.N . Khaldin, B .I . Khoroshavin, V.I . Chuev and N .I . Chumakov, Nucl. Instr. and Meth. 71 (1969) 201 . [2] I . Khono, K. Ikegami and T. Kageyama, Proc . 3rd Symp . on Ion Sources and Application Technology, Tokyo (1979) p. 199 . [3] E.D . Hudson, M.L . Mallory and R.S. Lord, IEEE Trans . Nucl . Sri. NS-23 (1976) 1065 . [4] W . Schmidt and R. Becker, GSI-Report 71-3 (1971) p . 46 .
596
Letter to the Editor
A MULTICHARGED LITHIUM PENNING ION SOURCE FOR THE ROSSENDORF CYCLOTRON U-120 J. DIETRICH, G. KERBER, H. ODRICH and W. NAUMANN
Academy of Sciences of the GDR, Central Institute for Nuclear Research, Rossendorf, GDR
Received
27
March
1985
and in revised form
30 July 1985
An internal hot cathode Penning ion source using a vaporizer without external heating was developed for the acceleration of lithium ions with the Rossendorf cyclotron U-120 . The Rossendorf fixed frequency cyclotron U-120 can be used to accelerate ions in the range of about 1 .5 A/Z _< 2.7 with a final energy of E = 27Z2 IA . For the acceleration of lithium ions with sufficient energy, an ion source is necessary which can produce triply charged lithium ions. A variety of ion sources for multiply charged ions are reported . Lithium ions can be produced in a Penning ion source by (1) vaporizing the solid in a furnace [1], (2) sputtering the material with ions accelerated from the discharge [2], and (3) by a process that involves ions unable to cross the first acceleration gap between the ion source and dee, which are accelerated back into the ion source where they sputter charge material into the arc [3]. An internal hot cathode Penning ion source was designed using a vaporizer without external heating. This ion source has a better reliability of operation in comparison with internal ion sources using a vaporizer with external heating . The geometrical layout of the ion source is shown in fig. 1 . The vaporizer (9) is directly assembled to the anode (1) and has a good thermal contact with the discharge chamber. Turning slots in the upper (13) and lower (14) parts of the anode cylinder gives a better thermal isolation between discharge chamber and water-cooled anode supports (15,16) . The anode and the vaporizer are made of stainless steel . The anode has a 6 mm bore diameter and its height is 90 mm, corresponding to an arc column volume of 2.5 cm3. The hot cathode (2) and the water-cooled anticathode (5) are made of tantalum. The extraction slit has a height of 10 mm and a width of 2 mm. The vaporizer is filled with about 1 g metallic lithium. For the acceleration of 7Li 3+ ions natural Li and for 6Li3+ ions enriched 6 Li is used . The temperature of the vaporizer is measured by a thermocouple (11) . Measures were taken to provide hermetic sealing of the discharge chamber in order to prevent lithium vapour from escaping in any other way 0168-9002/85/$03 .30 C Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
B
13
3
L0,11 .70 I
il
R
M
7 12
Î
417-
1010 M;
R
0
0.
11 16 8 501ntn
Fig . 1. Multicharged lithium ion source: (1) anode; (2) cathode ; (3) magnetic field; (4) discharge chamber; (5) anticathode ; (6) vapour feed; (7,8) gas inlets; (9) vaporizer ; (10) lithium ; (11) thermocouple; (12) lithium inlet; (13,14) turning slots; (15,16) anode supports ; (17) extraction slit; (18) accelerating electrode . than through the extraction slit. The source has operated under pulsed conditions with a frequency of 50 Hz and a pulse duration from 8 to 18 ms. The pulsed arc voltage is synchronized with the rf dee voltage. At the beginning of the operation of the ion source the discharge chamber was heated by the discharge struck on hydrogen gas. As the vapour of lithium entered into the discharge chamber, the feeding of the hydrogen gas gradually ceased. The lithium vapour gets into the discharge chamber through the vapour feed (6) with a diameter of 3 mm and a length of 5 mm. The vaporizer is mainly heated by the discharge power. Special shielding from the rf dee voltage is not necessary.
J. Dietrich et al. / A multicharged Li Penning ton source
In our case for heating the vaporizer an average electrical power of about 1 kW is needed to feed enough lithium vapour into the discharge chamber. It is known that for efficient production of triply charged lithium ions an arc voltage of about 330 V is needed [4] . Our experiments showed that -an arc current of 5 A is necessary in order to get a high intensity Li 3+ beam . Using these optimum parameters under do arc conditions the heating of the vaporizer will be too high . Therefore, pulsed arc operation is advantageous . By reduction of the pulse duration of the arc voltage the heating power is decreased without significant loss of external Li 3+ intensity (fig. 2). In such a way the lithium evaporation (vapour flow rate) can be controlled by means of the pulse duration . Fig . 3 shows the temperature of the vaporizer as a function of time after ignition of the arc discharge in the case of an arc power of about 800 W . The arrow indicates the moment of switching on the rf dee voltage . An additional increase of the vaporizer temperature occurs . This can be explained by the fact that ions return to the source because they are unable to cross the source-to-dee gap before the polarity of the dee voltage reverses . The rf phase at which ions leave the source also determines whether they return to the source. Rf heating also contributes to an increase of the vaporizer temperature . The lithium consumption is about 0 .1 g/h. The lifetime of the source amounts to about 10 h at the maximum intensity of Li 3 + ions . It is limited by the complete consumption of lithium. The operation of the cyclotron has been carried out at an rf dee voltage of 100 kV . Extracted beam currents up to 100 nA for 6 Li 3+ at 40 .5 MeV and for 7 Li 3+ at 34 .7 MeV have been obtained .
u
5
10
rM
15
597
20
Fig. 2 . Temperature of the vaporizer vs pulse duration. 900
800
700
600
References
400
300 1
5
i
10
I
t Imin]
15
Fig . 3. Temperature of the vaporizer vs time.
I
20
[11 P .I . Vasiliev, N .I . Venikov, D.V . Zevjakin, A .A . Ogloblin, N.N . Khaldin, B .I . Khoroshavin, V.I . Chuev and N .I . Chumakov, Nucl. Instr. and Meth. 71 (1969) 201 . [2] I . Khono, K. Ikegami and T. Kageyama, Proc . 3rd Symp . on Ion Sources and Application Technology, Tokyo (1979) p. 199 . [3] E.D . Hudson, M.L . Mallory and R.S. Lord, IEEE Trans . Nucl . Sri. NS-23 (1976) 1065 . [4] W . Schmidt and R. Becker, GSI-Report 71-3 (1971) p . 46 .