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Self controlled pulsing of a Penning ion source
NUCLEAR
INSTRUMENTS
AND METHODS
66 (1968)
IO5-IO8;
© NORTH-HOLLAND
PUBLISHING
CO.
SELF C O N T R O L L E D P U L S I N G OF A P E N N I N G I O N S O U R C E S. M A R G H I T U , A. R A D U a n d I. E. T E O D O R E S C U Institute f o r A t o m i c Physics, Bucharest, R o m a n i a
Received 7 J u n e 1968 A P e n n i n g ion source in a pulsed operation is presented. The distinguishing feature of the pulse circuit is its simplicity a n d reliability. T h e regulation o f s o m e source p a r a m e t e r s (repetition frequency, pulse d u r a t i o n a n d discharge current amplitude) is
based on the interdependence between the pulsing circuit a n d ion source operating conditions, the use o f a synchronizing generator being not necessary.
1. Introduction
In a previous work 7) a small size Penning ion source was investigated in steady-state operation and the conditions required for obtaining a high discharge current at a relatively low voltage, were pointed out. In the present work a description is given of the same ion source, pulse-operated by means of a simple selfcontrolled pulsing arrangement which can be applied almost without any change to every accelerator.
Ion sources of the Penning type, with oscillating electrons in a magnetic field, remarkable for their simplicity, are widely used in low energy accelerators, particularly in accelerators intended for neutron production. A great number of experimental investigations or routine operations, such as the study of nuclei or of short lifetime states, radioactivation analysis, oil welllogging, etc., can be effected under the best conditions by means of a pulsed beam with pulse durations in the range of tens or hundreds of microseconds. Such operating conditions can be obtained either by pulsing the ion source or the acceleration voltage, or by interrupting the continuous beam, before or after acceleration1). It is considered that within the mentioned time range, the most suitable procedure is to pulse the ion source, since in this way a discharge current may be obtained and consequently a much higher intensity of the beam extracted during the impulse, than would be the case under dc conditions 2' 3). Thus, in contrast to other pulsing methods, the mean intensity remains comparable or even greater than in the case of steady state operation. In addition, the high discharge current increases the monoatomic component of the beam, so that the Penning source becomes in this respect competitive with the high frequency source, when the accelerator is used as a low voltage neutron generator 4- 6).
Rf.N
Zs
Zs
2. The pulsing system In a small accelerator with the ion source located in the high potential electrode, the available space and power are limited and the pulse length, repetition rate and discharge current adjustment are subject to several constructive restrictions. It is recommended that the pulsing system should consist of a few simple and reliable elements of small overall dimensions and high efficiency. The duration of the discharge pulse cannot be less than 5-10/~s owing to the ionization time required by the source. All these considerations have led to the selection of a circuit diagram consisting of a pulse-forming network and an ionization switch. Fig. 1 shows two alternatives of the basic circuit diagram. The separation element Z~ may be either induc-
P E ,V
Rs = 2 0 K_(2
l
L
O-2Kv II
I
A
U
- -
P.~N
a)
~:0 SCOfl~
b)
Fig. 1. Alternative of the pulse circuit. Zs - separation e l e m e n t ; P F N - pulse f o r m i n g n e t w o r k ; T - t h y r a t r o n ; S - ion source; SG - synchro-generator; Rr, - ballast resistor.
-~
cope
Fig. 2. The self-controlled pulsing circuit,
105
106
s. MARGHITU et al.
Us
--
TABLE l
0sz
_7+_
2
1. Self-firing voltage (v) 800 2. Extraction hole diameter (mm) 2.2 3. Gas flow (Ncma/h) 10 4. Source pressure (mTorr) 6.5 5. Magnetic field (Oe) 500 6. Supply voltage (V) 1300 7. Repetition frequency (Hz) 140 8. Discharge current (A) 3.5 9. Pulse-length (~us) 60
.........
800 3.5 18 4 500 1800 6.3 60
500 3.5 l0 1.8 500 I 120 200 2.2 60
a -...
_.
Fig. 3. Repetition frequency variation by supplied voltage. t i r e or resistive. In the first case the P E N is charged resonantly, the required supply voltage being 1.8 times smaller than the line voltage while the charging effio/ ciency is a b o u t 90,,o. W h e n a resistive s e p a r a t i o n elem e n t is used, the circuit is simpler and has smaller dimensions but the charging efficiency does not exO/ ceed 50/o. The circuit shown in fig. la, is simpler and owing to the absence o f the ballast resistor Rb, it has a higher discharge efficiency. The pulse length m a y be changed in steps (when the accelerator is not operating) by connecting the corr e s p o n d i n g n u m b e r of P F N cells. The a d j u s t m e n t o f the repetition frequency is a -
60
chieved by varying the frequency o f the synchrog e n e r a t o r SG. Beyond certain frequency limits, it is necessary to m o d i f y the value of the s e p a r a t i o n element. The circuit with an inductive Zs involves as a rule the connection ot the latter in series with a voltage-supp o r t i n g diode. O u r experiments were effected with the basic circuit shown in fig. la, including a resistive s e p a r a t i o n element. The circuit was substantially simplified by means
Fig. 5. Discharge current pulse shape for magnetic field 400 Oe. Hor. : 20 lts/cm; Vert. : 0.5 A/cm.
"~ 50
ca
30
20
I0
300
~OY
500
5#Y
700
800
90Y 3ource vol{aqe,V
Fig. 4. Discharge current vs source voltage in steady-state operation.
Fig. 6. Discharge current pulse shape for magnetic field 500 Oe. Hor.: 201ts/cm; Vert.: 1 A/cm.
SELF C O N T R O L L E D
Fig. 7. Discharge current pulse shape for magnetic field 700 Oe. Hor.: 20ps/cm; Vert.: 1 A/cm.
PULSING
107
The repetition frequency is step regulated by modifying the time-constant (changing the separation resistance Rs). For given circuit element values and operating conditions of the source, the frequency may be continuously adjusted by the supply voltage variation as shown in fig. 3. When the line voltage UL attains the thyratron sparking value UST, an impulse whose amplitude is ½UL appears at the source terminals. The minimum frequency is theoretically obtained when the supply voltage U~ has a minimum value which equals LIST,(curve 1). By increasing the supply voltage, the breakdown voltage is attained sooner and the repetition frequency increases (curve 2). The main-synchronized operation of the circuit may be obtained by superimposing a small a.c. signal upon the grid polarization voltage. This signal could be for instance the bias rectifier ripple.
3. Experimental
Fig. 8. Discharge current pulse shape for magnetic field 1000 Oe. Hot.: 20/~s/cm; Vert.: 1 A/cm.
of self-controlled (voltage-controlled) keying, which allowed to eliminate the trigger (fig. 2). The important feature of the circuit is due to the fact that immediately after the thyratron breakdown the whole voltage of the PFN is applied on the source, since the firing conditions are particularly favourable. The commutation element can be a hydrogen thyratron or simply a spark-gap. An S-08-llI-i thyratron was used ( U = 8 0 0 V; IpCak=50 A; Im~a.=0.1 A; /max = 800 Hz) which allowed the ion source to be operated under all the required conditions without overcharging the tube. The self-firing voltage of thyratrons without grid polarization is subject to a statistical spread. It may be reduced by applying a small bias voltage which is adjusted to each particular tube. The PFN is a lumped-constant line with identical cells, having characteristic impedance of 300~ (approximately equal to that of the fired source) and a delay time of 7.5/~s per cell.
The source operation parameters concerning pressure and magnetic field have been obtained by extrapolating the steady-state characteristic shown in fig. 4. As it may be seen in the figure, the slope of the right hand branch of the curves becomes greater as the pressure and the magnetic field increase; for instance on the curve with p = 6 . S x 10 -3 Torr, H = 500 Oe, it attains the value 0.2 mA/V in steady-state and over 2 mA/V in pulsed operation (table 1, column 1). Table 1 shows a few exemples of pulsed operation obtained with two self-firing thyratron voltages. Figs. 5-8 show various forms of the discharge current pulse for different values of the magnetic field, all the other conditions being unchanged. For small values of the magnetic field (fig. 5), the avalanche formation time being large, the current pulse appears to be delayed with respect to the voltage pulse; its form and occurrence instant, show large fluctuations as well as the repetition frequency (about 17%). As the field increases, the delay and the frequency fluctuation decrease (about 3% in fig. 6 and fig. 7) while the current amplitude increases. However, a high frequency incoherent oscillation appears, at first during the risetime and extending for higher values of the field over the whole flat-topped part of the pulse. Increasing the pressure has the same effect. This suggests the existence of a negative dynamic resistance in the range of high current discharge characteristic, which amplifies the thermal noise or the plasma oscillations; the phenomenon is all the more marked as the plasma density is higher irrespective of the way the latter might have been obtained: by increasing the field, or the pressure.
108
s. MARGHI]'U et al.
These oscillations can be a t t e n u a t e d by a d e q u a t e d a m p i n g circuits b u t it w o u l d be interesting to examine whether their presence has or not a f a v o u r a b l e effect on the ionic structure of the extracted beam, t a k i n g into a c c o u n t the fact t h a t the presence of high frequency c o m p o n e n t s in p l a s m a oscillations indicated an increase o f the electron t e m p e r a t u r e and o f the i o n i z a t i o n degree. The pressure a n d magnetic field variations d e t e r m i n e a frequency v a r i a t i o n as a consequence of the modific a t i o n of the source ignition voltage. A t the limits o f the c o n t i n u o u s frequency regulation range the ion source a n d the t h y r a t r o n may not deionize as a consequence either o f the source resistance increase and o f the line-to-load mismatch, or of a current increase at very high supply voltages, which prevent the d e i o n i z a t i o n both o f the ion source and of the t h y r a t r o n .
4. Conclusions The m e a s u r e m e n t s effected have p r o v e d that small size ion sources o f the Penning t y p e can be used with high efficiency in a pulsed o p e r a t i o n w i t h o u t requiring a high power pulse generator, an excessive gas cons u m p t i o n or a very strong magnetic field, p e r m i t t i n g thus to realize a p e r m a n e n t m a g n e t ion source. Two different self-controlled pulsing circuits of reduced size and high reliability have been tried.
The circuit can be m i n i a t u r i z e d by using a static s p a r k - g a p , so as it could be successfully a p p l i e d also to n e u t r o n g e n e r a t o r tubes. A c o n t i n u o u s v a r i a t i o n o f the frequency can be obtained within a large range j u s t by regulating the supply voltage. In our experiments, which were not a i m e d at peak performances, a c o n t i n u o u s frequency range between 100 and 600 Hz and discharge currents up to 6 A were obtained. The frequency stability was a b o u t 3 4 % which is an a c c e p t a b l e value for experiments where no synchroniz a t i o n to an external time reference s t a n d a r d is required or where the s y n c h r o n i z a t i o n signal is taken f r o m the beam or s e c o n d a r y r a d i a t i o n s pulse.
References 1) j. Neiler and V. Good, in Fast neutron physics 1 (Interscience, 1960). e) I. E. Teodorescu, Acceleratoare de particule incarcate (Academia R. S. Romania, 1967) p. 184. a) I. E. Teodorescu, S. Marghitu and I. Luchian, Automatica si Electronica 8 (1964) 256. 4) I. E. Teodorescu, P. Croitoru, S. Marghitu and A. Radu, Rev. Roum. Phys. 12 (1967) 393. ~) J. L. Nagy, Nucl. Instr. and Meth. 32 (1965) 229. 6) I. E. Teodorescu and S. Marghitu, St. Cerc. Fiz. 17 (1965) 775. 7) I. E. Teodorescu and S. Marghitu, Conf. Plasma physics and technology (Bucharest, Nov. 1967).
INSTRUMENTS
AND METHODS
66 (1968)
IO5-IO8;
© NORTH-HOLLAND
PUBLISHING
CO.
SELF C O N T R O L L E D P U L S I N G OF A P E N N I N G I O N S O U R C E S. M A R G H I T U , A. R A D U a n d I. E. T E O D O R E S C U Institute f o r A t o m i c Physics, Bucharest, R o m a n i a
Received 7 J u n e 1968 A P e n n i n g ion source in a pulsed operation is presented. The distinguishing feature of the pulse circuit is its simplicity a n d reliability. T h e regulation o f s o m e source p a r a m e t e r s (repetition frequency, pulse d u r a t i o n a n d discharge current amplitude) is
based on the interdependence between the pulsing circuit a n d ion source operating conditions, the use o f a synchronizing generator being not necessary.
1. Introduction
In a previous work 7) a small size Penning ion source was investigated in steady-state operation and the conditions required for obtaining a high discharge current at a relatively low voltage, were pointed out. In the present work a description is given of the same ion source, pulse-operated by means of a simple selfcontrolled pulsing arrangement which can be applied almost without any change to every accelerator.
Ion sources of the Penning type, with oscillating electrons in a magnetic field, remarkable for their simplicity, are widely used in low energy accelerators, particularly in accelerators intended for neutron production. A great number of experimental investigations or routine operations, such as the study of nuclei or of short lifetime states, radioactivation analysis, oil welllogging, etc., can be effected under the best conditions by means of a pulsed beam with pulse durations in the range of tens or hundreds of microseconds. Such operating conditions can be obtained either by pulsing the ion source or the acceleration voltage, or by interrupting the continuous beam, before or after acceleration1). It is considered that within the mentioned time range, the most suitable procedure is to pulse the ion source, since in this way a discharge current may be obtained and consequently a much higher intensity of the beam extracted during the impulse, than would be the case under dc conditions 2' 3). Thus, in contrast to other pulsing methods, the mean intensity remains comparable or even greater than in the case of steady state operation. In addition, the high discharge current increases the monoatomic component of the beam, so that the Penning source becomes in this respect competitive with the high frequency source, when the accelerator is used as a low voltage neutron generator 4- 6).
Rf.N
Zs
Zs
2. The pulsing system In a small accelerator with the ion source located in the high potential electrode, the available space and power are limited and the pulse length, repetition rate and discharge current adjustment are subject to several constructive restrictions. It is recommended that the pulsing system should consist of a few simple and reliable elements of small overall dimensions and high efficiency. The duration of the discharge pulse cannot be less than 5-10/~s owing to the ionization time required by the source. All these considerations have led to the selection of a circuit diagram consisting of a pulse-forming network and an ionization switch. Fig. 1 shows two alternatives of the basic circuit diagram. The separation element Z~ may be either induc-
P E ,V
Rs = 2 0 K_(2
l
L
O-2Kv II
I
A
U
- -
P.~N
a)
~:0 SCOfl~
b)
Fig. 1. Alternative of the pulse circuit. Zs - separation e l e m e n t ; P F N - pulse f o r m i n g n e t w o r k ; T - t h y r a t r o n ; S - ion source; SG - synchro-generator; Rr, - ballast resistor.
-~
cope
Fig. 2. The self-controlled pulsing circuit,
105
106
s. MARGHITU et al.
Us
--
TABLE l
0sz
_7+_
2
1. Self-firing voltage (v) 800 2. Extraction hole diameter (mm) 2.2 3. Gas flow (Ncma/h) 10 4. Source pressure (mTorr) 6.5 5. Magnetic field (Oe) 500 6. Supply voltage (V) 1300 7. Repetition frequency (Hz) 140 8. Discharge current (A) 3.5 9. Pulse-length (~us) 60
.........
800 3.5 18 4 500 1800 6.3 60
500 3.5 l0 1.8 500 I 120 200 2.2 60
a -...
_.
Fig. 3. Repetition frequency variation by supplied voltage. t i r e or resistive. In the first case the P E N is charged resonantly, the required supply voltage being 1.8 times smaller than the line voltage while the charging effio/ ciency is a b o u t 90,,o. W h e n a resistive s e p a r a t i o n elem e n t is used, the circuit is simpler and has smaller dimensions but the charging efficiency does not exO/ ceed 50/o. The circuit shown in fig. la, is simpler and owing to the absence o f the ballast resistor Rb, it has a higher discharge efficiency. The pulse length m a y be changed in steps (when the accelerator is not operating) by connecting the corr e s p o n d i n g n u m b e r of P F N cells. The a d j u s t m e n t o f the repetition frequency is a -
60
chieved by varying the frequency o f the synchrog e n e r a t o r SG. Beyond certain frequency limits, it is necessary to m o d i f y the value of the s e p a r a t i o n element. The circuit with an inductive Zs involves as a rule the connection ot the latter in series with a voltage-supp o r t i n g diode. O u r experiments were effected with the basic circuit shown in fig. la, including a resistive s e p a r a t i o n element. The circuit was substantially simplified by means
Fig. 5. Discharge current pulse shape for magnetic field 400 Oe. Hor. : 20 lts/cm; Vert. : 0.5 A/cm.
"~ 50
ca
30
20
I0
300
~OY
500
5#Y
700
800
90Y 3ource vol{aqe,V
Fig. 4. Discharge current vs source voltage in steady-state operation.
Fig. 6. Discharge current pulse shape for magnetic field 500 Oe. Hor.: 201ts/cm; Vert.: 1 A/cm.
SELF C O N T R O L L E D
Fig. 7. Discharge current pulse shape for magnetic field 700 Oe. Hor.: 20ps/cm; Vert.: 1 A/cm.
PULSING
107
The repetition frequency is step regulated by modifying the time-constant (changing the separation resistance Rs). For given circuit element values and operating conditions of the source, the frequency may be continuously adjusted by the supply voltage variation as shown in fig. 3. When the line voltage UL attains the thyratron sparking value UST, an impulse whose amplitude is ½UL appears at the source terminals. The minimum frequency is theoretically obtained when the supply voltage U~ has a minimum value which equals LIST,(curve 1). By increasing the supply voltage, the breakdown voltage is attained sooner and the repetition frequency increases (curve 2). The main-synchronized operation of the circuit may be obtained by superimposing a small a.c. signal upon the grid polarization voltage. This signal could be for instance the bias rectifier ripple.
3. Experimental
Fig. 8. Discharge current pulse shape for magnetic field 1000 Oe. Hot.: 20/~s/cm; Vert.: 1 A/cm.
of self-controlled (voltage-controlled) keying, which allowed to eliminate the trigger (fig. 2). The important feature of the circuit is due to the fact that immediately after the thyratron breakdown the whole voltage of the PFN is applied on the source, since the firing conditions are particularly favourable. The commutation element can be a hydrogen thyratron or simply a spark-gap. An S-08-llI-i thyratron was used ( U = 8 0 0 V; IpCak=50 A; Im~a.=0.1 A; /max = 800 Hz) which allowed the ion source to be operated under all the required conditions without overcharging the tube. The self-firing voltage of thyratrons without grid polarization is subject to a statistical spread. It may be reduced by applying a small bias voltage which is adjusted to each particular tube. The PFN is a lumped-constant line with identical cells, having characteristic impedance of 300~ (approximately equal to that of the fired source) and a delay time of 7.5/~s per cell.
The source operation parameters concerning pressure and magnetic field have been obtained by extrapolating the steady-state characteristic shown in fig. 4. As it may be seen in the figure, the slope of the right hand branch of the curves becomes greater as the pressure and the magnetic field increase; for instance on the curve with p = 6 . S x 10 -3 Torr, H = 500 Oe, it attains the value 0.2 mA/V in steady-state and over 2 mA/V in pulsed operation (table 1, column 1). Table 1 shows a few exemples of pulsed operation obtained with two self-firing thyratron voltages. Figs. 5-8 show various forms of the discharge current pulse for different values of the magnetic field, all the other conditions being unchanged. For small values of the magnetic field (fig. 5), the avalanche formation time being large, the current pulse appears to be delayed with respect to the voltage pulse; its form and occurrence instant, show large fluctuations as well as the repetition frequency (about 17%). As the field increases, the delay and the frequency fluctuation decrease (about 3% in fig. 6 and fig. 7) while the current amplitude increases. However, a high frequency incoherent oscillation appears, at first during the risetime and extending for higher values of the field over the whole flat-topped part of the pulse. Increasing the pressure has the same effect. This suggests the existence of a negative dynamic resistance in the range of high current discharge characteristic, which amplifies the thermal noise or the plasma oscillations; the phenomenon is all the more marked as the plasma density is higher irrespective of the way the latter might have been obtained: by increasing the field, or the pressure.
108
s. MARGHI]'U et al.
These oscillations can be a t t e n u a t e d by a d e q u a t e d a m p i n g circuits b u t it w o u l d be interesting to examine whether their presence has or not a f a v o u r a b l e effect on the ionic structure of the extracted beam, t a k i n g into a c c o u n t the fact t h a t the presence of high frequency c o m p o n e n t s in p l a s m a oscillations indicated an increase o f the electron t e m p e r a t u r e and o f the i o n i z a t i o n degree. The pressure a n d magnetic field variations d e t e r m i n e a frequency v a r i a t i o n as a consequence of the modific a t i o n of the source ignition voltage. A t the limits o f the c o n t i n u o u s frequency regulation range the ion source a n d the t h y r a t r o n may not deionize as a consequence either o f the source resistance increase and o f the line-to-load mismatch, or of a current increase at very high supply voltages, which prevent the d e i o n i z a t i o n both o f the ion source and of the t h y r a t r o n .
4. Conclusions The m e a s u r e m e n t s effected have p r o v e d that small size ion sources o f the Penning t y p e can be used with high efficiency in a pulsed o p e r a t i o n w i t h o u t requiring a high power pulse generator, an excessive gas cons u m p t i o n or a very strong magnetic field, p e r m i t t i n g thus to realize a p e r m a n e n t m a g n e t ion source. Two different self-controlled pulsing circuits of reduced size and high reliability have been tried.
The circuit can be m i n i a t u r i z e d by using a static s p a r k - g a p , so as it could be successfully a p p l i e d also to n e u t r o n g e n e r a t o r tubes. A c o n t i n u o u s v a r i a t i o n o f the frequency can be obtained within a large range j u s t by regulating the supply voltage. In our experiments, which were not a i m e d at peak performances, a c o n t i n u o u s frequency range between 100 and 600 Hz and discharge currents up to 6 A were obtained. The frequency stability was a b o u t 3 4 % which is an a c c e p t a b l e value for experiments where no synchroniz a t i o n to an external time reference s t a n d a r d is required or where the s y n c h r o n i z a t i o n signal is taken f r o m the beam or s e c o n d a r y r a d i a t i o n s pulse.
References 1) j. Neiler and V. Good, in Fast neutron physics 1 (Interscience, 1960). e) I. E. Teodorescu, Acceleratoare de particule incarcate (Academia R. S. Romania, 1967) p. 184. a) I. E. Teodorescu, S. Marghitu and I. Luchian, Automatica si Electronica 8 (1964) 256. 4) I. E. Teodorescu, P. Croitoru, S. Marghitu and A. Radu, Rev. Roum. Phys. 12 (1967) 393. ~) J. L. Nagy, Nucl. Instr. and Meth. 32 (1965) 229. 6) I. E. Teodorescu and S. Marghitu, St. Cerc. Fiz. 17 (1965) 775. 7) I. E. Teodorescu and S. Marghitu, Conf. Plasma physics and technology (Bucharest, Nov. 1967).