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An electron bombardment weak-field ionizer for a polarized ion source
NUCLEAR INSTRUMENTS
A N D M E T H O D S 5 8 (I968) 303-306; ~) N O R T H - H O L L A N D
PUBLISHING
CO.
AN ELECTRON B O M B A R D M E N T WEAK-FIELD IONIZER FOR A P O L A R I Z E D ION S O U R C E G. A. VASIL'EV and E. A. GLASOV
P. N. Lebedev Physical Institute o f the USSR Academy o f Sciences, Moscow, USSR Received 4 September 1967 A n electron b o m b a r d m e n t weak-field ionizer is described which is used in the source o f polarized ions. T h e ionizer is characterized by high efficiency (2 x 10 a for h y d r o g e n a t o m s ) a n d low power
dissipation (120 W). A 3-keY ion beam produced h a s 1.5 m m dia. after being focused at a distance 250 m m f r o m the ionizer.
1. Introduction
diameter. The potential depression produced by a space charge inside the anode serves to focus positive ions into a well-collimated beam. It must be noted, however, that as the injected electron current is increased, a virtual cathode appears, which prevents electrons from reaching the near-axis region. The more transparent is the grid, lhe more is the mean number of oscillations performed by electrons before they are trapped by the anode and so the lesser is the current in the outer circuit which is sufficient for virtual cathode formation. To determine the value of this current we can to a first approximation consider the current density inside the anode to be constant and equal to the density of the injection current, that crosses the thin cylindrical sheet S, indicated in fig. 1. The probability for electrons to survive after first crossing the grid is equal to q(q is the grid transparency). These first survivers cross the sheet S twice. After crossing the grid for the second time the same probability becomes q2 and the second survivers will be rejected back by the cathode field practically along the same trajectories, that is without loss. Now they have to cross the sheet S twice again. Continuing this process one finds, that every electron emitted from the cathode crosses the sheet S N times where N = 2q+2q2+2q3+ . . . . 2 q ( 1 - q ) -1. So for the current density we have IN/(2~zal) (I is the anode current, measured in outer circuit, a and l are respectively the radius and the length of the anode cylinder). To go forth we can use the well-known formula for a maximum stable current in the beam with axial symmetry assuming constant current density2), which in our case takes the form of
Intensity and quality of an ion beam produced by a polarized ion source (following the atomic beam method) depend greatly on the ionizer. At present an ionizer efficiency of the order of 10 - 3 iS generally achieved, but the emittance is still too large and major losses are encountered when matching the source to an accelerator. If ionization is to be carried out in a weak ( ~ 10 G) magnetic field, the flow of electrons in the ionizer must be formed electrostatically. Here a cylindrical electrode geometry offers several advantagest). In such a device the neutral beam passes along the axis of a grid-like cylinder, which acts as an anode (fig. 1). Electrons are injected transversally from a coaxial cathode of greater
A c
I
t\ \
•
/7
!!
-
/ ;)" /.
•
2+..
.:;" - ,
lma~ = 32.4X
S Fig. I. The ionizer (conceptual design). C - c a t h o d e ; A grid-like anode; S-cylindrical sheet for which the current density is
calculated. 303
IO-6V+.Zl/(aN),
(I)
where lm~x is in A and V is the anode voltage in V. For the ionizer described below the measured Im,X (without magnetic field) coincides with the value computed from eq. (1), the deviation being not more than 15%. The virtual cathode regime (when I > Ima×)is marked
304
G.A. VASIL'EV AND E. A. GLASOV
2. The electrode arrangement A view of the ionizer design is shown in fig. 2, where insulators and minor details are omitted. Cathode 1 is LaB6 deposited on the inner surface of a tantalum tube with 10 mm, i.d., wall thickness 0.5 m m and length 24 m m (fig. 3). A n o d e 2, placed coaxially inside the cathode represents a cylindrical grid made of tungsten wire 0.1 m m in dia. The grid consists of 15 parallel wires equally spaced at a diameter of 6,4 m m and a helix with a pitch of 0.5 mm, the whole structure forming a tubular basket (fig. 3). The geometrical transparency of the grid is q = 0.74 and hence N = 5.7. To make such a grid a copper rod with suitable grooves was used and then removed by etching. The grid fringes are reinforced with tantalum cuffs. The heating of the cathode is accomplished firstly by passing an alternating current t h r o u g h the grid. After some outgassing, when the cathode accepts the working temperature ( ~ 1400 ° K), an anode voltage is applied and the electron current begins to flow. At that m o m e n t the heating current can be turned off and cathode emission is further sustained, by anode dissipation only. The temperature o f the grid during operation is about 2000 ° K.
Fig. 2. Cross sectional view of the ionizer (schematic). The neutral beam passes along the axis from top to bottom. Positive potentials of the electrodes with respect to chamber are for: ( 1) - 2700 V, (2) and (4) 3000 V, (3) - 2800 V, (5) and (7) - 2000 V, (6)-from 2000 to 2500 V (focusing), (8)-0 V. by a low ion current (due to the reduced ionization volume) and therefore should be avoided. O f the cylindrical weak-field ionizers, following in the main the schema o f fig. 1, two designs have been reported3'4). They are characterized in particular by large power dissipation ( ~ 1 kW), so that water cooling is necessary. In this article we describe an effective low-power ionizer which is a part of our source of polarized protons and deuteronsS). This source is designed to operate in the high voltage terminal of a pressure tank Van de Graaff accelerator. The power dissipated by the ionizer is reduced by minimizing its dimensions and also by using the anode in the unusual capacity of cathode heater.
3. Characteristics Some features of the ionizer behaviour can be explained by current-voltage characteristics shown in fig. 4. The coordinates are I and V ~-. If the cathode temperature is high enough, it is the space charge between cathode and anode, that limits the anode current (straight line 1). However, dimensions and transparency of the anode are chosen in such a way that a virtual cathode rises inside the anode, which rising is accompanied by a four-times drop in ion current. To the right of the line 1 the electron current is temperature-limited, The straight line 2, which represents formula (1) indicates the threshold of the virtual
Fig. 3. Cathode (left) and anode of the ionizer.
AN ELECTRON
BOMBARDEMENT
WEAK-FIELD
IONIZER
305
electrodes 3 and 4 effectively controls the whole ionization region. To form an ion beam and to inject it into an accelerator, electrodes 5, 6, 7 and 8 are used. Of them only electrode 6 has a variable potential and serves to adjust the cross-over position. A 3-keV beam focused at a distance of 250 m m from the ionizer, has 1.5 m m dia. The ionizer displayed no sensitivity to weak magnetic fields of any direction up to 20 G.
0
U~
Fig. 4. Current-voltage characteristics of the ionizer. Sectioning indicates the virtual cathode region. cathode rising. To the right of it, there is no virtual cathode and a smooth potential pit exists inside the anode. That is the region where the ionizer normally operates according to equation IV= P(I). Here P(I) is the anode dissipation power necessary to sustain an emission current L The graph of this equation is curve 3, which has a negative slope. To make the total resistance positive, some means of current stabilization must be provided. In practice the anode current I n for the operating point A (fig. 4) is adjusted to be 10-15°;:o less than the critical current which corresponds to the virtual cathode rising (point B). Typical values are I a = 0.4 A, VA = 300 V. Hence the total power dissipated in the ionizer chamber is only 120 W and there is no need for forced cooling. 4. Ion extraction and focusing Positive ions produced inside the anode are extracted by electrode 3 (fig. 2), which is negative ( - 2 0 0 V) with respect to the anode. At the opposite side of the ionizer there is a similar electrode 4 which has anode potential and serves as ion rejector. The joint action of both electrodes produces a slight change in the potential minimum along the ionizer. Though this change is hard to measure, its effect on ion extraction can be easily demonstrated. To do this one should make the rejector also negative with respect to the anode. As a result the ion current in the needed direction degrades sharply and can completely vanish if the "rejector" is more negative than the extractor. That is to say, each of the
Fig. 5. A complete assembly of the ionizer. The upper electrode, not shown in fig. 2, has chamber potential and serves for centring.
306
G . A . V A S I L ' E V AND E. A. GLASOV
5. Construction Four ceramic rods of 5 mm dia. constitute the ionizer skeleton (fig. 5). All metallic details except cathode and anode are made of non-magnetic stainless steel. Special gauges were used to achieve axial symmetry of electrodes.
6. Vacuum conditions The ionizer chamber is equipped with Penning type ion-getter pumps (60 l/sec for H2). Outgassing was carried on at 10 -5 Torr. The only heater was the ionizer itself, so the wall temperature never raised above 60 ° C. In the course of outgassing, the pressure was reduced to 10 -6 Torr and after argon treatment to (2-3) × 10 -8 Torr. The H + ion current (without neutral beam) measured with the help of a magnetic analyser was (5-7) x 10 -9 A at the lowest pressure.
7. Efficiency The ionizer efficiency, i.e. the ionization probability for single passing of an atom is jal/ev, where j is the current density, l is the length of the ionization region,
e is the electronic charge, a and v are respectively the ionization cross-section and mean velocity of the atoms. The calculated efficiency for hydrogen atoms is 2 x 10-3 in agreement with the experimental results.
8. Operating experience The described ionizer was in operation for a period of a year showing good stability. Life in excess of 100 h has been attained with failure usually due to anode destruction. The authors are grateful to Dr. G. A. Kudintseva for preparing the LaB 6 cathodes.
References 1) j. M. Dickson, Progr. Nucl. Techn. Instr. 1 (1965) 105. ") J. R. Pierce, Theory and design of electron beams (D. Van Nostrand, Princeton, N.J. 1954). 3) D. A. G. Broad, A. P. Banford and J. M. Dickson, Experientia, Suppl. 12 (1966) 76. 4) G. Clausnitzer, Nucl. Instr. and Meth. 23 (1963) 309. 5) I. J. Barit, G. A. Vasil'ev, E. A. Glasov, V. V. Jolkin, Nucl. Instr. and Meth. 57 (1967) 160.
A N D M E T H O D S 5 8 (I968) 303-306; ~) N O R T H - H O L L A N D
PUBLISHING
CO.
AN ELECTRON B O M B A R D M E N T WEAK-FIELD IONIZER FOR A P O L A R I Z E D ION S O U R C E G. A. VASIL'EV and E. A. GLASOV
P. N. Lebedev Physical Institute o f the USSR Academy o f Sciences, Moscow, USSR Received 4 September 1967 A n electron b o m b a r d m e n t weak-field ionizer is described which is used in the source o f polarized ions. T h e ionizer is characterized by high efficiency (2 x 10 a for h y d r o g e n a t o m s ) a n d low power
dissipation (120 W). A 3-keY ion beam produced h a s 1.5 m m dia. after being focused at a distance 250 m m f r o m the ionizer.
1. Introduction
diameter. The potential depression produced by a space charge inside the anode serves to focus positive ions into a well-collimated beam. It must be noted, however, that as the injected electron current is increased, a virtual cathode appears, which prevents electrons from reaching the near-axis region. The more transparent is the grid, lhe more is the mean number of oscillations performed by electrons before they are trapped by the anode and so the lesser is the current in the outer circuit which is sufficient for virtual cathode formation. To determine the value of this current we can to a first approximation consider the current density inside the anode to be constant and equal to the density of the injection current, that crosses the thin cylindrical sheet S, indicated in fig. 1. The probability for electrons to survive after first crossing the grid is equal to q(q is the grid transparency). These first survivers cross the sheet S twice. After crossing the grid for the second time the same probability becomes q2 and the second survivers will be rejected back by the cathode field practically along the same trajectories, that is without loss. Now they have to cross the sheet S twice again. Continuing this process one finds, that every electron emitted from the cathode crosses the sheet S N times where N = 2q+2q2+2q3+ . . . . 2 q ( 1 - q ) -1. So for the current density we have IN/(2~zal) (I is the anode current, measured in outer circuit, a and l are respectively the radius and the length of the anode cylinder). To go forth we can use the well-known formula for a maximum stable current in the beam with axial symmetry assuming constant current density2), which in our case takes the form of
Intensity and quality of an ion beam produced by a polarized ion source (following the atomic beam method) depend greatly on the ionizer. At present an ionizer efficiency of the order of 10 - 3 iS generally achieved, but the emittance is still too large and major losses are encountered when matching the source to an accelerator. If ionization is to be carried out in a weak ( ~ 10 G) magnetic field, the flow of electrons in the ionizer must be formed electrostatically. Here a cylindrical electrode geometry offers several advantagest). In such a device the neutral beam passes along the axis of a grid-like cylinder, which acts as an anode (fig. 1). Electrons are injected transversally from a coaxial cathode of greater
A c
I
t\ \
•
/7
!!
-
/ ;)" /.
•
2+..
.:;" - ,
lma~ = 32.4X
S Fig. I. The ionizer (conceptual design). C - c a t h o d e ; A grid-like anode; S-cylindrical sheet for which the current density is
calculated. 303
IO-6V+.Zl/(aN),
(I)
where lm~x is in A and V is the anode voltage in V. For the ionizer described below the measured Im,X (without magnetic field) coincides with the value computed from eq. (1), the deviation being not more than 15%. The virtual cathode regime (when I > Ima×)is marked
304
G.A. VASIL'EV AND E. A. GLASOV
2. The electrode arrangement A view of the ionizer design is shown in fig. 2, where insulators and minor details are omitted. Cathode 1 is LaB6 deposited on the inner surface of a tantalum tube with 10 mm, i.d., wall thickness 0.5 m m and length 24 m m (fig. 3). A n o d e 2, placed coaxially inside the cathode represents a cylindrical grid made of tungsten wire 0.1 m m in dia. The grid consists of 15 parallel wires equally spaced at a diameter of 6,4 m m and a helix with a pitch of 0.5 mm, the whole structure forming a tubular basket (fig. 3). The geometrical transparency of the grid is q = 0.74 and hence N = 5.7. To make such a grid a copper rod with suitable grooves was used and then removed by etching. The grid fringes are reinforced with tantalum cuffs. The heating of the cathode is accomplished firstly by passing an alternating current t h r o u g h the grid. After some outgassing, when the cathode accepts the working temperature ( ~ 1400 ° K), an anode voltage is applied and the electron current begins to flow. At that m o m e n t the heating current can be turned off and cathode emission is further sustained, by anode dissipation only. The temperature o f the grid during operation is about 2000 ° K.
Fig. 2. Cross sectional view of the ionizer (schematic). The neutral beam passes along the axis from top to bottom. Positive potentials of the electrodes with respect to chamber are for: ( 1) - 2700 V, (2) and (4) 3000 V, (3) - 2800 V, (5) and (7) - 2000 V, (6)-from 2000 to 2500 V (focusing), (8)-0 V. by a low ion current (due to the reduced ionization volume) and therefore should be avoided. O f the cylindrical weak-field ionizers, following in the main the schema o f fig. 1, two designs have been reported3'4). They are characterized in particular by large power dissipation ( ~ 1 kW), so that water cooling is necessary. In this article we describe an effective low-power ionizer which is a part of our source of polarized protons and deuteronsS). This source is designed to operate in the high voltage terminal of a pressure tank Van de Graaff accelerator. The power dissipated by the ionizer is reduced by minimizing its dimensions and also by using the anode in the unusual capacity of cathode heater.
3. Characteristics Some features of the ionizer behaviour can be explained by current-voltage characteristics shown in fig. 4. The coordinates are I and V ~-. If the cathode temperature is high enough, it is the space charge between cathode and anode, that limits the anode current (straight line 1). However, dimensions and transparency of the anode are chosen in such a way that a virtual cathode rises inside the anode, which rising is accompanied by a four-times drop in ion current. To the right of the line 1 the electron current is temperature-limited, The straight line 2, which represents formula (1) indicates the threshold of the virtual
Fig. 3. Cathode (left) and anode of the ionizer.
AN ELECTRON
BOMBARDEMENT
WEAK-FIELD
IONIZER
305
electrodes 3 and 4 effectively controls the whole ionization region. To form an ion beam and to inject it into an accelerator, electrodes 5, 6, 7 and 8 are used. Of them only electrode 6 has a variable potential and serves to adjust the cross-over position. A 3-keV beam focused at a distance of 250 m m from the ionizer, has 1.5 m m dia. The ionizer displayed no sensitivity to weak magnetic fields of any direction up to 20 G.
0
U~
Fig. 4. Current-voltage characteristics of the ionizer. Sectioning indicates the virtual cathode region. cathode rising. To the right of it, there is no virtual cathode and a smooth potential pit exists inside the anode. That is the region where the ionizer normally operates according to equation IV= P(I). Here P(I) is the anode dissipation power necessary to sustain an emission current L The graph of this equation is curve 3, which has a negative slope. To make the total resistance positive, some means of current stabilization must be provided. In practice the anode current I n for the operating point A (fig. 4) is adjusted to be 10-15°;:o less than the critical current which corresponds to the virtual cathode rising (point B). Typical values are I a = 0.4 A, VA = 300 V. Hence the total power dissipated in the ionizer chamber is only 120 W and there is no need for forced cooling. 4. Ion extraction and focusing Positive ions produced inside the anode are extracted by electrode 3 (fig. 2), which is negative ( - 2 0 0 V) with respect to the anode. At the opposite side of the ionizer there is a similar electrode 4 which has anode potential and serves as ion rejector. The joint action of both electrodes produces a slight change in the potential minimum along the ionizer. Though this change is hard to measure, its effect on ion extraction can be easily demonstrated. To do this one should make the rejector also negative with respect to the anode. As a result the ion current in the needed direction degrades sharply and can completely vanish if the "rejector" is more negative than the extractor. That is to say, each of the
Fig. 5. A complete assembly of the ionizer. The upper electrode, not shown in fig. 2, has chamber potential and serves for centring.
306
G . A . V A S I L ' E V AND E. A. GLASOV
5. Construction Four ceramic rods of 5 mm dia. constitute the ionizer skeleton (fig. 5). All metallic details except cathode and anode are made of non-magnetic stainless steel. Special gauges were used to achieve axial symmetry of electrodes.
6. Vacuum conditions The ionizer chamber is equipped with Penning type ion-getter pumps (60 l/sec for H2). Outgassing was carried on at 10 -5 Torr. The only heater was the ionizer itself, so the wall temperature never raised above 60 ° C. In the course of outgassing, the pressure was reduced to 10 -6 Torr and after argon treatment to (2-3) × 10 -8 Torr. The H + ion current (without neutral beam) measured with the help of a magnetic analyser was (5-7) x 10 -9 A at the lowest pressure.
7. Efficiency The ionizer efficiency, i.e. the ionization probability for single passing of an atom is jal/ev, where j is the current density, l is the length of the ionization region,
e is the electronic charge, a and v are respectively the ionization cross-section and mean velocity of the atoms. The calculated efficiency for hydrogen atoms is 2 x 10-3 in agreement with the experimental results.
8. Operating experience The described ionizer was in operation for a period of a year showing good stability. Life in excess of 100 h has been attained with failure usually due to anode destruction. The authors are grateful to Dr. G. A. Kudintseva for preparing the LaB 6 cathodes.
References 1) j. M. Dickson, Progr. Nucl. Techn. Instr. 1 (1965) 105. ") J. R. Pierce, Theory and design of electron beams (D. Van Nostrand, Princeton, N.J. 1954). 3) D. A. G. Broad, A. P. Banford and J. M. Dickson, Experientia, Suppl. 12 (1966) 76. 4) G. Clausnitzer, Nucl. Instr. and Meth. 23 (1963) 309. 5) I. J. Barit, G. A. Vasil'ev, E. A. Glasov, V. V. Jolkin, Nucl. Instr. and Meth. 57 (1967) 160.