- Email: [email protected]
Recent ion source progress at texas a&m university
1046
Nuclear Instruments and Methods in Physics Research B40/41 (1989) 1046-1048 North-Holland, Amsterdam
RECENT ION SOURCE PROGRESS S.M. ELLIOTT, Deportment
A. NASSIRI
AT TEXAS A&M
UNIVERSITY
*, Y.Q. PANG, J.B. BENTON, T.S. ELLIOTT
of Physics, Texas A&M University, College Station,
and P.A. WEBB
Texas 77843-4242, USA
Cesium sputtering and MEWA II (MEtal Vapor Vacuum Arc) ion sources have been constructed and operated successfully. The Middleton-type sputtering source is being developed for l2 C accelerator mass spectroscopy (AMS) applications. A spherical ionizer, CO, gas target and glow discharge desorption apparatus are incorporated into the source. A glow discharge method of reducing carbon background between sample measurements is being investigated. A design for a 90 o double focussing magnetic spectrometer is presented which will be used as a test facility for further ion source development.
1. High intensity negative ion source In order
to study generation
of carbon
negative
ion
beams suitable for AMS, a cesium sputtering
source, as illustrated in fig. 1, was constructed. It is based on the high intensity source design of Middleton [l]. Positive cesium ions are produced at the surface of a spherical ionizer operating at about 1100 o C, formed into a beam and accelerated to a target incorporating the sample to be measured. The target material is sputtered from its cesium-coated, low work function surface. A substantial fraction of the emitted particles are negative ions and are formed into a beam and accelerated through an aperture in the ionizer toward the extractor gap. The spherical ionizer geometry [2,3] provides improved emittance figures over the cylindrical-type ionizer commonly in use [4]. The target system uses a capillary tube to
* Currently at the Texas Accelerator Center, The Woodlands, Texas 77380, USA.
Fig. 1. High intensity negative ion source with gas target.
0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
admit CO2 gas onto the surface of a solid titanium getter [5]. The source is constructed of all inorganic materials except the o-ring in the target sliding seal. This seal is to be replaced by an extendable metal bellows assembly. The source has easily produced beam currents in excess of 10 l.tA for 160 12C and Cu ions. The beam divergence half angle has’ been measured at about 0.5 o with 16 kV extraction potential.
2. Glow discharge desorption After termination of sample gas flow with a CO, target a reduced but substantial beam current persists, possibly due, in part, to recoil implantation of carbon into the target [6]. The deposited carbon layer adjacent to the target and ionizer may provide a flux of carbon to the target as well, contaminating subsequent samples, though the authors have not observed this. The desorption of carbon and carbon compounds from the target and adjacent surfaces by Ar/reactive gas plasmas is being investigated. The Ar is used to support the plasma discharge and to provide ion bombardment induced desorption of the source surfaces [7]. H,, 0, or other reactive gases are used to convert elemental carbon to volatile compounds chemically, such as CH, or CO and CO,. Fluorine or fluorine-based compounds are being studied to promote etching of the titanium getter to expedite the removal of the carbonrich surface of the target. The TiF, effluent of this etching process is also volatile, thus avoiding target fouling. In order to study the effects of a glow discharge on the residual emmision of beam current a gas mixture is admitted into the source through the target capillary tube. A dc potential is applied between the target and ionizer, with the target connected to the negative power
SM. Elliott et al. / Recent ion source progress
1047
supply terminal. A continuous flow of gas is maintained at a suitable pressure by throttling the vacuum pump. With the ionizer off, a bright region in the discharge is observed at the surface of the target. The gas jet from the target orifice is also visible and the plasma is seen to penetrate into the ionizer region. Preliminary measurements have been made of the reduction of “C emission using a 90% Ar, 10% H, gas mixture. With a pressure of 2 X 10-l Torr the discharge potential was about 300 V at 200 mA. The ionizer current and Cs boiler temperature were kept at their normal settings. Maintaining the discharge at these parameters for 30 min (1.8 kW mm) reduced the “C emission to less than half its previous value.
3. MEWA
II positive ion source
The MEWA (MEtal Vapor Vacuum Arc) ion source has been described elsewhere [8]. Briefly, this source produces a highly ionized plasma between the cathode and the anode, the latter having an aperture through which a portion of the plasma passes. The plasma is then expanded onto the multi-aperture extractor grid. The arc supply is a pulse line with a Ssection LC network of 0.5 Q impedence and a pulse length of 120 ps. This line is charged to a potential of up to 350 V. A surface arc discharge between the trigger electrode and the cathode is initiated by a high voltage pulse. This closes the anode to cathode circuit, discharging the pulse line, which produces the plasma. The extraction system consists of three multi-aperture grids connected as an accel-decel lens. Early attempts to operate the source were not successful due to triggering difficulties. One attempt resulted in a trigger current of about 40 A, but no arc current was measured. An investigation of the trigger/ cathode assembly revealed the presence of arc spots on the cathode mounting stem. This indicated that the trigger discharge was to the rear of the cathode instead
;z
:
L
0:
s
6-
E 3
ARC:
180
V.
160
A.
2
Hz
l
120
).WC
4-
$ i
6olRoE
2: 0 0
1111,11/I,III,,1111,1111,1111 5 10
EXTRACTION
15
20
VOLTAGE
25
30
(kV)
Fig. 2. Measured Fe beam current for all charge states from the MEWA
II ion source.
Fig. 3. Schematic drawing of ion source test system presently under construction.
of in the cathode/anode region. An improved trigger/ cathode assembly was tested, resulting in an arc current up to 270 A. With the new pulse line type arc supply installed a 120 JLS duration beam current of 1 mA was detected on both a calibrated pulse transformer and a Faraday cup. The source was operated with an extraction potential of 10 kV, an arc current of 175 A and a repetition rate of 10 Hz. With a larger gap and improved alignment of the extractor grids, Fe beam current of about 7 mA was detected at 30 kV extraction potential. The arc supply was set to 180 V at 120 ps duration and 2 Hz repetition rate. Detected beam current as a function of extraction potential is shown in fig. 2. The beam current displayed is integrated over all the charge states in the beam.
4. Ion source test system Both positive and negative ion sources will be tested on a magnetic spectrometer system presently under construction, as shown in fig. 3. The beam line is composed of a 90’ double focussing sector magnet of 30 cm bending radius. The beam from the source is focussed through x-y adjustable slits by a three cylinder Einzel lens. Electrostatically suppressed Faraday cups or pulse transformers following the object and image slits detect the beam. The vacuum system utilizes two 1000 l/s cryo-pumps and presently maintains a pressure of less than 5 X lo-’ Torr during testing of sources. The extractor supply is rated at *lOO kV and 30 mA with 0.01% per hour stability. The system includes a quadrupole mass spectrometer residual gas analyzer (RGA) with an electron multiplier detector. An VII. ACCELERATOR
TECHNOLOGY
1048 emittance
SM. Elliott et al. / Recent ion source progress
measuring instrument is planned for the fu-
ture. The authors wish to thank Mr. J. Finney and Mr. C. Lake for superb design and drafting with only our ideas and rough sketches for input. Thanks are also due to Mr. J. Schwede and the physics machine shop for transforming these drawings into fine, finished components. We would also like to thank Drs. P.M. McIntyre, D.D. DiBitonto, D.A. Church and W.P. Kirk for their support in this work. Finally the authors are greatly in debt to Dr. R. Middleton and Mr. J. Klein of the University of Pennsylvania and Dr. LG. Brown of the Lawrence Berkeley Laboratory for their help and many illuminating discussions.
References 111 R. Middleton, Nucl. Instr. and Meth. 214 (1983) 139. 121 N.R. White, Nucl. Instr. and Meth. 206 (1983) 15.
131 R. Middleton, Proc. Workshop on Technology in Accelerator Mass Spectroscopy (1986) eds. R.E.M. Hedges and T. Hall, Oxford, England (1987) p. 82. 141G.D. Alton, J.W. McConnell, S. Tajima and G.J. Nelson, Nucl. Instr. and Meth. B24/25 (1987) 826. 151 R. Middleton, Nucl. Instr. and Meth. BS (1984) 193. 161 C.R. Bronk and R.E.M. Hedges, Nucl. Instr. and Meth. B29 (1987) 45. [71R. Calder, A. Grillot, F. Le Nomand and A. Mathewson, Proc. 7th Int. Vacuum Contr. and 3rd Int. Conf. on Solid Surfaces, Vienna, Austria (1977) p. 231. 181 LG. Brown, J.E. Galvin, B.F. Gavin and R.A. McGill, Rev. Sci. Instr. 57 (1986) 1069.
Nuclear Instruments and Methods in Physics Research B40/41 (1989) 1046-1048 North-Holland, Amsterdam
RECENT ION SOURCE PROGRESS S.M. ELLIOTT, Deportment
A. NASSIRI
AT TEXAS A&M
UNIVERSITY
*, Y.Q. PANG, J.B. BENTON, T.S. ELLIOTT
of Physics, Texas A&M University, College Station,
and P.A. WEBB
Texas 77843-4242, USA
Cesium sputtering and MEWA II (MEtal Vapor Vacuum Arc) ion sources have been constructed and operated successfully. The Middleton-type sputtering source is being developed for l2 C accelerator mass spectroscopy (AMS) applications. A spherical ionizer, CO, gas target and glow discharge desorption apparatus are incorporated into the source. A glow discharge method of reducing carbon background between sample measurements is being investigated. A design for a 90 o double focussing magnetic spectrometer is presented which will be used as a test facility for further ion source development.
1. High intensity negative ion source In order
to study generation
of carbon
negative
ion
beams suitable for AMS, a cesium sputtering
source, as illustrated in fig. 1, was constructed. It is based on the high intensity source design of Middleton [l]. Positive cesium ions are produced at the surface of a spherical ionizer operating at about 1100 o C, formed into a beam and accelerated to a target incorporating the sample to be measured. The target material is sputtered from its cesium-coated, low work function surface. A substantial fraction of the emitted particles are negative ions and are formed into a beam and accelerated through an aperture in the ionizer toward the extractor gap. The spherical ionizer geometry [2,3] provides improved emittance figures over the cylindrical-type ionizer commonly in use [4]. The target system uses a capillary tube to
* Currently at the Texas Accelerator Center, The Woodlands, Texas 77380, USA.
Fig. 1. High intensity negative ion source with gas target.
0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
admit CO2 gas onto the surface of a solid titanium getter [5]. The source is constructed of all inorganic materials except the o-ring in the target sliding seal. This seal is to be replaced by an extendable metal bellows assembly. The source has easily produced beam currents in excess of 10 l.tA for 160 12C and Cu ions. The beam divergence half angle has’ been measured at about 0.5 o with 16 kV extraction potential.
2. Glow discharge desorption After termination of sample gas flow with a CO, target a reduced but substantial beam current persists, possibly due, in part, to recoil implantation of carbon into the target [6]. The deposited carbon layer adjacent to the target and ionizer may provide a flux of carbon to the target as well, contaminating subsequent samples, though the authors have not observed this. The desorption of carbon and carbon compounds from the target and adjacent surfaces by Ar/reactive gas plasmas is being investigated. The Ar is used to support the plasma discharge and to provide ion bombardment induced desorption of the source surfaces [7]. H,, 0, or other reactive gases are used to convert elemental carbon to volatile compounds chemically, such as CH, or CO and CO,. Fluorine or fluorine-based compounds are being studied to promote etching of the titanium getter to expedite the removal of the carbonrich surface of the target. The TiF, effluent of this etching process is also volatile, thus avoiding target fouling. In order to study the effects of a glow discharge on the residual emmision of beam current a gas mixture is admitted into the source through the target capillary tube. A dc potential is applied between the target and ionizer, with the target connected to the negative power
SM. Elliott et al. / Recent ion source progress
1047
supply terminal. A continuous flow of gas is maintained at a suitable pressure by throttling the vacuum pump. With the ionizer off, a bright region in the discharge is observed at the surface of the target. The gas jet from the target orifice is also visible and the plasma is seen to penetrate into the ionizer region. Preliminary measurements have been made of the reduction of “C emission using a 90% Ar, 10% H, gas mixture. With a pressure of 2 X 10-l Torr the discharge potential was about 300 V at 200 mA. The ionizer current and Cs boiler temperature were kept at their normal settings. Maintaining the discharge at these parameters for 30 min (1.8 kW mm) reduced the “C emission to less than half its previous value.
3. MEWA
II positive ion source
The MEWA (MEtal Vapor Vacuum Arc) ion source has been described elsewhere [8]. Briefly, this source produces a highly ionized plasma between the cathode and the anode, the latter having an aperture through which a portion of the plasma passes. The plasma is then expanded onto the multi-aperture extractor grid. The arc supply is a pulse line with a Ssection LC network of 0.5 Q impedence and a pulse length of 120 ps. This line is charged to a potential of up to 350 V. A surface arc discharge between the trigger electrode and the cathode is initiated by a high voltage pulse. This closes the anode to cathode circuit, discharging the pulse line, which produces the plasma. The extraction system consists of three multi-aperture grids connected as an accel-decel lens. Early attempts to operate the source were not successful due to triggering difficulties. One attempt resulted in a trigger current of about 40 A, but no arc current was measured. An investigation of the trigger/ cathode assembly revealed the presence of arc spots on the cathode mounting stem. This indicated that the trigger discharge was to the rear of the cathode instead
;z
:
L
0:
s
6-
E 3
ARC:
180
V.
160
A.
2
Hz
l
120
).WC
4-
$ i
6olRoE
2: 0 0
1111,11/I,III,,1111,1111,1111 5 10
EXTRACTION
15
20
VOLTAGE
25
30
(kV)
Fig. 2. Measured Fe beam current for all charge states from the MEWA
II ion source.
Fig. 3. Schematic drawing of ion source test system presently under construction.
of in the cathode/anode region. An improved trigger/ cathode assembly was tested, resulting in an arc current up to 270 A. With the new pulse line type arc supply installed a 120 JLS duration beam current of 1 mA was detected on both a calibrated pulse transformer and a Faraday cup. The source was operated with an extraction potential of 10 kV, an arc current of 175 A and a repetition rate of 10 Hz. With a larger gap and improved alignment of the extractor grids, Fe beam current of about 7 mA was detected at 30 kV extraction potential. The arc supply was set to 180 V at 120 ps duration and 2 Hz repetition rate. Detected beam current as a function of extraction potential is shown in fig. 2. The beam current displayed is integrated over all the charge states in the beam.
4. Ion source test system Both positive and negative ion sources will be tested on a magnetic spectrometer system presently under construction, as shown in fig. 3. The beam line is composed of a 90’ double focussing sector magnet of 30 cm bending radius. The beam from the source is focussed through x-y adjustable slits by a three cylinder Einzel lens. Electrostatically suppressed Faraday cups or pulse transformers following the object and image slits detect the beam. The vacuum system utilizes two 1000 l/s cryo-pumps and presently maintains a pressure of less than 5 X lo-’ Torr during testing of sources. The extractor supply is rated at *lOO kV and 30 mA with 0.01% per hour stability. The system includes a quadrupole mass spectrometer residual gas analyzer (RGA) with an electron multiplier detector. An VII. ACCELERATOR
TECHNOLOGY
1048 emittance
SM. Elliott et al. / Recent ion source progress
measuring instrument is planned for the fu-
ture. The authors wish to thank Mr. J. Finney and Mr. C. Lake for superb design and drafting with only our ideas and rough sketches for input. Thanks are also due to Mr. J. Schwede and the physics machine shop for transforming these drawings into fine, finished components. We would also like to thank Drs. P.M. McIntyre, D.D. DiBitonto, D.A. Church and W.P. Kirk for their support in this work. Finally the authors are greatly in debt to Dr. R. Middleton and Mr. J. Klein of the University of Pennsylvania and Dr. LG. Brown of the Lawrence Berkeley Laboratory for their help and many illuminating discussions.
References 111 R. Middleton, Nucl. Instr. and Meth. 214 (1983) 139. 121 N.R. White, Nucl. Instr. and Meth. 206 (1983) 15.
131 R. Middleton, Proc. Workshop on Technology in Accelerator Mass Spectroscopy (1986) eds. R.E.M. Hedges and T. Hall, Oxford, England (1987) p. 82. 141G.D. Alton, J.W. McConnell, S. Tajima and G.J. Nelson, Nucl. Instr. and Meth. B24/25 (1987) 826. 151 R. Middleton, Nucl. Instr. and Meth. BS (1984) 193. 161 C.R. Bronk and R.E.M. Hedges, Nucl. Instr. and Meth. B29 (1987) 45. [71R. Calder, A. Grillot, F. Le Nomand and A. Mathewson, Proc. 7th Int. Vacuum Contr. and 3rd Int. Conf. on Solid Surfaces, Vienna, Austria (1977) p. 231. 181 LG. Brown, J.E. Galvin, B.F. Gavin and R.A. McGill, Rev. Sci. Instr. 57 (1986) 1069.