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Improvements of the Jena AMS system
Nuclear Instruments and Methods in Physics Research B 268 (2010) 902–905
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Improvements of the Jena AMS system Axel Steinhof *, Istvan Hejja, Thomas Wagner Max-Planck Institut für Biogeochemie, Jena, Germany
a r t i c l e
i n f o
Article history: Available online 13 October 2009 Keywords: AMS system Slit stabilization Cockroft–Walton generator High-voltage rectifiers
a b s t r a c t A new slit stabilization circuit and a modified generator drive were installed on an AMS system based on a 3 MV TandetronTM, produced by High Voltage Engineering Europa (HVEE). Furthermore our test procedure used at the Jena AMS system for the rectifiers of the Cockroft–Walton generator is presented. Ó 2009 Published by Elsevier B.V.
1. Introduction The Jena AMS system is an accelerator mass spectrometer of the third generation, operating at a terminal voltage of 2.5 MV, produced by a solid state Cockroft–Walton generator, and applying the so-called recombinator for simultaneous injection of the three carbon isotopes. After the first system was built up at the National Ocean Sciences AMS Facility at Woods Hole, Massachusetts [1], the company HVEE made this spectrometer under the tradename TandetronTM a very common system [2]. Detailed information on the design, characteristics and performance can be found elsewhere [3,4]. The AMS system at the Max-Planck Institute für Biogeochemie in Jena (Germany) passed its acceptance test in 2003. Since then some details were improved to make the operation work more smoothly. In this article the modifications of the slit stabilization circuit, of the generator drive and of the procedure for the test of the rectifiers of the Cockroft–Walton generator are presented. Fig. 1 shows the schematic of the AMS system arrangement with emphasis on the components described in this article. 2. Slit stabilization The slit-stabilization controller (SSC) regulates the terminal voltage responding to the ion current difference sensed between two isolated picking-up plates, called the C12side and C14side slits, inside the off-center 13C Faraday cup behind the 110° magnet (see Fig. 1). Maximum ±110 kV voltage shift can be reached by means of the control circuit with a time constant of about 2 s. In the original design by HVEE the SSC was (i) a simple proportional controller (type P), which (ii) used the difference of the currents * Corresponding author. Address: Max-Planck Institut für Biogeochemie, HansKnöll-Str. 10, 07745 Jena, Germany. Tel.: +49 3641 57 6450; fax: +49 3641 57 7450. E-mail address: [email protected] (A. Steinhof). 0168-583X/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.nimb.2009.10.060
of the two slits inside the 13C cup, denoted IC12side and IC14side. Such a design has some intrinsic disadvantages; first it causes, for reasonable strength of the P factor, oscillations of the terminal voltage and second the magnitude of the regulation depends on the value of the currents. Instead a new PID type digital controller has been designed for the Jena AMS system. The new SSC is based on a microcontroller and uses the term given by the following equation as input signal:
regulation signal ¼
IC12side IC14side IC12side þ IC14side
The regulation signal of the new SSC is given in percent, the regulation signal of the HVEE-SSC is in principle a current differences, but will be given in HVU (High Voltage Unit). We operated the HVEE-SSC adjusted to 22 nA/HVU, since at that value the oscillations were not too strong. Because of secondary electron ejected from the picking-up plates this factor is 2–3 times smaller for an ion beam. Fig. 2 shows a block diagram of the new SSC. For the AMS data acquisition the 13C current (‘‘C13 output” in Fig. 2) is put together, as before, by the currents of both slits combined with the current of the cup main body itself. The circuit has two output signals transferred via fiber optics: the input signal for the PID controller (see equation) is sent to the main AMS control computer and the output signal of the PID controller is sent to the Cockroft–Walton generator for the terminal voltage. The operating program and the operating parameters of the new SSC can be modified via a standard RS-232 serial communication interface. The operating program is written in a high level computer language (C). Furthermore beside the standard P, I, D functions of such controllers there are other features implemented such as value limitations, digital noise filtering, or a ‘‘beam interruption watchdog”. The ‘‘beam interruption watchdog” is an adjustable threshold below which the settings of the terminal
A. Steinhof et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 902–905
903
Fig. 1. Schematic drawing of the Jena AMS system indicating the parts described in this article.
Fig. 2. Schematic of the slit stabilization circuit. The microcontroller is a FUJITSU MB90F543GSPFR microcontroller. The ADCs used for the measurement of the 3 currents are separate 16 bit ADCs. The current of the main cup body (denoted ‘C13 input’) is connected to an ADC additionally to have the option to use IC12side + IC14side + IC13input as nominator of the signal.
voltage are preserved. At present, we set this threshold to 30 nA, while operating at 400–450 nA. This is very helpful during target changes: in the process of the target leaving or reaching the sputter position the ion beams are irregular and without ion beam the regulation signal is determined by noise. Due to the preservation of the terminal settings the optimal value is quickly reached. The aim of the slit stabilization is to stabilize the ion beam position. For optimization and comparison we use the oscillations of the regulation signal, which we have to relate to the slope of the regulation signals versus the terminal voltage (Fig. 3). The oscillations result from electronic noise, variations of the ion beam position or energy, both intrinsic and induced by the SSC, and in case of the HVEE circuit, intensity fluctuations of the ion beam. We measured amplitudes of oscillations of ±20 HVU with the HVEE device
and ±8% for the new device, which is partly based on a lower noise level. These amplitudes have to be related to the slope of the regulation signal versus terminal voltage (Fig. 3), which were 24.5 HVU/kV for the HVEE device (at 420 mA 13C3+ current) and 39.8%/kV for the new device. The stabilization of the new device is therefore much better; it shows smaller oscillations at a larger slope of the regulation signal. This is an advantage if the plateau of the transmission profile (Fig. 3) gets smaller, e.g. due to larger currents or narrower slit settings for lower background. 3. Test of the rectifiers The terminal voltage of the accelerator is generated by a solidstate parallel-fed Cockroft–Walton generator [3]. In the Cockroft–
904
A. Steinhof et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 902–905
Fig. 3. Scan of the terminal voltage. The regulation signal of the HVEE SSC (‘RS old’) was measured in a different scan and given in HVU (High Voltage Units, see text). The regulation signal of the new SSC (‘RS new’) is given in percent.
Walton generator used at the Jena AMS system there are 80 stacks with 18 rectifier assemblies each. The rectifier assembly consists of 2 high voltage diodes of type SL1200 in series in potting material. The rectifiers are specified for a PRV (peak reverse voltage) of 12 kV. Due to problems with the Cockroft–Walton generator all the rectifiers were tested. Tests were started with a forward voltage drop measurement applying a forward current of 3.4 mA, but then switched to a current measurement at a high reverse voltage. Later, as the AMS system was operating again, both methods were investigated with our 150 new spare rectifiers. The rectifiers could be sorted out in 3 different groups: (i) ‘failed’ – reverse current of more than 10 lA at an applied reverse voltage of 10 kV, (ii) ‘for restricted usage’ – less than 10 lA at a voltage of 18 kV, and (iii) ‘good’ – a reversed current of less than 1 lA at 24 kV. The groups
of the defect, the limited suitable, and the good ones consist of 6, 38, and 106 pieces. In contrast to the common opinion it was found according to the test results, that the simple forward voltage drop test is not truly able to verify applicability of the rectifiers (see Fig. 4).
4. Modifications of the generator drive The turbo molecular pump for the gas stripper has to be driven potential free in the accelerator vessel (see Fig. 1). Therefore a generator driven by rotating shafts is used as power supply. In the case of the Jena AMS system, 3 shafts of approximately 750 mm each are connected in series by cardanic couplings. The original HVEE
Fig. 4. The number of high voltage rectifiers versus the voltage drop at a forward current of 3.4 mA. The numbers are given in fraction of the respective group. The qualification according to defect, limited suitable, or good was done according to the test with reverse, high voltage (see text).
A. Steinhof et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 902–905
905
Fig. 5. The two different mount principles of the sprocket wheel to the shaft. The sprocket wheel is a part of a double cardanic curved-tooth gear coupling Bowex M (KTR, Rheine Germany). In the mount with the screws (top figure) the screw heads were milled for a tight running fit to give a form closure with the acrilic glass. The bottom figure shows the parallel keys (black area) mounted with the actual shafts. The acrylic glass is indicated by the hetching lines orientated from top left to bottom right.
shafts were made of glass-fiber reinforced plastic (GFRP) with a diameter of 24 mm, but they proved to be failing in a mechanical breakdown (probably due to too much axial freedom of the bearing). After contacting other labs with similar machines we came to the conclusion, that even HVEE has been changing the design for the shafts with every newly built machine, so we investigated different alternatives: GFRP shafts (CGTEC, Gunzenhausen, Germany) – too high surge current, also such shafts of the same deliverer are used in a similar 3 MV TandetronTM at the Jena University [5], shafts made of glass-fiber reinforced PEEK (TECAPEEK GF30, Ensinger, Nufringen, Germany) with a diameter of 30 mm – mechanical rigidity too small, tube type shafts made of acrylic glass (Ø40 5 mm) (Röhm, Darmstadt, Germany), driving gears mounted by screws (see Fig. 5) – dilated fits due to mechanical tensions, rod type shafts made of acrylic glass (Ø22 mm), driving gears mounted by parallel keys as of the original design – being used for nearly 2 years, but finally abandoned because of softness on the limit,
running version – Ø40 mm rod type shafts of acrylic glass (see Fig. 5). In order to avoid the high torsion material stress the original HVEE simple ‘switch on & go’ abrupt starting switch was replaced by a more sophisticated running up electronic, which utilizes a frequency inverter (FR-S 500, Mitsubishi Electric), that makes the shafts reach the final rotating speed in 90 s. The running version is now operating for nearly two years without any problems. Therefore we recommend (i) the mount principle as shown in Fig. 5 bottom, (ii) shafts made out of acrylic glass with a diameter between 30 and 40 mm, and (iii) a soft-start electronic as described above. References [1] [2] [3] [4]
K.H. Purser, T.H. Smick, R.K. Purser, Nucl. Instr. and Meth. B 52 (1990) 263. A. Gottgang, M. Klein, D.J.W. Mous, Radiocarbon 43 (2001) 149. K.H. Purser, Radiocarbon 34 (1992) 458. M.-J. Nadeau, A.E. Litherland, A. Rieck, P.M. Grootes, Nucl. Instr. and Meth. B 223–224 (2004) 346. [5] Frank Jehn, Private communication, Friedrich-Schiller-Universität, Jena, Germany, 2005.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Improvements of the Jena AMS system Axel Steinhof *, Istvan Hejja, Thomas Wagner Max-Planck Institut für Biogeochemie, Jena, Germany
a r t i c l e
i n f o
Article history: Available online 13 October 2009 Keywords: AMS system Slit stabilization Cockroft–Walton generator High-voltage rectifiers
a b s t r a c t A new slit stabilization circuit and a modified generator drive were installed on an AMS system based on a 3 MV TandetronTM, produced by High Voltage Engineering Europa (HVEE). Furthermore our test procedure used at the Jena AMS system for the rectifiers of the Cockroft–Walton generator is presented. Ó 2009 Published by Elsevier B.V.
1. Introduction The Jena AMS system is an accelerator mass spectrometer of the third generation, operating at a terminal voltage of 2.5 MV, produced by a solid state Cockroft–Walton generator, and applying the so-called recombinator for simultaneous injection of the three carbon isotopes. After the first system was built up at the National Ocean Sciences AMS Facility at Woods Hole, Massachusetts [1], the company HVEE made this spectrometer under the tradename TandetronTM a very common system [2]. Detailed information on the design, characteristics and performance can be found elsewhere [3,4]. The AMS system at the Max-Planck Institute für Biogeochemie in Jena (Germany) passed its acceptance test in 2003. Since then some details were improved to make the operation work more smoothly. In this article the modifications of the slit stabilization circuit, of the generator drive and of the procedure for the test of the rectifiers of the Cockroft–Walton generator are presented. Fig. 1 shows the schematic of the AMS system arrangement with emphasis on the components described in this article. 2. Slit stabilization The slit-stabilization controller (SSC) regulates the terminal voltage responding to the ion current difference sensed between two isolated picking-up plates, called the C12side and C14side slits, inside the off-center 13C Faraday cup behind the 110° magnet (see Fig. 1). Maximum ±110 kV voltage shift can be reached by means of the control circuit with a time constant of about 2 s. In the original design by HVEE the SSC was (i) a simple proportional controller (type P), which (ii) used the difference of the currents * Corresponding author. Address: Max-Planck Institut für Biogeochemie, HansKnöll-Str. 10, 07745 Jena, Germany. Tel.: +49 3641 57 6450; fax: +49 3641 57 7450. E-mail address: [email protected] (A. Steinhof). 0168-583X/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.nimb.2009.10.060
of the two slits inside the 13C cup, denoted IC12side and IC14side. Such a design has some intrinsic disadvantages; first it causes, for reasonable strength of the P factor, oscillations of the terminal voltage and second the magnitude of the regulation depends on the value of the currents. Instead a new PID type digital controller has been designed for the Jena AMS system. The new SSC is based on a microcontroller and uses the term given by the following equation as input signal:
regulation signal ¼
IC12side IC14side IC12side þ IC14side
The regulation signal of the new SSC is given in percent, the regulation signal of the HVEE-SSC is in principle a current differences, but will be given in HVU (High Voltage Unit). We operated the HVEE-SSC adjusted to 22 nA/HVU, since at that value the oscillations were not too strong. Because of secondary electron ejected from the picking-up plates this factor is 2–3 times smaller for an ion beam. Fig. 2 shows a block diagram of the new SSC. For the AMS data acquisition the 13C current (‘‘C13 output” in Fig. 2) is put together, as before, by the currents of both slits combined with the current of the cup main body itself. The circuit has two output signals transferred via fiber optics: the input signal for the PID controller (see equation) is sent to the main AMS control computer and the output signal of the PID controller is sent to the Cockroft–Walton generator for the terminal voltage. The operating program and the operating parameters of the new SSC can be modified via a standard RS-232 serial communication interface. The operating program is written in a high level computer language (C). Furthermore beside the standard P, I, D functions of such controllers there are other features implemented such as value limitations, digital noise filtering, or a ‘‘beam interruption watchdog”. The ‘‘beam interruption watchdog” is an adjustable threshold below which the settings of the terminal
A. Steinhof et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 902–905
903
Fig. 1. Schematic drawing of the Jena AMS system indicating the parts described in this article.
Fig. 2. Schematic of the slit stabilization circuit. The microcontroller is a FUJITSU MB90F543GSPFR microcontroller. The ADCs used for the measurement of the 3 currents are separate 16 bit ADCs. The current of the main cup body (denoted ‘C13 input’) is connected to an ADC additionally to have the option to use IC12side + IC14side + IC13input as nominator of the signal.
voltage are preserved. At present, we set this threshold to 30 nA, while operating at 400–450 nA. This is very helpful during target changes: in the process of the target leaving or reaching the sputter position the ion beams are irregular and without ion beam the regulation signal is determined by noise. Due to the preservation of the terminal settings the optimal value is quickly reached. The aim of the slit stabilization is to stabilize the ion beam position. For optimization and comparison we use the oscillations of the regulation signal, which we have to relate to the slope of the regulation signals versus the terminal voltage (Fig. 3). The oscillations result from electronic noise, variations of the ion beam position or energy, both intrinsic and induced by the SSC, and in case of the HVEE circuit, intensity fluctuations of the ion beam. We measured amplitudes of oscillations of ±20 HVU with the HVEE device
and ±8% for the new device, which is partly based on a lower noise level. These amplitudes have to be related to the slope of the regulation signal versus terminal voltage (Fig. 3), which were 24.5 HVU/kV for the HVEE device (at 420 mA 13C3+ current) and 39.8%/kV for the new device. The stabilization of the new device is therefore much better; it shows smaller oscillations at a larger slope of the regulation signal. This is an advantage if the plateau of the transmission profile (Fig. 3) gets smaller, e.g. due to larger currents or narrower slit settings for lower background. 3. Test of the rectifiers The terminal voltage of the accelerator is generated by a solidstate parallel-fed Cockroft–Walton generator [3]. In the Cockroft–
904
A. Steinhof et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 902–905
Fig. 3. Scan of the terminal voltage. The regulation signal of the HVEE SSC (‘RS old’) was measured in a different scan and given in HVU (High Voltage Units, see text). The regulation signal of the new SSC (‘RS new’) is given in percent.
Walton generator used at the Jena AMS system there are 80 stacks with 18 rectifier assemblies each. The rectifier assembly consists of 2 high voltage diodes of type SL1200 in series in potting material. The rectifiers are specified for a PRV (peak reverse voltage) of 12 kV. Due to problems with the Cockroft–Walton generator all the rectifiers were tested. Tests were started with a forward voltage drop measurement applying a forward current of 3.4 mA, but then switched to a current measurement at a high reverse voltage. Later, as the AMS system was operating again, both methods were investigated with our 150 new spare rectifiers. The rectifiers could be sorted out in 3 different groups: (i) ‘failed’ – reverse current of more than 10 lA at an applied reverse voltage of 10 kV, (ii) ‘for restricted usage’ – less than 10 lA at a voltage of 18 kV, and (iii) ‘good’ – a reversed current of less than 1 lA at 24 kV. The groups
of the defect, the limited suitable, and the good ones consist of 6, 38, and 106 pieces. In contrast to the common opinion it was found according to the test results, that the simple forward voltage drop test is not truly able to verify applicability of the rectifiers (see Fig. 4).
4. Modifications of the generator drive The turbo molecular pump for the gas stripper has to be driven potential free in the accelerator vessel (see Fig. 1). Therefore a generator driven by rotating shafts is used as power supply. In the case of the Jena AMS system, 3 shafts of approximately 750 mm each are connected in series by cardanic couplings. The original HVEE
Fig. 4. The number of high voltage rectifiers versus the voltage drop at a forward current of 3.4 mA. The numbers are given in fraction of the respective group. The qualification according to defect, limited suitable, or good was done according to the test with reverse, high voltage (see text).
A. Steinhof et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 902–905
905
Fig. 5. The two different mount principles of the sprocket wheel to the shaft. The sprocket wheel is a part of a double cardanic curved-tooth gear coupling Bowex M (KTR, Rheine Germany). In the mount with the screws (top figure) the screw heads were milled for a tight running fit to give a form closure with the acrilic glass. The bottom figure shows the parallel keys (black area) mounted with the actual shafts. The acrylic glass is indicated by the hetching lines orientated from top left to bottom right.
shafts were made of glass-fiber reinforced plastic (GFRP) with a diameter of 24 mm, but they proved to be failing in a mechanical breakdown (probably due to too much axial freedom of the bearing). After contacting other labs with similar machines we came to the conclusion, that even HVEE has been changing the design for the shafts with every newly built machine, so we investigated different alternatives: GFRP shafts (CGTEC, Gunzenhausen, Germany) – too high surge current, also such shafts of the same deliverer are used in a similar 3 MV TandetronTM at the Jena University [5], shafts made of glass-fiber reinforced PEEK (TECAPEEK GF30, Ensinger, Nufringen, Germany) with a diameter of 30 mm – mechanical rigidity too small, tube type shafts made of acrylic glass (Ø40 5 mm) (Röhm, Darmstadt, Germany), driving gears mounted by screws (see Fig. 5) – dilated fits due to mechanical tensions, rod type shafts made of acrylic glass (Ø22 mm), driving gears mounted by parallel keys as of the original design – being used for nearly 2 years, but finally abandoned because of softness on the limit,
running version – Ø40 mm rod type shafts of acrylic glass (see Fig. 5). In order to avoid the high torsion material stress the original HVEE simple ‘switch on & go’ abrupt starting switch was replaced by a more sophisticated running up electronic, which utilizes a frequency inverter (FR-S 500, Mitsubishi Electric), that makes the shafts reach the final rotating speed in 90 s. The running version is now operating for nearly two years without any problems. Therefore we recommend (i) the mount principle as shown in Fig. 5 bottom, (ii) shafts made out of acrylic glass with a diameter between 30 and 40 mm, and (iii) a soft-start electronic as described above. References [1] [2] [3] [4]
K.H. Purser, T.H. Smick, R.K. Purser, Nucl. Instr. and Meth. B 52 (1990) 263. A. Gottgang, M. Klein, D.J.W. Mous, Radiocarbon 43 (2001) 149. K.H. Purser, Radiocarbon 34 (1992) 458. M.-J. Nadeau, A.E. Litherland, A. Rieck, P.M. Grootes, Nucl. Instr. and Meth. B 223–224 (2004) 346. [5] Frank Jehn, Private communication, Friedrich-Schiller-Universität, Jena, Germany, 2005.