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First experience with a capacitative regulation system for a 6 MV Van de Graaff accelerator
Nuclear Instruments and Methods in Physics Research A268 (1988) 442-444 North-Holland, Amsterdam
FIRST EXPERIENCE WITH A CAPACITATIVE REGULATION SYSTEM FOR A 6 MV VAN DE GRAAFF ACCELERATOR P. ARNDT, R. MICHAELSEN, M. TEICHMANN and K. ZIEGLER Hahn -Meitner-Institut Berlin GmbH, Ghemcker Sir. 100, D-1000 Berlin 39, Germany
H. HEINZE Fa. Projekt Elektronik GmbH, Stubenrauchstr. 72, 1000 Berlin 41, Germany
The energy regulation system of the Berlin 6 MV CN Van de Graaff accelerator was completed by addition of a fast capacitative terminal voltage regulation sytem. As a result the terminal voltage ripple is reduced by more than one order of magnitude. 1. Introduction The energy stability of electrostatic accelerators normally is controlled by slit current measurements in combination with a corona current regulation system . Due to the finite travel time of the electrons between the corona probe and the high voltage terminal the speed of the corona response is limited . As a result the terminal voltage ripple that is mainly due to irregular charging and decharging of the terminal cannot be smoothed totally by the corona control . For the installation of the heavy ion accelerator facility VICKSI [1] the Berlin 6 MV CN Van de Graaff machine had been converted to a heavy ion injector [2] for a k =120 separated-magnet isochronous cyclotron. As the relatively high terminal ripple might severely Fig. 2. 720 spark gap screws protect the ring insulators and the driving amplifier . affect the wanted stability and quality of the VICKSI beam we recently decided to install a fast Capacitative terminal voltage Regulation System (CRS) that would exhibit no delay in response . 2. Mechanical setup
Fig. 1. The new regulating ring is mounted at the wall of the pressure tank just below the corona probe position. 0168-9002/88/$03 .50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
The idea and the first test results with a prototype setup have already been reported at the '86 SNEAP meeting [3]. The capacitative coupling to the terminal voltage is performed by a 20 cm wide stainless steel ring that is installed close to the wall of the pressure tank (fig. 1) opposite to the terminal . The ring is insulated against the tankwall and connected to a driving amplifier via a high voltage feedthrough . Two sets of 360
P. Arndt et al. / Capacitattoe regulation system for 6 MV Van de Graaff Tankwal( Spinning
443
Terminal Ripple
lin Fig. 3 . General layout of the new regulation system. spark gap screws, attached around the perimeter of the ring (fig. 2) protect the ring insulators as well as the driving amplifier.
3 . Electronic circuits The general layout of the new regulation setup is shown in fig . 3 . The CRS works in parallel to the conventional corona current regulation system. The amplifier (fig . 4) works with two type 4PR250C tubes in push/pull mode with capacitive coupling. An IC-preamplifier regulates the signal amplitude and the centre voltage of the tubes by driving the Lower Driving Unit (active U/I converter), which controls the lower tube and limits the negative output current . The upper tube is driven from the lower tube by the Upper Driving Unit (active I/-I converter with offset) . The Upper Driving Unit provides current limiting for positive output current . The supply of the upper tube and the Upper Driving Unit is done by a low-capacitance filament winding which has a 50 kV isolation . This new
Fig. 4 . Schematic layout of the high voltage amplifier.
50msldiv.
Fig . 5 . Terminal ripple reduction by the use of the new capacitative regulation system demonstrated for different accelerated beams. (a) 36Ar3+ beam, terminal voltage = 4.2 MV and cupcurrent = 420 nA ; (b) 4°Ar 2+ , 3 .2 MV and 7 juA ; (c) 4°Ar 2+ , 5 .62 MV and 6 .5 p A. III . CHARGING/INSULATING/RELATED TECHNOLOGY
P. Arndt et al. / Capacitatiue regulation system for 6 MV Van de Graaff
4. Results and conclusions
a) without regulation
-l--7-
--~
bl with regulation
Fig. 6. Comparison of time-of-flight-shifts for the terminal bunched beam, measured about 30 m downstream the beamline behind the CN at the stripper position. (a) Without capacitative regulation - the steps are caused by different delays introduced for time calibration. (b) With regulation switched on . principle saves driving- and inverter tubes and makes possible an exact current limit of both tubes. Operating the amplifier at a peak-to-peak voltage of 40 kV into a load capacitance of 2000 pF a bandwidth of 300 Hz was obtained .
The effect of the Capacitative Regulation System is best demonstrated by comparing the terminal ripple as measured with the capacitive pickup with and without the CRS working. Fig. 5 shows for three different terminal voltages, beams, and beam currents the typical ripple pictures without (upper trace) and with CRS in operation (lower trace). With the CRS a reduction of the ripple by an order of magnitude is obtained . The effect of the higher stability on the beam is demonstrated in fig. 6. It shows the reduction of the time of flight shifts of the beam bunches measured 30 m downstream from the accelerator. Note the different vertical scale for the two traces. Again an improvement of the stability by an order of magnitude is achieved . The new CRS with the final amplifier version has now been in normal operation for two months and established a dramatic increase of stability for the VICKSI beams. The result is the successful completion of some critical experiments of our customers. The success of our capacitive regulation may encourage the colleagues of other electrostatic accelerator laboratories to try similar solutions . References
[11 Vorschlag für eine Van de Graaff-Zyklotron-Kombination zur Beschleunigung mittelschwerer Ionen, VICKSI, HMI Report B 118 (Berlin, 1972). [21 D. Hilscher, D. Renner and B. Spellmeyer, Revue Phys. Appl . 12 (1977) 13378. [31 P. Arndt and K. Ziegler, Proc. 1986 SNEAP, to be published by World Scientific Publishing .
FIRST EXPERIENCE WITH A CAPACITATIVE REGULATION SYSTEM FOR A 6 MV VAN DE GRAAFF ACCELERATOR P. ARNDT, R. MICHAELSEN, M. TEICHMANN and K. ZIEGLER Hahn -Meitner-Institut Berlin GmbH, Ghemcker Sir. 100, D-1000 Berlin 39, Germany
H. HEINZE Fa. Projekt Elektronik GmbH, Stubenrauchstr. 72, 1000 Berlin 41, Germany
The energy regulation system of the Berlin 6 MV CN Van de Graaff accelerator was completed by addition of a fast capacitative terminal voltage regulation sytem. As a result the terminal voltage ripple is reduced by more than one order of magnitude. 1. Introduction The energy stability of electrostatic accelerators normally is controlled by slit current measurements in combination with a corona current regulation system . Due to the finite travel time of the electrons between the corona probe and the high voltage terminal the speed of the corona response is limited . As a result the terminal voltage ripple that is mainly due to irregular charging and decharging of the terminal cannot be smoothed totally by the corona control . For the installation of the heavy ion accelerator facility VICKSI [1] the Berlin 6 MV CN Van de Graaff machine had been converted to a heavy ion injector [2] for a k =120 separated-magnet isochronous cyclotron. As the relatively high terminal ripple might severely Fig. 2. 720 spark gap screws protect the ring insulators and the driving amplifier . affect the wanted stability and quality of the VICKSI beam we recently decided to install a fast Capacitative terminal voltage Regulation System (CRS) that would exhibit no delay in response . 2. Mechanical setup
Fig. 1. The new regulating ring is mounted at the wall of the pressure tank just below the corona probe position. 0168-9002/88/$03 .50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
The idea and the first test results with a prototype setup have already been reported at the '86 SNEAP meeting [3]. The capacitative coupling to the terminal voltage is performed by a 20 cm wide stainless steel ring that is installed close to the wall of the pressure tank (fig. 1) opposite to the terminal . The ring is insulated against the tankwall and connected to a driving amplifier via a high voltage feedthrough . Two sets of 360
P. Arndt et al. / Capacitattoe regulation system for 6 MV Van de Graaff Tankwal( Spinning
443
Terminal Ripple
lin Fig. 3 . General layout of the new regulation system. spark gap screws, attached around the perimeter of the ring (fig. 2) protect the ring insulators as well as the driving amplifier.
3 . Electronic circuits The general layout of the new regulation setup is shown in fig . 3 . The CRS works in parallel to the conventional corona current regulation system. The amplifier (fig . 4) works with two type 4PR250C tubes in push/pull mode with capacitive coupling. An IC-preamplifier regulates the signal amplitude and the centre voltage of the tubes by driving the Lower Driving Unit (active U/I converter), which controls the lower tube and limits the negative output current . The upper tube is driven from the lower tube by the Upper Driving Unit (active I/-I converter with offset) . The Upper Driving Unit provides current limiting for positive output current . The supply of the upper tube and the Upper Driving Unit is done by a low-capacitance filament winding which has a 50 kV isolation . This new
Fig. 4 . Schematic layout of the high voltage amplifier.
50msldiv.
Fig . 5 . Terminal ripple reduction by the use of the new capacitative regulation system demonstrated for different accelerated beams. (a) 36Ar3+ beam, terminal voltage = 4.2 MV and cupcurrent = 420 nA ; (b) 4°Ar 2+ , 3 .2 MV and 7 juA ; (c) 4°Ar 2+ , 5 .62 MV and 6 .5 p A. III . CHARGING/INSULATING/RELATED TECHNOLOGY
P. Arndt et al. / Capacitatiue regulation system for 6 MV Van de Graaff
4. Results and conclusions
a) without regulation
-l--7-
--~
bl with regulation
Fig. 6. Comparison of time-of-flight-shifts for the terminal bunched beam, measured about 30 m downstream the beamline behind the CN at the stripper position. (a) Without capacitative regulation - the steps are caused by different delays introduced for time calibration. (b) With regulation switched on . principle saves driving- and inverter tubes and makes possible an exact current limit of both tubes. Operating the amplifier at a peak-to-peak voltage of 40 kV into a load capacitance of 2000 pF a bandwidth of 300 Hz was obtained .
The effect of the Capacitative Regulation System is best demonstrated by comparing the terminal ripple as measured with the capacitive pickup with and without the CRS working. Fig. 5 shows for three different terminal voltages, beams, and beam currents the typical ripple pictures without (upper trace) and with CRS in operation (lower trace). With the CRS a reduction of the ripple by an order of magnitude is obtained . The effect of the higher stability on the beam is demonstrated in fig. 6. It shows the reduction of the time of flight shifts of the beam bunches measured 30 m downstream from the accelerator. Note the different vertical scale for the two traces. Again an improvement of the stability by an order of magnitude is achieved . The new CRS with the final amplifier version has now been in normal operation for two months and established a dramatic increase of stability for the VICKSI beams. The result is the successful completion of some critical experiments of our customers. The success of our capacitive regulation may encourage the colleagues of other electrostatic accelerator laboratories to try similar solutions . References
[11 Vorschlag für eine Van de Graaff-Zyklotron-Kombination zur Beschleunigung mittelschwerer Ionen, VICKSI, HMI Report B 118 (Berlin, 1972). [21 D. Hilscher, D. Renner and B. Spellmeyer, Revue Phys. Appl . 12 (1977) 13378. [31 P. Arndt and K. Ziegler, Proc. 1986 SNEAP, to be published by World Scientific Publishing .