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A self-energizing beam position stabilizer for low voltage accelerators
NUCLEAR INSTRUMENTS
A SELF-ENERGIZING
AND METHODS 72 (1969)
BEAM P O S I T I O N
220-222;
© NORTH-HOLLAND
P U B L I S H I N G CO.
STABILIZER FOR L O W VOLTAGE ACCELERATORS* D. A. S. WALKER +
Research Institute Jor Physics, Stockholm 50, Sweden Received 10 April 1969 Two resistors, connected between a pair of pickup pins and ground, develope voltages which are proportional to the current striking each pin. These voltages are fed back to a pair of electrostatic deflection plates to maintain stable beam position about
the pins. Experimental results obtained with an isotope separator are reported and possible applications to higher energy facilities are briefly discussed.
Most experiments involving accelerators require an energy or position stabilizer to maintain maximum beam current at the target. In this regard, the application of pin stabilization systems to isotope separators has been widespread. This stabilizer often involves some form of differential amplification with the compensation applied to a pair of electrostatic deflection plates. This paper describes a simple position stabilizing system employing the conventional method of beam position sensing (i.e. pickup pins). The correction voltages, generated by resistors connected between each pickup pin and ground, are however fed directly back to the deflection plates thus eliminating the need for high gain differential amplifiers. It is desirable to operate with negative potentials since any positive potential along the beam path tends to extract electrons from the beam plasma. With the present system these conditions could be easily obtained by inserting a negative potential between the resistors and ground. Stabilizers employing electron tubes often involve quiescent voltages of several hundred V (positive). The lower voltages used in the present deflection system ( < 0.1% of the maximum acceleration potential) make it more suitable than other methods of electrostatic stabilization. This system was recently investigated in connection with the isotope separator at this Institute 1) and the results are reported together with a brief discussion of the possibilities of extending this method to higher energies.
deflected towards pin A because of the magnetic field. The increased current flowing from pin A through RL to ground produces an additional voltage (EL) which is fed back to stabilizer plate C. This action maintains the beam in a symmetrical position about pins A and B. The position of the pickup pins and deflection plates were identical to that described by Thulin2). The stabilizing network could be placed in or out of the system (electrically) by means of a switch (not shown in fig. l) located at the end or the pin supporting arm, outside the target chamber. Two spot galvanometers and an electrometer were used to monitor the pin and target currents respectively. The beam position could be observed on a fluorescent screen. The l0 s ohm stabilizing resistors 3) were mounted in a shielded box and thoroughly cleaned with alcohol. The time
The system: The basic system, as applied to an isotope separator, is shown in fig. 1. If, for example, the acceleration potential decreases, the beam is * This research has been sponsored, in part, by the Advanced Electronic Devices Branch, Air Force Avionics Laboratory, through the European Office of Aerospace Research, OAR, United States Air Force under contract F-61052-68-C-0062. t On leave of absence from the Research Chemistry Branch, Atomic Energy of Canada Ltd., Chalk River, Ontario, Canada.
220
DEFLECTION " ~C~" PLATES(C,D) L
~
- .
~
MAGNET
RL=RR
PICKUP PINS(A,B) . Iw____._h.iB RL@ (~ (
RR
} //////// Fig. 1. The basic stabilizing system: showing the target (T), the pickup pins (A and B), the stabilizing resistors (RL and Rx) and the deflection plates (C and D). Mcters IL, lr~ and Ix monitor the pin and target currents.
A
S E L F - E N E R G I Z I N G
B E A M
constant of this system was calculated to be about 0.2 msec. For optimum performance the pins were arranged :such that a small portion of the beam profile strikes each pin thereby producing low initial pin currents and correspondingly low correction voltages. This arrangement had no visible effects on the beam focus. The voltage coefficients of resistors of large value ( > 107ohm) are often nonlinear and should be determined beforehand. I f resistors of the same manufacture are used this effect should be selfcancelling. In any installation a low leakage rotary ,,;witch for selecting several values of resistance could be incorporated to handle a wider range of currents and aid in determining the optimum value of resistance for a particular application. Shielded cable should be used for all external connections.
Experimental results: Three sets of measurements were performed with a 12C beam at an acceleration voltage of 55 keV and a target current of approximately 2ktA. The pickup pin spacing was 5 mm. Under these conditions, taking the geometrical characteristics of the separator into consideration, a deflection potential of approximately 70 V was required to deflect the beam a distance of 5 m m at the pickup pins. The average pin current was initially adjusted to 0.75/tA which corresponded to a potential of 75 V on each plate. The following observations were made: 1. The beam could be forced to assume a position
Impu[sf,
P O S I T I O N
221
S T A B I L I Z E R
approximately _+3 m m of its mean unstabilized position by manual horizontal movement of the pin supporting arm; 2. The pins were moved out of the beam path, in both directions, and, by switching the stabilizer on, the beam was forced into a symmetrical position about the pins from a distance of 5 m m ; 3. The pins were centered on an unstabilized beam and the acceleration voltage was varied. The target current remained constant for changes of approximately _+70 V. Increasing the pin current, by defocusing the beam, permitted a correspondingly wider change in the acceleration potential. All the above observations are consistent with the initial experimental conditions (viz. 5 m m of deflection for a 75 V deflection potential). A similar system was previously used by the author at the Chalk River Laboratories 4) to maintain an analysed 1.5 MeV beam of electrons, produced by a van de Graaff, in the centre of a 5 m m aperture. For that application two 5 x l0 s ohm resistors 4) were used to provide about I kV deflection potentials. The time constant of that network was sufficiently short ( ~ 25 msec) to produce adequate correction for the 10 keV terminal voltage variations. The differential amplifier normally used for beam position stabilization in an isotope separator has been replaced by a pair of resistors. Experiments indicate the same order of stability over a restricted range of currents. This range however, can easily be extended,
Ep = 3.88 MeV
Kanal
100C EK=i3keV
50C
.." .
Ex=i76 key
•'", "" ' ..-.,...'... ,'..'....'-" "" •. "..%......
i
J
i
,
J
50
i
i
,
,
..
~......"
i
100
.
i
,
i
i
1
~"
150 Kanalzahl
Fig. 2. Calculated graphs showing the pin current required to deflect a beam of given energy a distance of 5 mm for several values of stabilizing resistance. Conditions: deflection plate length is twice the plate spacing, pickup pin to deflection plate distance is 4.4 m.
222
D.A.S.
within practical limits, to handle beam currents varying over several orders of magnitude. This system behaves in an identical manner to the electron tube stabilizer to the extent that, even if the beam is initially outside the pins, a correction voltage of the proper polarity will be obtained as long as the fringing field of the beam strikes either of the pins. It is apparent that this simple method could be applied to other acceleration facilities at different energies. This is examined in fig. 2. This figure shows the pickup pin current required to maintain a beam of given energy within a distance of 5 mm, as a function of stabilizing resistance, where the deflection plate length is twice the plate spacing and the plate to pin distance is 4.4 m. These are the conditions used in many isotope separators but similar graphs are easily calculated for other accelerator facilities. From fig. 2a it is apparent that a wide range of resistance values
WALKER
could be used with low energy facilities (i.e. < 500 keV). Using the same configuration at higher energies (fig. 2b) requires a compromise between pin current intensity and maximum practical value of resistance, the latter being limited by the leakage currents in the external circuitry. I wish to thank S. Borg for his continued interest in this work and for his many valuable comments. The assistance of R. Buchta during the installation and testing of this apparatus is greatfully acknowledged. References 1) T. Alviiger and J. Uhler, Arkiv Fysik 13 (1958) 145. 2) S. Thulin, Arkiv Fysik 9 (1954) 107. ~) Victoreen Instrument Co., Cleveland, Ohio, U.S.A., Model RX-l. 4) Resistance Products Co., Harrisburg, Pa., U.S.A.
A SELF-ENERGIZING
AND METHODS 72 (1969)
BEAM P O S I T I O N
220-222;
© NORTH-HOLLAND
P U B L I S H I N G CO.
STABILIZER FOR L O W VOLTAGE ACCELERATORS* D. A. S. WALKER +
Research Institute Jor Physics, Stockholm 50, Sweden Received 10 April 1969 Two resistors, connected between a pair of pickup pins and ground, develope voltages which are proportional to the current striking each pin. These voltages are fed back to a pair of electrostatic deflection plates to maintain stable beam position about
the pins. Experimental results obtained with an isotope separator are reported and possible applications to higher energy facilities are briefly discussed.
Most experiments involving accelerators require an energy or position stabilizer to maintain maximum beam current at the target. In this regard, the application of pin stabilization systems to isotope separators has been widespread. This stabilizer often involves some form of differential amplification with the compensation applied to a pair of electrostatic deflection plates. This paper describes a simple position stabilizing system employing the conventional method of beam position sensing (i.e. pickup pins). The correction voltages, generated by resistors connected between each pickup pin and ground, are however fed directly back to the deflection plates thus eliminating the need for high gain differential amplifiers. It is desirable to operate with negative potentials since any positive potential along the beam path tends to extract electrons from the beam plasma. With the present system these conditions could be easily obtained by inserting a negative potential between the resistors and ground. Stabilizers employing electron tubes often involve quiescent voltages of several hundred V (positive). The lower voltages used in the present deflection system ( < 0.1% of the maximum acceleration potential) make it more suitable than other methods of electrostatic stabilization. This system was recently investigated in connection with the isotope separator at this Institute 1) and the results are reported together with a brief discussion of the possibilities of extending this method to higher energies.
deflected towards pin A because of the magnetic field. The increased current flowing from pin A through RL to ground produces an additional voltage (EL) which is fed back to stabilizer plate C. This action maintains the beam in a symmetrical position about pins A and B. The position of the pickup pins and deflection plates were identical to that described by Thulin2). The stabilizing network could be placed in or out of the system (electrically) by means of a switch (not shown in fig. l) located at the end or the pin supporting arm, outside the target chamber. Two spot galvanometers and an electrometer were used to monitor the pin and target currents respectively. The beam position could be observed on a fluorescent screen. The l0 s ohm stabilizing resistors 3) were mounted in a shielded box and thoroughly cleaned with alcohol. The time
The system: The basic system, as applied to an isotope separator, is shown in fig. 1. If, for example, the acceleration potential decreases, the beam is * This research has been sponsored, in part, by the Advanced Electronic Devices Branch, Air Force Avionics Laboratory, through the European Office of Aerospace Research, OAR, United States Air Force under contract F-61052-68-C-0062. t On leave of absence from the Research Chemistry Branch, Atomic Energy of Canada Ltd., Chalk River, Ontario, Canada.
220
DEFLECTION " ~C~" PLATES(C,D) L
~
- .
~
MAGNET
RL=RR
PICKUP PINS(A,B) . Iw____._h.iB RL@ (~ (
RR
} //////// Fig. 1. The basic stabilizing system: showing the target (T), the pickup pins (A and B), the stabilizing resistors (RL and Rx) and the deflection plates (C and D). Mcters IL, lr~ and Ix monitor the pin and target currents.
A
S E L F - E N E R G I Z I N G
B E A M
constant of this system was calculated to be about 0.2 msec. For optimum performance the pins were arranged :such that a small portion of the beam profile strikes each pin thereby producing low initial pin currents and correspondingly low correction voltages. This arrangement had no visible effects on the beam focus. The voltage coefficients of resistors of large value ( > 107ohm) are often nonlinear and should be determined beforehand. I f resistors of the same manufacture are used this effect should be selfcancelling. In any installation a low leakage rotary ,,;witch for selecting several values of resistance could be incorporated to handle a wider range of currents and aid in determining the optimum value of resistance for a particular application. Shielded cable should be used for all external connections.
Experimental results: Three sets of measurements were performed with a 12C beam at an acceleration voltage of 55 keV and a target current of approximately 2ktA. The pickup pin spacing was 5 mm. Under these conditions, taking the geometrical characteristics of the separator into consideration, a deflection potential of approximately 70 V was required to deflect the beam a distance of 5 m m at the pickup pins. The average pin current was initially adjusted to 0.75/tA which corresponded to a potential of 75 V on each plate. The following observations were made: 1. The beam could be forced to assume a position
Impu[sf,
P O S I T I O N
221
S T A B I L I Z E R
approximately _+3 m m of its mean unstabilized position by manual horizontal movement of the pin supporting arm; 2. The pins were moved out of the beam path, in both directions, and, by switching the stabilizer on, the beam was forced into a symmetrical position about the pins from a distance of 5 m m ; 3. The pins were centered on an unstabilized beam and the acceleration voltage was varied. The target current remained constant for changes of approximately _+70 V. Increasing the pin current, by defocusing the beam, permitted a correspondingly wider change in the acceleration potential. All the above observations are consistent with the initial experimental conditions (viz. 5 m m of deflection for a 75 V deflection potential). A similar system was previously used by the author at the Chalk River Laboratories 4) to maintain an analysed 1.5 MeV beam of electrons, produced by a van de Graaff, in the centre of a 5 m m aperture. For that application two 5 x l0 s ohm resistors 4) were used to provide about I kV deflection potentials. The time constant of that network was sufficiently short ( ~ 25 msec) to produce adequate correction for the 10 keV terminal voltage variations. The differential amplifier normally used for beam position stabilization in an isotope separator has been replaced by a pair of resistors. Experiments indicate the same order of stability over a restricted range of currents. This range however, can easily be extended,
Ep = 3.88 MeV
Kanal
100C EK=i3keV
50C
.." .
Ex=i76 key
•'", "" ' ..-.,...'... ,'..'....'-" "" •. "..%......
i
J
i
,
J
50
i
i
,
,
..
~......"
i
100
.
i
,
i
i
1
~"
150 Kanalzahl
Fig. 2. Calculated graphs showing the pin current required to deflect a beam of given energy a distance of 5 mm for several values of stabilizing resistance. Conditions: deflection plate length is twice the plate spacing, pickup pin to deflection plate distance is 4.4 m.
222
D.A.S.
within practical limits, to handle beam currents varying over several orders of magnitude. This system behaves in an identical manner to the electron tube stabilizer to the extent that, even if the beam is initially outside the pins, a correction voltage of the proper polarity will be obtained as long as the fringing field of the beam strikes either of the pins. It is apparent that this simple method could be applied to other acceleration facilities at different energies. This is examined in fig. 2. This figure shows the pickup pin current required to maintain a beam of given energy within a distance of 5 mm, as a function of stabilizing resistance, where the deflection plate length is twice the plate spacing and the plate to pin distance is 4.4 m. These are the conditions used in many isotope separators but similar graphs are easily calculated for other accelerator facilities. From fig. 2a it is apparent that a wide range of resistance values
WALKER
could be used with low energy facilities (i.e. < 500 keV). Using the same configuration at higher energies (fig. 2b) requires a compromise between pin current intensity and maximum practical value of resistance, the latter being limited by the leakage currents in the external circuitry. I wish to thank S. Borg for his continued interest in this work and for his many valuable comments. The assistance of R. Buchta during the installation and testing of this apparatus is greatfully acknowledged. References 1) T. Alviiger and J. Uhler, Arkiv Fysik 13 (1958) 145. 2) S. Thulin, Arkiv Fysik 9 (1954) 107. ~) Victoreen Instrument Co., Cleveland, Ohio, U.S.A., Model RX-l. 4) Resistance Products Co., Harrisburg, Pa., U.S.A.