REVIEW OF SCIENTIFIC INSTRUMENTS 85, 125106 (2014)
Nondestructive synchronous beam current monitor Michel Kireeff Covoa) Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA
(Received 12 September 2014; accepted 15 November 2014; published online 9 December 2014) A fast current transformer is mounted after the deflectors of the Berkeley 88-Inch Cyclotron. The measured signal is amplified and connected to the input of a lock-in amplifier. The lock-in amplifier performs a synchronous detection of the signal at the cyclotron second harmonic frequency. The magnitude of the signal detected is calibrated against a Faraday cup and corresponds to the beam intensity. It has exceptional resolution, long term stability, and can measure the beam current leaving the cyclotron as low as 1 nA. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4902903] I. INTRODUCTION
The 88-Inch Cyclotron at Lawrence Berkeley National Laboratory is a sector-focused cyclotron with both light- and heavy-ion capabilities that supports a local research program in nuclear science and is the home of the Berkeley Accelerator Space Effects Facility. The cyclotron has three ion sources that have led to progressively higher intensities and charge states of heavier ions. The ions produced by the ion sources are axially injected inside the 88-Inch Cyclotron. After the ions enter the cyclotron, they are accelerated by a radio frequency (RF) electric field and held to a spiral trajectory by a static magnetic field. The RF field makes the ions to bunch up into packets. The ions gain velocity and the orbit increases with radius. The ions that are not synchronized with the RF are lost. The cyclotron operates in the frequency range of 5.5– 16.5 MHz, but it can operate using harmonic acceleration, so the energy range of the machine is limited only by the capabilities of the magnet, not the RF system. Early on it was realized that the variable frequency of the cyclotron translated to a mass resolution of 1/3000, meaning that the cyclotron could separate most ions of near identical mass-to-charge ratio emanating from the ion source. The combination of cyclotron and electron cyclotron resonance ion sources provides the unique ability to run “cocktails” of ions. A cocktail is a mixture of ions of near identical chargeto-mass ratio.1 During the cyclotron operation, the ions are tuned out of the source together. The cyclotron acts as a charge-to-mass analyzer to separate them, providing different ion species and charge states for energy variable experiments. The wideband driven RF system for the 88-Inch Cyclotron offers fast beam tuning,2 allowing users to switch back and forth between several ion species of the same cocktail with small adjustments of the accelerator frequency, so a new beam does not require retuning the whole accelerator and is accomplished in approximately 1 min. Sections II and III will review recent nondestructive techniques to measure the beam intensity and show the design and commissioning of a synchronous beam current monitor that uses a fast beam current transformer connected to a locka) Electronic mail: [email protected].
0034-6748/2014/85(12)/125106/5/$30.00
in amplifier. The large bandwidth and short rise time of the transformer allows exceptional high resolution current measurements. The transformer is installed after the exit deflector to measure the beam current of the cyclotron down to 1 nA. The measured current is acquired, stored in files, and displayed in a computer screen. II. NONDESTRUCTIVE TECHNIQUES
Monitoring the intensity without obstructing the beam is required to maintain the cyclotron efficiency and not disturb the experiments. Current measurements down to few nanoamps range is an extremely challenging task. The noise can easily mask the beam current signal to be measured. Several nondestructive techniques to monitor the beam intensity are described below, such as wall current monitors, beam current transformers, and capacitive monitors. A resistive wall monitor from FERMILAB Main Injector measures the image charge that flows along the beam lines with a 1 ohm gap impedance.3 Currents not associated with the image current can interfere with measurements, so noise reduction techniques such as shielding and microwave absorbers are used to increase the signal-to-noise ratio. A Fast Current Transformer (FCT) and a SR844 lock-in amplifier are installed in the E6 beamline of RIKEN.4 The signal vector of the beam current that is 90◦ off-phase with the noise is measured in order to decrease the noise. The assumption is that the noise phase is constant and does not coincide with the phase of the beam current signal. Otherwise, the beam current monitor (BCM) will need to be moved to another location. The cyclotron beam is also chopped at 0.01 Hz with duty cycle of 50% to enhance the visibility. A lowintensity beam of 3 nA is measured. A current transformer with a ceramic break is also used in the Tevatron at FERMILAB to measure the transfer line beam intensities.5 The signal is processed with a field programmable gate array and the main source of noise identified is the analog-to-digital and digital-to-analog conversions. At the cyclotron in the University of Jyväskylä, a capacitive probe is connected to a very narrow phase doubleheterodyne receiver with in-phase and quadrature-phase demodulator. It measures the amplitude of the beam current signal at the cyclotron frequency.6 The current intensity of 100 nA is measured using this technique. A capacitive probe
85, 125106-1
© 2014 AIP Publishing LLC
125106-2
Michel Kireeff Covo
Rev. Sci. Instrum. 85, 125106 (2014)
FIG. 1. Fast current transformer. (a) Fast current transformer mounted inside the vacuum chamber is shown at the left side and a shield at the right side. (b) Assembly mounted in the staging line after the cyclotron deflectors.
is also connected to a SR844 lock-in amplifier in the RIBF at RIKEN and measures a beam current down to 10 nA.7 This paper shows a system made mostly with commercial components. It combines the high gain of the FCT with the selectivity of the lock-in amplifier to extract a faint beam intensity signal from an extremely noisy environment such as a cyclotron laboratory, similar to the hardware given in Ref. 4. However, rather than relying on the stability of the noise phase or chopping the beam to improve the signal ratio, a meticulous work is done to suppress the noise from the system and permit synchronous detection of the beam bunch signal at the cyclotron second harmonic frequency intensity down to 1 nA intensity. The noise level is decreased by filtering the common-mode noise, placing a low noise amplifier close to the sensor, using a hard line cable for long transmission, and filtering the high frequency noise signals that would add in the square wave detecting lock-in. This technique allows outstanding performance of the BCM and will be discussed next. III. BEAM CURRENT MONITOR A. Fast current transformer
The fast current transformer, model FCT-082-05:1-H from Bergoz Instrumentation,8 is shown in Figure 1. It is a toroidal core made of cobalt-based nanocrystalline and amorphous alloys of 82 mm inner diameter and a five turn coil wound, which has a gain of 5 V/A. The bandwidth of the FCT is 32 KHz to 700 MHz with typical rise time of 500 ps. The FCT is mounted inside a spherical octagon ultrahigh vacuum chamber with a floating coaxial feedthrough, Figure 1(a). The vacuum chamber is installed in the staging line after the cyclotron deflectors, Figure 1(b). Therefore, any beam leaving the cyclotron will cross it and the wall current will be diverted around the outside of the device. A shield, grounded only upstream, is installed to protect the FCT from beam losses.
The signal is then sent to the control room by a low loss heliax cable and filtered by another common-mode choke, which is made with two turns of cable around a ferrite toroid of material 43. The signal is finally connected to the input of the lockin amplifier model SR844 from Stanford Research Systems.9 The lock-in amplifier performs synchronous detection and can discriminate extremely low amplitude signal from noise. A low-pass filter model BLP-70+ is added before the lock-in amplifier to increase the signal-to-noise ratio by filtering high frequency noises generated by the cyclotron. It reduces the noise contribution to the signal measured because the SR844 is a square wave detecting lock-in that adds all the signals at the odd harmonics of the reference signal. The sharp bunch shape with a duty cycle of ∼9% present in the cyclotron creates many harmonics10 with only ∼30% of the total spectral power concentrated in its fundamental.11 Therefore, the harmonic content can be measured because of the fast response of the FCT. Although the cyclotron laboratory noise is higher at the fundamental cyclotron frequency, the noise decreases with increasing frequency. For that reason synchronous detection with the second harmonic is chosen. The signal from the cyclotron RF generator is connected to the frequency doubler, model RK-3 from Mini Circuits, and generates the second harmonic, which is used as a reference signal for the lock-in amplifier. The SR844 has an internal harmonic detection of the 2F component, however the external reference signal assures that the lock-in amplifier meets its highest technical specifications. The noise measured without beam crossing the FCT has components generated by the RF system. Large phase noise variations are observed even in small frequency adjustments during a cocktail run. Therefore, the amplitude is used to measure the beam intensity, rather than the phase, because it is less sensitive to variations.
B. Hardware and system
Figure 2 shows the BCM system. The signal of the FCT is filtered by a common-mode choke, which consists of four turns of BNC cable around three ferrite toroids of material 43. The signal is then strengthen by a high linearity ultraband low noise amplifier, model RLNA01M03GA from RF-Lambda, which has a gain of 38 dB and 1.5 dB noise figure.
IV. MEASUREMENTS A. Short term performance
Figure 3 shows beam current measurements of a high intensity 219.2 MeV 48 Ca+11 ion beam development run. The intensity of the beam is decreased from 3.4 uA to 12 nA using
125106-3
Michel Kireeff Covo
Rev. Sci. Instrum. 85, 125106 (2014)
FIG. 2. BCM system. The FCT measures the beam current. The signal is amplified by a low noise amplifier and transmitted to a lock-in amplifier. The reference signal for the lock-in amplifier is obtained by doubling the RF frequency of the cyclotron. The lock-in amplifier detects the second harmonic component of the beam spectrum.
FIG. 3. Beam current measurements of 219.2 MeV 48 Ca+11 ion beam. (a) FC versus BCM measurements with a power trend line fitting. (b) BCM and FC current measurements over time in linear scale. (c) BCM and FC current measurements over time in logarithmic scale.
125106-4
Michel Kireeff Covo
Rev. Sci. Instrum. 85, 125106 (2014)
FIG. 4. Beam current measurements of 50 MeV H+ ion beam. (a) FC versus BCM measurements with a power trend line fitting. (b) BCM and FC current measurements in linear scale. (c) BCM and FC current measurements in logarithmic scale.
an attenuator system that has screens and is located in the low energy injection line.12 Figure 3(a) shows the current measured with a Faraday cup (FC) in the ordinate and the signal measured with the BCM in the abscissa. A power trend line fits the data and shows a coefficient close to 1. The small nonlinearity is attributed to space charge longitudinal relaxation of the beam. Figure 3(b) shows the FC signal with a blue solid circle marker, using the fitting equation obtained in Figure 3(a), and the BCM signal with a red solid triangle marker that are plotted over time. Figure 3(c) shows the ordinate in log scale to display the current variations down to 10 nA. Figure 4 shows beam current measurements of a low intensity 50 MeV H+ ion beam development run. The intensity of the beam is decreased from 20 nA to 1 nA using attenuators. Figure 4(a) shows measurements of the beam current obtained with a FC in the ordinate and the BCM signal measured in the abscissa. The power trend line has a similar nonlinearity observed in Figure 3(a). Figure 4(b) shows the FC with a blue solid circle marker, using the fitting equation obtained in Figure 4(a), and the BCM signal with a red solid triangle marker that are plotted over time. Figure 4(c) shows the ordinate in log scale to display current variations down to 1 nA with resolved intensity of ∼0.2 nA.
Figures 3 and 4 show that the data measured with the two different instruments (i.e., FC and BCM) are in excellent agreement. At the beam current of 1nA, the scattering of the Faraday cup measurements was ∼0.05 nA and the scattering of the beam current monitor measurements was ∼0.1 nA, so the agreement down to 1 nA can be assured. The output of the BCM is connected to a computer that acquires, displays, and stores the data in files. The BCM is cross-calibrated with a FC measurement at the beginning of each run and a linear correlation is assumed.
B. Long term performance
Possible variations of the time structure of the external beam, which may happen over time or when making small tuning adjustments on the cyclotron, could modify the harmonic components of the beam bunch spectrum. Figure 5 shows the data from a run of 244 MeV 50 Ti+12 ion beam during a period of 50 h on 03/09/2014 keeping the same amplitude calibration of the BCM. The BCM is plotted using a blue solid triangle marker and the primary ordinate at the left. A dedicated measurement with a Faraday cup is not feasible because it would require a very long cyclotron beam
125106-5
Michel Kireeff Covo
Rev. Sci. Instrum. 85, 125106 (2014)
compared with measurements of the neutron dose, produced by the beam inside the experimental area, and show excellent long term stability of the BCM system. It gives confidence that possible changes in the time structure of the external beam over time or due to small tuning adjustments do not affect the measurements.
ACKNOWLEDGMENTS
FIG. 5. Long term BCM measurement of 244 MeV 50 Ti+12 ion beam, using the left ordinate, and experimental area neutron dose, using the right ordinate.
time; therefore the neutron dose, concurrently measured at the experimental area after the beam hits the target, is plotted using a red solid circle marker and the secondary ordinate at the right. The time is shown in the abscissa. Assuming that the neutron dose is proportional to the beam intensity, the plot shows the excellent BCM long term performance without a significant variation of the calibration factor from the beam current monitor. The outliner points are generated during RF breakdowns or when the RF is turned off so the experimenters can get inside of the cave and make adjustments to the equipment. V. CONCLUSIONS
A FCT is mounted after the deflectors of the 88-Inch Cyclotron. The signal is filtered, amplified, and sent to a lock-in amplifier that performs synchronous detection at the second harmonic of the cyclotron RF. The beam current is varied with attenuators located in the axial injection. A power trend line shows the best fit of the FC versus the BCM data. The beam current signal, obtained using the best fit equation, is plotted against the FC measurement and shows remarkable agreement down to 1nA. A BCM system is then implemented assuming linear correlation because the coefficient of the power trend line equation is almost 1. The amplitude of the vector detected is calibrated against a FC measurement, giving the beam intensity. The data recorded by the BPM system during two days are
The author would like to thank Brien Ninemire, Catherine R. Siero, Thomas Gimpel, and Scott M. Small for the cyclotron operation support, Adrian Hodgkinson, Thomas Perry, and John P. Garcia for the mechanical support, and Mark Regis and Brendan Ford for the electronic support. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Award No. DE-AC02-05CH11231. 1 R.
D. Leitner-Wutte, M. A. McMahan, D. Argento, T. Gimpel, A. Guy, J. Morel, R. Siero, R. Thatcher, and C. M. Lyneis, “Heavy ion cocktail beams at the 88-Inch Cyclotron,” in Proceedings of the 15th International Workshop on ECR Ion Sources (ECRIS’02), Jyväskylä, Finland, 12–14 June 2002, http://www.jacow.org. 2 M. Kireeff Covo, Int. J. Microw. Wirel. Technol. 4, 553 (2012). 3 B. Fellenz and J. Crisp, “An improved resistive wall monitor,” in Proceedings of Beam Instrumentation Workshop, Stanford, California, USA, 4–7 May 1998, AIP Conf. Proc. 451, 446 (1998). 4 M. Wada, Y. Ishida, T. Nakamura, A. Takamine, A. Yoshida, and Y. Yamazaki, “Nondestructive intensity monitor for cyclotron beams,” RIKEN Accel. Prog. Rep. 38 (2005), available at http://www.nishina.riken.jp/ researcher/APR/Document/ProgressReport_vol_38.pdf. 5 J. Crisp, B. Fellenz, J. Fitzgerald, D. Heikkinen, and M. A. Ibrahim, “Operation of the intensity monitors in beam transport lines at Fermilab during Run II,” J. Instrum. 6, T10001 (2011). 6 J. Gustafsson, Nucl. Instrum. Methods Phys. Res., Sect. A 385, 189 (1997). 7 R. Koyama, N. Sakamoto, M. Fujimaki, N. Fukunishi, A. Goto, M. Hemmi, M. Kase, K. Suda, T. Watanabe, K. Yamada, and O. Kamigaito, Nucl. Instrum. Methods Phys. Res., Sect. A 729, 788 (2013). 8 Bergoz Precision Beam Instrumentation: Fast current transformer datasheet. 9 Stanford Research Systems: SR844 lock-in amplifier user’s manual. 10 M. Kireeff Covo, “Nondestructive beam current monitor for the 88Inch Cyclotron,” in Proceedings of the International Particle Accelerator Conference (IPAC’14), Dresden, Germany, 15–20 June 2014, http://www.jacow.org, p. 3738. 11 K. V. Ettinger and F. R. Stewart, “A non-intercepting cyclotron beam monitor,” in Proceedings of the International Cyclotron Conference (CYCLOTRONS’69), London, UK, 17–20 September 1969, http:// www.jacow.org, p. 403. 12 R. F. Burton, D. J. Clark, and C. M. Lyneis, Nucl. Instrum. Methods Phys. Res., Sect. A 270, 198 (1988).
Review of Scientific Instruments is copyrighted by the American Institute of Physics (AIP). Redistribution of journal material is subject to the AIP online journal license and/or AIP copyright. For more information, see http://ojps.aip.org/rsio/rsicr.jsp
Nondestructive synchronous beam current monitor Michel Kireeff Covoa) Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, USA
(Received 12 September 2014; accepted 15 November 2014; published online 9 December 2014) A fast current transformer is mounted after the deflectors of the Berkeley 88-Inch Cyclotron. The measured signal is amplified and connected to the input of a lock-in amplifier. The lock-in amplifier performs a synchronous detection of the signal at the cyclotron second harmonic frequency. The magnitude of the signal detected is calibrated against a Faraday cup and corresponds to the beam intensity. It has exceptional resolution, long term stability, and can measure the beam current leaving the cyclotron as low as 1 nA. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4902903] I. INTRODUCTION
The 88-Inch Cyclotron at Lawrence Berkeley National Laboratory is a sector-focused cyclotron with both light- and heavy-ion capabilities that supports a local research program in nuclear science and is the home of the Berkeley Accelerator Space Effects Facility. The cyclotron has three ion sources that have led to progressively higher intensities and charge states of heavier ions. The ions produced by the ion sources are axially injected inside the 88-Inch Cyclotron. After the ions enter the cyclotron, they are accelerated by a radio frequency (RF) electric field and held to a spiral trajectory by a static magnetic field. The RF field makes the ions to bunch up into packets. The ions gain velocity and the orbit increases with radius. The ions that are not synchronized with the RF are lost. The cyclotron operates in the frequency range of 5.5– 16.5 MHz, but it can operate using harmonic acceleration, so the energy range of the machine is limited only by the capabilities of the magnet, not the RF system. Early on it was realized that the variable frequency of the cyclotron translated to a mass resolution of 1/3000, meaning that the cyclotron could separate most ions of near identical mass-to-charge ratio emanating from the ion source. The combination of cyclotron and electron cyclotron resonance ion sources provides the unique ability to run “cocktails” of ions. A cocktail is a mixture of ions of near identical chargeto-mass ratio.1 During the cyclotron operation, the ions are tuned out of the source together. The cyclotron acts as a charge-to-mass analyzer to separate them, providing different ion species and charge states for energy variable experiments. The wideband driven RF system for the 88-Inch Cyclotron offers fast beam tuning,2 allowing users to switch back and forth between several ion species of the same cocktail with small adjustments of the accelerator frequency, so a new beam does not require retuning the whole accelerator and is accomplished in approximately 1 min. Sections II and III will review recent nondestructive techniques to measure the beam intensity and show the design and commissioning of a synchronous beam current monitor that uses a fast beam current transformer connected to a locka) Electronic mail: [email protected].
0034-6748/2014/85(12)/125106/5/$30.00
in amplifier. The large bandwidth and short rise time of the transformer allows exceptional high resolution current measurements. The transformer is installed after the exit deflector to measure the beam current of the cyclotron down to 1 nA. The measured current is acquired, stored in files, and displayed in a computer screen. II. NONDESTRUCTIVE TECHNIQUES
Monitoring the intensity without obstructing the beam is required to maintain the cyclotron efficiency and not disturb the experiments. Current measurements down to few nanoamps range is an extremely challenging task. The noise can easily mask the beam current signal to be measured. Several nondestructive techniques to monitor the beam intensity are described below, such as wall current monitors, beam current transformers, and capacitive monitors. A resistive wall monitor from FERMILAB Main Injector measures the image charge that flows along the beam lines with a 1 ohm gap impedance.3 Currents not associated with the image current can interfere with measurements, so noise reduction techniques such as shielding and microwave absorbers are used to increase the signal-to-noise ratio. A Fast Current Transformer (FCT) and a SR844 lock-in amplifier are installed in the E6 beamline of RIKEN.4 The signal vector of the beam current that is 90◦ off-phase with the noise is measured in order to decrease the noise. The assumption is that the noise phase is constant and does not coincide with the phase of the beam current signal. Otherwise, the beam current monitor (BCM) will need to be moved to another location. The cyclotron beam is also chopped at 0.01 Hz with duty cycle of 50% to enhance the visibility. A lowintensity beam of 3 nA is measured. A current transformer with a ceramic break is also used in the Tevatron at FERMILAB to measure the transfer line beam intensities.5 The signal is processed with a field programmable gate array and the main source of noise identified is the analog-to-digital and digital-to-analog conversions. At the cyclotron in the University of Jyväskylä, a capacitive probe is connected to a very narrow phase doubleheterodyne receiver with in-phase and quadrature-phase demodulator. It measures the amplitude of the beam current signal at the cyclotron frequency.6 The current intensity of 100 nA is measured using this technique. A capacitive probe
85, 125106-1
© 2014 AIP Publishing LLC
125106-2
Michel Kireeff Covo
Rev. Sci. Instrum. 85, 125106 (2014)
FIG. 1. Fast current transformer. (a) Fast current transformer mounted inside the vacuum chamber is shown at the left side and a shield at the right side. (b) Assembly mounted in the staging line after the cyclotron deflectors.
is also connected to a SR844 lock-in amplifier in the RIBF at RIKEN and measures a beam current down to 10 nA.7 This paper shows a system made mostly with commercial components. It combines the high gain of the FCT with the selectivity of the lock-in amplifier to extract a faint beam intensity signal from an extremely noisy environment such as a cyclotron laboratory, similar to the hardware given in Ref. 4. However, rather than relying on the stability of the noise phase or chopping the beam to improve the signal ratio, a meticulous work is done to suppress the noise from the system and permit synchronous detection of the beam bunch signal at the cyclotron second harmonic frequency intensity down to 1 nA intensity. The noise level is decreased by filtering the common-mode noise, placing a low noise amplifier close to the sensor, using a hard line cable for long transmission, and filtering the high frequency noise signals that would add in the square wave detecting lock-in. This technique allows outstanding performance of the BCM and will be discussed next. III. BEAM CURRENT MONITOR A. Fast current transformer
The fast current transformer, model FCT-082-05:1-H from Bergoz Instrumentation,8 is shown in Figure 1. It is a toroidal core made of cobalt-based nanocrystalline and amorphous alloys of 82 mm inner diameter and a five turn coil wound, which has a gain of 5 V/A. The bandwidth of the FCT is 32 KHz to 700 MHz with typical rise time of 500 ps. The FCT is mounted inside a spherical octagon ultrahigh vacuum chamber with a floating coaxial feedthrough, Figure 1(a). The vacuum chamber is installed in the staging line after the cyclotron deflectors, Figure 1(b). Therefore, any beam leaving the cyclotron will cross it and the wall current will be diverted around the outside of the device. A shield, grounded only upstream, is installed to protect the FCT from beam losses.
The signal is then sent to the control room by a low loss heliax cable and filtered by another common-mode choke, which is made with two turns of cable around a ferrite toroid of material 43. The signal is finally connected to the input of the lockin amplifier model SR844 from Stanford Research Systems.9 The lock-in amplifier performs synchronous detection and can discriminate extremely low amplitude signal from noise. A low-pass filter model BLP-70+ is added before the lock-in amplifier to increase the signal-to-noise ratio by filtering high frequency noises generated by the cyclotron. It reduces the noise contribution to the signal measured because the SR844 is a square wave detecting lock-in that adds all the signals at the odd harmonics of the reference signal. The sharp bunch shape with a duty cycle of ∼9% present in the cyclotron creates many harmonics10 with only ∼30% of the total spectral power concentrated in its fundamental.11 Therefore, the harmonic content can be measured because of the fast response of the FCT. Although the cyclotron laboratory noise is higher at the fundamental cyclotron frequency, the noise decreases with increasing frequency. For that reason synchronous detection with the second harmonic is chosen. The signal from the cyclotron RF generator is connected to the frequency doubler, model RK-3 from Mini Circuits, and generates the second harmonic, which is used as a reference signal for the lock-in amplifier. The SR844 has an internal harmonic detection of the 2F component, however the external reference signal assures that the lock-in amplifier meets its highest technical specifications. The noise measured without beam crossing the FCT has components generated by the RF system. Large phase noise variations are observed even in small frequency adjustments during a cocktail run. Therefore, the amplitude is used to measure the beam intensity, rather than the phase, because it is less sensitive to variations.
B. Hardware and system
Figure 2 shows the BCM system. The signal of the FCT is filtered by a common-mode choke, which consists of four turns of BNC cable around three ferrite toroids of material 43. The signal is then strengthen by a high linearity ultraband low noise amplifier, model RLNA01M03GA from RF-Lambda, which has a gain of 38 dB and 1.5 dB noise figure.
IV. MEASUREMENTS A. Short term performance
Figure 3 shows beam current measurements of a high intensity 219.2 MeV 48 Ca+11 ion beam development run. The intensity of the beam is decreased from 3.4 uA to 12 nA using
125106-3
Michel Kireeff Covo
Rev. Sci. Instrum. 85, 125106 (2014)
FIG. 2. BCM system. The FCT measures the beam current. The signal is amplified by a low noise amplifier and transmitted to a lock-in amplifier. The reference signal for the lock-in amplifier is obtained by doubling the RF frequency of the cyclotron. The lock-in amplifier detects the second harmonic component of the beam spectrum.
FIG. 3. Beam current measurements of 219.2 MeV 48 Ca+11 ion beam. (a) FC versus BCM measurements with a power trend line fitting. (b) BCM and FC current measurements over time in linear scale. (c) BCM and FC current measurements over time in logarithmic scale.
125106-4
Michel Kireeff Covo
Rev. Sci. Instrum. 85, 125106 (2014)
FIG. 4. Beam current measurements of 50 MeV H+ ion beam. (a) FC versus BCM measurements with a power trend line fitting. (b) BCM and FC current measurements in linear scale. (c) BCM and FC current measurements in logarithmic scale.
an attenuator system that has screens and is located in the low energy injection line.12 Figure 3(a) shows the current measured with a Faraday cup (FC) in the ordinate and the signal measured with the BCM in the abscissa. A power trend line fits the data and shows a coefficient close to 1. The small nonlinearity is attributed to space charge longitudinal relaxation of the beam. Figure 3(b) shows the FC signal with a blue solid circle marker, using the fitting equation obtained in Figure 3(a), and the BCM signal with a red solid triangle marker that are plotted over time. Figure 3(c) shows the ordinate in log scale to display the current variations down to 10 nA. Figure 4 shows beam current measurements of a low intensity 50 MeV H+ ion beam development run. The intensity of the beam is decreased from 20 nA to 1 nA using attenuators. Figure 4(a) shows measurements of the beam current obtained with a FC in the ordinate and the BCM signal measured in the abscissa. The power trend line has a similar nonlinearity observed in Figure 3(a). Figure 4(b) shows the FC with a blue solid circle marker, using the fitting equation obtained in Figure 4(a), and the BCM signal with a red solid triangle marker that are plotted over time. Figure 4(c) shows the ordinate in log scale to display current variations down to 1 nA with resolved intensity of ∼0.2 nA.
Figures 3 and 4 show that the data measured with the two different instruments (i.e., FC and BCM) are in excellent agreement. At the beam current of 1nA, the scattering of the Faraday cup measurements was ∼0.05 nA and the scattering of the beam current monitor measurements was ∼0.1 nA, so the agreement down to 1 nA can be assured. The output of the BCM is connected to a computer that acquires, displays, and stores the data in files. The BCM is cross-calibrated with a FC measurement at the beginning of each run and a linear correlation is assumed.
B. Long term performance
Possible variations of the time structure of the external beam, which may happen over time or when making small tuning adjustments on the cyclotron, could modify the harmonic components of the beam bunch spectrum. Figure 5 shows the data from a run of 244 MeV 50 Ti+12 ion beam during a period of 50 h on 03/09/2014 keeping the same amplitude calibration of the BCM. The BCM is plotted using a blue solid triangle marker and the primary ordinate at the left. A dedicated measurement with a Faraday cup is not feasible because it would require a very long cyclotron beam
125106-5
Michel Kireeff Covo
Rev. Sci. Instrum. 85, 125106 (2014)
compared with measurements of the neutron dose, produced by the beam inside the experimental area, and show excellent long term stability of the BCM system. It gives confidence that possible changes in the time structure of the external beam over time or due to small tuning adjustments do not affect the measurements.
ACKNOWLEDGMENTS
FIG. 5. Long term BCM measurement of 244 MeV 50 Ti+12 ion beam, using the left ordinate, and experimental area neutron dose, using the right ordinate.
time; therefore the neutron dose, concurrently measured at the experimental area after the beam hits the target, is plotted using a red solid circle marker and the secondary ordinate at the right. The time is shown in the abscissa. Assuming that the neutron dose is proportional to the beam intensity, the plot shows the excellent BCM long term performance without a significant variation of the calibration factor from the beam current monitor. The outliner points are generated during RF breakdowns or when the RF is turned off so the experimenters can get inside of the cave and make adjustments to the equipment. V. CONCLUSIONS
A FCT is mounted after the deflectors of the 88-Inch Cyclotron. The signal is filtered, amplified, and sent to a lock-in amplifier that performs synchronous detection at the second harmonic of the cyclotron RF. The beam current is varied with attenuators located in the axial injection. A power trend line shows the best fit of the FC versus the BCM data. The beam current signal, obtained using the best fit equation, is plotted against the FC measurement and shows remarkable agreement down to 1nA. A BCM system is then implemented assuming linear correlation because the coefficient of the power trend line equation is almost 1. The amplitude of the vector detected is calibrated against a FC measurement, giving the beam intensity. The data recorded by the BPM system during two days are
The author would like to thank Brien Ninemire, Catherine R. Siero, Thomas Gimpel, and Scott M. Small for the cyclotron operation support, Adrian Hodgkinson, Thomas Perry, and John P. Garcia for the mechanical support, and Mark Regis and Brendan Ford for the electronic support. This material is based upon work supported by the U.S. Department of Energy, Office of Science, Office of Nuclear Physics under Award No. DE-AC02-05CH11231. 1 R.
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