- Email: [email protected]
Automated cyclotron tuning using beam phase measurements
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 568 (2006) 532–536 www.elsevier.com/locate/nima
Automated cyclotron tuning using beam phase measurements J.H. Timmer, H. Ro¨cken, T. Stephani, C. Baumgarten, A. Geisler ACCEL Instruments GmbH, Friedrich-Ebert-Str. 1, 51429 Bergisch Gladbach, Germany Received 27 June 2006; received in revised form 31 July 2006; accepted 1 August 2006 Available online 31 August 2006
Abstract The ACCEL K250 superconducting cyclotron is specifically designed for the use in proton therapy systems. The compact medical 250 MeV proton accelerator fulfils all present and future beam requirements for fast scanning treatment systems and is delivered as a turn key system; no operator is routinely required. During operation of the cyclotron heat dissipation of the RF system induces a small drift in iron temperature. This temperature drift slightly detunes the magnetic field and small corrections must be made. A non-destructive beam phase detector has been developed to measure and quantify the effect of a magnetic field drift. Signal calculations were made and the design of the capacitive pickup probe was optimised to cover the desired beam current range. Measurements showed a very good agreement with the calculated signals and beam phase can be measured with currents down to 3 nA. The measured phase values are used as input for a feedback loop controlling the current in the superconducting coil. The magnetic field of the cyclotron is tuned automatically and online to maintain a fixed beam phase. Extraction efficiency is thereby optimised continuously and activation of the cyclotron is minimised. The energy and position stability of the extracted beam are well within specification. r 2006 Elsevier B.V. All rights reserved. PACS: 29.20.Hm; 84.71.Ba; 87.56.v; 29.27.Ac Keywords: Cyclotron; Proton therapy; Extraction efficiency; Energy stability; Beam phase; Capacitive pickup
1. Introduction ACCEL Instruments GmbH is a leading supplier of accelerator equipment and systems for research, industry and healthcare. For application in the field of proton therapy ACCEL developed a new 250 MeV superconducting cyclotron in cooperation with the National Superconducting Cyclotron Laboratory in Michigan, USA [1]. The first two cyclotrons of this type have been commissioned successfully and are integrated in proton therapy facilities, one at the Paul Scherrer Institute (PSI) in Switzerland and one at the Rinecker Proton Therapy Center (RPTC) in Germany [2]. The superconducting cyclotron was specifically designed for a medical environment and is delivered as a turn key system. This means that apart from a daily setup and quality checks no operator is routinely required. One Corresponding author. Tel.: +49 2204 84 2500; fax: +49 2204 84 2501.
E-mail address: [email protected] (J.H. Timmer). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.08.005
typical cyclotron effect normally corrected for by an operator is the drift of the magnetic field caused by the heating of the iron by the RF structure. The power consumption of the RF resonators in the ACCEL cyclotron equals 115 kW and a considerable amount of this power is dissipated inside the cyclotron as heat loss. The temperature of the cyclotron iron is actively stabilised but small temperature changes after morning start-up cannot be avoided. Small changes in the contribution of the iron to the magnetic field can cause a sub-optimal isochronous field and thereby a reduced extraction efficiency. Changes in the iron contribution can also cause changes in the energy and position of the extracted particles. The effect of the iron heating can be compensated by adjusting the current in the superconducting coil. In many applications it might be sufficient to let an operator make the adjustments but this is not desirable for a medical accelerator. Optimal energy and position stability combined with high and stable extraction efficiency require online and automated tuning.
ARTICLE IN PRESS J.H. Timmer et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 532–536
For this purpose the phase of the extracted beam with respect to the RF master oscillator is measured using a capacitive pickup probe. The pickup probe is positioned as the first beam line element behind the cyclotron. The magnetic field is tuned for perfect isochronism by a feedback loop that regulates the coil current such that the beam phase has a fixed value. This paper describes the design of the beam phase detector and the effect of the phase regulation on the performance of the superconducting cyclotron. 2. Signal calculation Capacitive pickup probes are in use at several accelerator facilities around the world. Application of a pickup probe for online stabilisation of a medical cyclotron poses a challenge because of the low proton current available for the measurement. The beam intensity extracted from the cyclotron varies between 1 nA and 1 mA, depending on the setting and corresponding transmission of the Energy Selection System (ESS). In order to cover this large dynamic range detailed signal estimations were made and the pickup design was optimised. The basic layout of the probe is shown in Fig. 1. A conducting cylindrical pickup is mounted inside the beam tube. Beam bunches passing through the cylinder induce a mirror charge on the wall of the pickup. The generated voltage Q=C drives a current through a 50 O resistor and the signal is further processed using a preamplifier as shown in the equivalent circuit. The potential generated or the amount of current flowing can be calculated from first principles. We followed the elegant analysis of Cee [3] but added the effect of the capacitance between the pickup and the beam tube. The voltage induced on the isolated pickup surface—represented by the voltage source V in the
equivalent circuit—can be described by 2sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi I0 L Dt 2 4 V ðtÞ ¼ bc t þ R2 2 2 2 Dt f b c C ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s L Dt 2 bc t þ R2 2 2 s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi L Dt 2 2 bc t þ þR 2 2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3 L Dt 2 bc t þ þ R2 5, þ 2 2
533
ð1Þ
where I0 is the average beam current, f the bunch frequency, Dt the time for a bunch with a uniform current distribution to pass, L is the length of the pickup cylinder, b c the velocity of the bunch and R the radius of the pickup cylinder. Eq. (1) shows a linear dependence between the voltage and the average beam current I0 and indicates that the capacitance of the pickup cylinder to ground should be minimised in order to maximise the signal from the detector. The voltage over the 50 Ohm resistor is then equal to VR ¼
V Z C =Z R þ 1
(2)
and in the limit C-0 the results of Cee are reproduced. The voltage pulses generated by the charge bunches can be described as a Fourier sum of the fundamental RF frequency (72 MHz) and all higher harmonics. We chose to extract the phase information from the second harmonic as a considerable 72 MHz disturbance signal can be expected in the cyclotron bunker which would complicate the signal processing.
Fig. 1. Cut-view of the capacitive pickup probe and its equivalent circuit.
ARTICLE IN PRESS J.H. Timmer et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 532–536
534
300 V(t) VR
Voltage (nV)
200
100
0 -5
-3
-1
1
3
5
7
9
-100 Time (ns) Fig. 2. Calculated voltage pulse induced by a typical proton bunch of 1 nA beam at 250 MeV.
Fig. 3. Phase probe with preamplifier as first beam line element between vacuum valve and cyclotron at the Paul Scherrer Institute.
The layout of the probe was optimised in such a way that the second harmonic of the pulse was maximised. To do this a bunch length and longitudinal charge distribution must be assumed. Eq. (1) describes the voltage induced by a bunch with a uniform charge distribution, a composition of multiple of these bunches can be used to simulate a more advanced and realistic distribution. The bunch length was assumed to equal 81 of the RF cycle which equals 57 mm. Although both the charge distribution and the bunch length affect the shape of the voltage pulse as shown in Fig. 2 considerably, they have much less effect on the second harmonic of the pulse and were therefore not critical in the signal estimation. Taking the Fourier transformation of V ðtÞ the amplitude of the second harmonic was calculated to be 23.9 nV/nA of proton beam. If the capacitance of the pickup to ground is not taken into account the signal strength estimated is a factor 2 higher.
than 60 dB. Then the signal is mixed with the RF reference signal and finally converted into DC signals by an I/Q demodulator. The resulting values I and Q describe the phase difference between the measurement and the reference signal and the amplitude of the measurement signal. I and Q can be seen as the real and imaginary part of a vector inpthe complex plane. The amplitude A ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi corresponds to I 2 þ Q2 , the phase difference df can be derived from I ¼ A cosðdfÞ and Q ¼ A sinðdfÞ. The sampling rate of the I/Q demodulator is equal to 50 kHz and thousand I/Q samples are averaged to calculate one phase/amplitude pair. The control program is based on National Instruments LabVIEWTM and communication with the main control system is realised via a PROFIBUSs interface. The electronics were developed in close cooperation with Cryoelectra GmbH [4].
3. Detector realisation
4. Detector measurements
The pickup probe is installed as the first beam line element behind the cyclotron and the outer tube is part of the vacuum system (Fig. 3). The pickup cylinder is mounted in the outer tube with isolating positioning screws made of Vespels. The preamplifier is connected directly to the probe for the first filtering and amplification of the voltage pulses. The amplified signal is transported from the preamplifier to the data acquisition unit over a distance of 50 m with a special semi-rigid cable. Thereby it is possible to achieve an attenuation of about 120 dB of the severe disturbing radiation on the line. The shielding is especially important as the 72 MHz RF frequency and its higher harmonics are omnipresent in the cyclotron bunker and can interfere with the signal from the beam. In the data acquisition unit the pulses are filtered and amplified several times; the 144 MHz portion is passed and the fundamental frequency, the third and all higher harmonics are blocked. The harmonic rejection is more
Laboratory measurements were performed to determine the precision of the measurement system. Using a network analyser the first and second harmonics were generated as sinus signals to simulate the reference and probe signal. To simulate the real cyclotron including RF disturbance an interferer of 80 up to 60 dBm was introduced in the signal line. The accuracy of the phase measurement depends on the signal strength which in turn depends on the beam current. At maximum input power, equalling 1 mA of beam current, the absolute error is smaller 70.11. At the lowest possible detectable input power the absolute error is still smaller 71.21. The detector was then tested in combination with the cyclotron. The presence of a stable second harmonic disturbance signal with an amplitude of 800 nV made it necessary to implement a beam on/off calibration. After calibration a small unstable offset signal of about 35 nV remains. Phase could be measured with the precision
ARTICLE IN PRESS J.H. Timmer et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 532–536
535
1200
y = 25.3*x + 17.3
800
400
Horizontal Position Extracted Beam
80
4
40
0
0 0
-4 -30
0
20
Position (mm)
Extraction efficiency (%)
Amplitude (nV)
Extraction Efficiency
40
0
30
Change in coil current (mA)
Beam current (nA)
5. Cyclotron characteristics A typical ‘cold start’ of the cyclotron occurs when the cyclotron is started after a weekend shut down. After the RF amplifier is switched on, 115 kW is delivered to the cyclotron and the temperature rise of the cooling water in the liner is about 3 1C. The coil current correction needed to keep the magnetic field isochronous from a cold start to steady state is in the range of 100 mA which equals 10 Gauss at extraction radius. The time to reach steady state is 24 h. The slow cyclotron drift is corrected with a change in magnet coil current. The effects of a change in coil current— measured in a short period compared to the slow temperature drifts—are shown in Figs. 5 and 6. The dependence of extraction efficiency on coil current shown in Fig. 5 was measured at the PSI. The curve is strongly affected by the positions of the two electrostatic extraction deflectors and the magnetic ‘trim rods’ used to centre and extract the beam. Careful optimisation resulted in a curve with a flat top as shown in Fig. 5 with the extraction efficiency higher than 80% over a range of 15 mA. This efficiency is considered to be a very good result for a compact medical cyclotron and a validation of the extensive magnetic modelling done during the design phase. If the coil current is changed the beam position measured on the first profile monitor in the beam line also changes.
90
500 Measured phase Measured extracted proton energy
60
0
30
∆ Energy (keV)
predicted by the laboratory tests with beam currents as low as 3 nA. This is close to the minimum detectable signal (MDS) limit of 2 nA, as calculated for this system from the temperature-dependent base noise level (174 dBm), the minimum system bandwidth (10 kHz) and the noise figure of the preamplifier. The relation between beam current and detector output voltage was measured; results are shown in Fig. 4. The agreement between the measured detector sensitivity of 25.3 nV/nA and the estimated signal of 23.9 nV/nA based on Eq. (2) is very good.
Fig. 5. Cyclotron extraction efficiency and horizontal position of the extracted beam as function of changes in the magnet coil current.
Phase (°)
Fig. 4. Measured amplitude of the second harmonic as function of beam current.
-500 -20
0
20
Change in coil current (mA) Fig. 6. Measured beam phase and extracted particle energy as function of relative coil current.
This is due to the contribution of the coil to the magnetic field in the extraction channel and the changed scattering of protons by the deflector septum and cathode. The particle energy of the extracted beam was determined by measuring the range of protons in water. An energy variation of 600 keV at 250 MeV was measured over the range of coil current for which particles could be extracted [5]. This is roughly consistent with predictions based on particle tracking calculations. Using a slightly simplified model of the cyclotron a temperature drift of the iron and corresponding change in magnetic field can be regarded as an equidistant shift of the four curves in Figs. 5 and 6 parallel to the horizontal axes. The size of the change in the measured beam phase is an indicator for the change in coil current needed to move back to the original values of extraction efficiency, beam position and energy. 6. Results with automated tuning The phase and amplitude values are used as input for a feedback loop in the—PLC-based—main control system.
ARTICLE IN PRESS J.H. Timmer et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 532–536
536 15
3
Table 1 Specified and measured acceptance criteria for the medical cyclotron
2
Acceptance criterion
External beam position
10 1
0 5
Position (mm)
Realtive coil current (mA)
Relative coil current
-1
0 00:00
18:00
-2 36:00
Time (min)
Fig. 7. Development of the magnetic coil current and external position over more than half an hour with automated tuning activated.
Amplitude threshold value, averaging time, target phase value and maximum coil current change can be accessed as variables in the control system. The relationship between coil current and measured phase as shown in Fig. 6 is linearised and used to determine the needed step size in the coil current if the measured phase deviates from the targeted phase value. Typical traces of the coil current and the position of the extracted beam as measured by the first profile monitor are shown in Fig. 7. The phase-current feedback loop is active and adjusts the coil current 10 mA in half an hour while the position of the extracted beam remains constant. The use of beam phase information proved to be very successful for the stabilisation of the cyclotron. Specifications and measurements of the energy stability and position stability of the extracted beam as well as extraction efficiency are listed in Table 1. Magnetic field drifts can also be detected by NMR measurements or by monitoring of the iron temperature. These indicators proved to be not nearly as precise as the information obtained directly from the beam using the capacitive pickup probe. The superconducting coil of the ACCEL K250 cyclotron allows the use of the coil current as driving parameter instead of RF
Position stability Extracted beam (1s) Energy stability Range in water (1s) Equivalent energy Extraction efficiency
Specified
Measured
70.2 mm
70.2 mm
70.5 mm 7187 keV 480%
70.2 mm 775 keV 81–84%
frequency. As the iron of the K250 is fully saturated changes in the coil current are absolutely reproducible and non-linear effects as seen in cyclotrons with normal conducting coils are absent. As a result the feedback control loop behaves in a very stable and predictable manner. Switching off the RF system or the magnet overnight does not influence the performance of the cyclotron in the morning and operating costs can be reduced. An important benefit of the automated tuning is that the extraction efficiency is maximised continuously and online. Beam losses and activation in the extraction region due to a slightly detuned magnetic field are minimised. This facilitates maintenance and strongly improves deflector stability and lifetime. Automated magnetic field tuning thus reinforces the benefits of the high extraction efficiency cyclotron. References [1] M. Schillo, et al., Compact superconducting 250 MeV proton cyclotron for the PSI PROSCAN therapy project, in: Proceedings of the Cyclotrons and their Applications, East Lansing, 2001, pp. 37–39. [2] H.-U. Klein, et al., Nucl. Instr. and Meth. B 241 (2005) 721. [3] R. Cee, Entwicklung und Aufbau von Strahldiagnosesystemen fu¨r den Heidelberger Hochstrominjektor, Ph.D. Thesis, Ruprecht-Karls-Universita¨t Heideldberg, 2000, pp. 101–104. [4] B. Aminov, et al., Beam phase detector for proton therapy facilities, in: Proceedings of the Particle Accelerator Conference, Knoxville, 2005. [5] M. Schippers, et al., Effects of magnetic field of COMET on beam energy, PSI document P20/SJ85-601.1.
Nuclear Instruments and Methods in Physics Research A 568 (2006) 532–536 www.elsevier.com/locate/nima
Automated cyclotron tuning using beam phase measurements J.H. Timmer, H. Ro¨cken, T. Stephani, C. Baumgarten, A. Geisler ACCEL Instruments GmbH, Friedrich-Ebert-Str. 1, 51429 Bergisch Gladbach, Germany Received 27 June 2006; received in revised form 31 July 2006; accepted 1 August 2006 Available online 31 August 2006
Abstract The ACCEL K250 superconducting cyclotron is specifically designed for the use in proton therapy systems. The compact medical 250 MeV proton accelerator fulfils all present and future beam requirements for fast scanning treatment systems and is delivered as a turn key system; no operator is routinely required. During operation of the cyclotron heat dissipation of the RF system induces a small drift in iron temperature. This temperature drift slightly detunes the magnetic field and small corrections must be made. A non-destructive beam phase detector has been developed to measure and quantify the effect of a magnetic field drift. Signal calculations were made and the design of the capacitive pickup probe was optimised to cover the desired beam current range. Measurements showed a very good agreement with the calculated signals and beam phase can be measured with currents down to 3 nA. The measured phase values are used as input for a feedback loop controlling the current in the superconducting coil. The magnetic field of the cyclotron is tuned automatically and online to maintain a fixed beam phase. Extraction efficiency is thereby optimised continuously and activation of the cyclotron is minimised. The energy and position stability of the extracted beam are well within specification. r 2006 Elsevier B.V. All rights reserved. PACS: 29.20.Hm; 84.71.Ba; 87.56.v; 29.27.Ac Keywords: Cyclotron; Proton therapy; Extraction efficiency; Energy stability; Beam phase; Capacitive pickup
1. Introduction ACCEL Instruments GmbH is a leading supplier of accelerator equipment and systems for research, industry and healthcare. For application in the field of proton therapy ACCEL developed a new 250 MeV superconducting cyclotron in cooperation with the National Superconducting Cyclotron Laboratory in Michigan, USA [1]. The first two cyclotrons of this type have been commissioned successfully and are integrated in proton therapy facilities, one at the Paul Scherrer Institute (PSI) in Switzerland and one at the Rinecker Proton Therapy Center (RPTC) in Germany [2]. The superconducting cyclotron was specifically designed for a medical environment and is delivered as a turn key system. This means that apart from a daily setup and quality checks no operator is routinely required. One Corresponding author. Tel.: +49 2204 84 2500; fax: +49 2204 84 2501.
E-mail address: [email protected] (J.H. Timmer). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.08.005
typical cyclotron effect normally corrected for by an operator is the drift of the magnetic field caused by the heating of the iron by the RF structure. The power consumption of the RF resonators in the ACCEL cyclotron equals 115 kW and a considerable amount of this power is dissipated inside the cyclotron as heat loss. The temperature of the cyclotron iron is actively stabilised but small temperature changes after morning start-up cannot be avoided. Small changes in the contribution of the iron to the magnetic field can cause a sub-optimal isochronous field and thereby a reduced extraction efficiency. Changes in the iron contribution can also cause changes in the energy and position of the extracted particles. The effect of the iron heating can be compensated by adjusting the current in the superconducting coil. In many applications it might be sufficient to let an operator make the adjustments but this is not desirable for a medical accelerator. Optimal energy and position stability combined with high and stable extraction efficiency require online and automated tuning.
ARTICLE IN PRESS J.H. Timmer et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 532–536
For this purpose the phase of the extracted beam with respect to the RF master oscillator is measured using a capacitive pickup probe. The pickup probe is positioned as the first beam line element behind the cyclotron. The magnetic field is tuned for perfect isochronism by a feedback loop that regulates the coil current such that the beam phase has a fixed value. This paper describes the design of the beam phase detector and the effect of the phase regulation on the performance of the superconducting cyclotron. 2. Signal calculation Capacitive pickup probes are in use at several accelerator facilities around the world. Application of a pickup probe for online stabilisation of a medical cyclotron poses a challenge because of the low proton current available for the measurement. The beam intensity extracted from the cyclotron varies between 1 nA and 1 mA, depending on the setting and corresponding transmission of the Energy Selection System (ESS). In order to cover this large dynamic range detailed signal estimations were made and the pickup design was optimised. The basic layout of the probe is shown in Fig. 1. A conducting cylindrical pickup is mounted inside the beam tube. Beam bunches passing through the cylinder induce a mirror charge on the wall of the pickup. The generated voltage Q=C drives a current through a 50 O resistor and the signal is further processed using a preamplifier as shown in the equivalent circuit. The potential generated or the amount of current flowing can be calculated from first principles. We followed the elegant analysis of Cee [3] but added the effect of the capacitance between the pickup and the beam tube. The voltage induced on the isolated pickup surface—represented by the voltage source V in the
equivalent circuit—can be described by 2sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi I0 L Dt 2 4 V ðtÞ ¼ bc t þ R2 2 2 2 Dt f b c C ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s L Dt 2 bc t þ R2 2 2 s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi L Dt 2 2 bc t þ þR 2 2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3 L Dt 2 bc t þ þ R2 5, þ 2 2
533
ð1Þ
where I0 is the average beam current, f the bunch frequency, Dt the time for a bunch with a uniform current distribution to pass, L is the length of the pickup cylinder, b c the velocity of the bunch and R the radius of the pickup cylinder. Eq. (1) shows a linear dependence between the voltage and the average beam current I0 and indicates that the capacitance of the pickup cylinder to ground should be minimised in order to maximise the signal from the detector. The voltage over the 50 Ohm resistor is then equal to VR ¼
V Z C =Z R þ 1
(2)
and in the limit C-0 the results of Cee are reproduced. The voltage pulses generated by the charge bunches can be described as a Fourier sum of the fundamental RF frequency (72 MHz) and all higher harmonics. We chose to extract the phase information from the second harmonic as a considerable 72 MHz disturbance signal can be expected in the cyclotron bunker which would complicate the signal processing.
Fig. 1. Cut-view of the capacitive pickup probe and its equivalent circuit.
ARTICLE IN PRESS J.H. Timmer et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 532–536
534
300 V(t) VR
Voltage (nV)
200
100
0 -5
-3
-1
1
3
5
7
9
-100 Time (ns) Fig. 2. Calculated voltage pulse induced by a typical proton bunch of 1 nA beam at 250 MeV.
Fig. 3. Phase probe with preamplifier as first beam line element between vacuum valve and cyclotron at the Paul Scherrer Institute.
The layout of the probe was optimised in such a way that the second harmonic of the pulse was maximised. To do this a bunch length and longitudinal charge distribution must be assumed. Eq. (1) describes the voltage induced by a bunch with a uniform charge distribution, a composition of multiple of these bunches can be used to simulate a more advanced and realistic distribution. The bunch length was assumed to equal 81 of the RF cycle which equals 57 mm. Although both the charge distribution and the bunch length affect the shape of the voltage pulse as shown in Fig. 2 considerably, they have much less effect on the second harmonic of the pulse and were therefore not critical in the signal estimation. Taking the Fourier transformation of V ðtÞ the amplitude of the second harmonic was calculated to be 23.9 nV/nA of proton beam. If the capacitance of the pickup to ground is not taken into account the signal strength estimated is a factor 2 higher.
than 60 dB. Then the signal is mixed with the RF reference signal and finally converted into DC signals by an I/Q demodulator. The resulting values I and Q describe the phase difference between the measurement and the reference signal and the amplitude of the measurement signal. I and Q can be seen as the real and imaginary part of a vector inpthe complex plane. The amplitude A ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi corresponds to I 2 þ Q2 , the phase difference df can be derived from I ¼ A cosðdfÞ and Q ¼ A sinðdfÞ. The sampling rate of the I/Q demodulator is equal to 50 kHz and thousand I/Q samples are averaged to calculate one phase/amplitude pair. The control program is based on National Instruments LabVIEWTM and communication with the main control system is realised via a PROFIBUSs interface. The electronics were developed in close cooperation with Cryoelectra GmbH [4].
3. Detector realisation
4. Detector measurements
The pickup probe is installed as the first beam line element behind the cyclotron and the outer tube is part of the vacuum system (Fig. 3). The pickup cylinder is mounted in the outer tube with isolating positioning screws made of Vespels. The preamplifier is connected directly to the probe for the first filtering and amplification of the voltage pulses. The amplified signal is transported from the preamplifier to the data acquisition unit over a distance of 50 m with a special semi-rigid cable. Thereby it is possible to achieve an attenuation of about 120 dB of the severe disturbing radiation on the line. The shielding is especially important as the 72 MHz RF frequency and its higher harmonics are omnipresent in the cyclotron bunker and can interfere with the signal from the beam. In the data acquisition unit the pulses are filtered and amplified several times; the 144 MHz portion is passed and the fundamental frequency, the third and all higher harmonics are blocked. The harmonic rejection is more
Laboratory measurements were performed to determine the precision of the measurement system. Using a network analyser the first and second harmonics were generated as sinus signals to simulate the reference and probe signal. To simulate the real cyclotron including RF disturbance an interferer of 80 up to 60 dBm was introduced in the signal line. The accuracy of the phase measurement depends on the signal strength which in turn depends on the beam current. At maximum input power, equalling 1 mA of beam current, the absolute error is smaller 70.11. At the lowest possible detectable input power the absolute error is still smaller 71.21. The detector was then tested in combination with the cyclotron. The presence of a stable second harmonic disturbance signal with an amplitude of 800 nV made it necessary to implement a beam on/off calibration. After calibration a small unstable offset signal of about 35 nV remains. Phase could be measured with the precision
ARTICLE IN PRESS J.H. Timmer et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 532–536
535
1200
y = 25.3*x + 17.3
800
400
Horizontal Position Extracted Beam
80
4
40
0
0 0
-4 -30
0
20
Position (mm)
Extraction efficiency (%)
Amplitude (nV)
Extraction Efficiency
40
0
30
Change in coil current (mA)
Beam current (nA)
5. Cyclotron characteristics A typical ‘cold start’ of the cyclotron occurs when the cyclotron is started after a weekend shut down. After the RF amplifier is switched on, 115 kW is delivered to the cyclotron and the temperature rise of the cooling water in the liner is about 3 1C. The coil current correction needed to keep the magnetic field isochronous from a cold start to steady state is in the range of 100 mA which equals 10 Gauss at extraction radius. The time to reach steady state is 24 h. The slow cyclotron drift is corrected with a change in magnet coil current. The effects of a change in coil current— measured in a short period compared to the slow temperature drifts—are shown in Figs. 5 and 6. The dependence of extraction efficiency on coil current shown in Fig. 5 was measured at the PSI. The curve is strongly affected by the positions of the two electrostatic extraction deflectors and the magnetic ‘trim rods’ used to centre and extract the beam. Careful optimisation resulted in a curve with a flat top as shown in Fig. 5 with the extraction efficiency higher than 80% over a range of 15 mA. This efficiency is considered to be a very good result for a compact medical cyclotron and a validation of the extensive magnetic modelling done during the design phase. If the coil current is changed the beam position measured on the first profile monitor in the beam line also changes.
90
500 Measured phase Measured extracted proton energy
60
0
30
∆ Energy (keV)
predicted by the laboratory tests with beam currents as low as 3 nA. This is close to the minimum detectable signal (MDS) limit of 2 nA, as calculated for this system from the temperature-dependent base noise level (174 dBm), the minimum system bandwidth (10 kHz) and the noise figure of the preamplifier. The relation between beam current and detector output voltage was measured; results are shown in Fig. 4. The agreement between the measured detector sensitivity of 25.3 nV/nA and the estimated signal of 23.9 nV/nA based on Eq. (2) is very good.
Fig. 5. Cyclotron extraction efficiency and horizontal position of the extracted beam as function of changes in the magnet coil current.
Phase (°)
Fig. 4. Measured amplitude of the second harmonic as function of beam current.
-500 -20
0
20
Change in coil current (mA) Fig. 6. Measured beam phase and extracted particle energy as function of relative coil current.
This is due to the contribution of the coil to the magnetic field in the extraction channel and the changed scattering of protons by the deflector septum and cathode. The particle energy of the extracted beam was determined by measuring the range of protons in water. An energy variation of 600 keV at 250 MeV was measured over the range of coil current for which particles could be extracted [5]. This is roughly consistent with predictions based on particle tracking calculations. Using a slightly simplified model of the cyclotron a temperature drift of the iron and corresponding change in magnetic field can be regarded as an equidistant shift of the four curves in Figs. 5 and 6 parallel to the horizontal axes. The size of the change in the measured beam phase is an indicator for the change in coil current needed to move back to the original values of extraction efficiency, beam position and energy. 6. Results with automated tuning The phase and amplitude values are used as input for a feedback loop in the—PLC-based—main control system.
ARTICLE IN PRESS J.H. Timmer et al. / Nuclear Instruments and Methods in Physics Research A 568 (2006) 532–536
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3
Table 1 Specified and measured acceptance criteria for the medical cyclotron
2
Acceptance criterion
External beam position
10 1
0 5
Position (mm)
Realtive coil current (mA)
Relative coil current
-1
0 00:00
18:00
-2 36:00
Time (min)
Fig. 7. Development of the magnetic coil current and external position over more than half an hour with automated tuning activated.
Amplitude threshold value, averaging time, target phase value and maximum coil current change can be accessed as variables in the control system. The relationship between coil current and measured phase as shown in Fig. 6 is linearised and used to determine the needed step size in the coil current if the measured phase deviates from the targeted phase value. Typical traces of the coil current and the position of the extracted beam as measured by the first profile monitor are shown in Fig. 7. The phase-current feedback loop is active and adjusts the coil current 10 mA in half an hour while the position of the extracted beam remains constant. The use of beam phase information proved to be very successful for the stabilisation of the cyclotron. Specifications and measurements of the energy stability and position stability of the extracted beam as well as extraction efficiency are listed in Table 1. Magnetic field drifts can also be detected by NMR measurements or by monitoring of the iron temperature. These indicators proved to be not nearly as precise as the information obtained directly from the beam using the capacitive pickup probe. The superconducting coil of the ACCEL K250 cyclotron allows the use of the coil current as driving parameter instead of RF
Position stability Extracted beam (1s) Energy stability Range in water (1s) Equivalent energy Extraction efficiency
Specified
Measured
70.2 mm
70.2 mm
70.5 mm 7187 keV 480%
70.2 mm 775 keV 81–84%
frequency. As the iron of the K250 is fully saturated changes in the coil current are absolutely reproducible and non-linear effects as seen in cyclotrons with normal conducting coils are absent. As a result the feedback control loop behaves in a very stable and predictable manner. Switching off the RF system or the magnet overnight does not influence the performance of the cyclotron in the morning and operating costs can be reduced. An important benefit of the automated tuning is that the extraction efficiency is maximised continuously and online. Beam losses and activation in the extraction region due to a slightly detuned magnetic field are minimised. This facilitates maintenance and strongly improves deflector stability and lifetime. Automated magnetic field tuning thus reinforces the benefits of the high extraction efficiency cyclotron. References [1] M. Schillo, et al., Compact superconducting 250 MeV proton cyclotron for the PSI PROSCAN therapy project, in: Proceedings of the Cyclotrons and their Applications, East Lansing, 2001, pp. 37–39. [2] H.-U. Klein, et al., Nucl. Instr. and Meth. B 241 (2005) 721. [3] R. Cee, Entwicklung und Aufbau von Strahldiagnosesystemen fu¨r den Heidelberger Hochstrominjektor, Ph.D. Thesis, Ruprecht-Karls-Universita¨t Heideldberg, 2000, pp. 101–104. [4] B. Aminov, et al., Beam phase detector for proton therapy facilities, in: Proceedings of the Particle Accelerator Conference, Knoxville, 2005. [5] M. Schippers, et al., Effects of magnetic field of COMET on beam energy, PSI document P20/SJ85-601.1.