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Beam phase measurement system for the K130 cyclotron in Jyväskylä
Nuclear Instruments and Methods in Physics Research A 335 (1993) 417-423 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
Beam phase measurement system for the K130 cyclotron in Jyväskylä b and K. Kaski a b, E. Liukkonen Tampere University of Technology, Microelectronics Lab., P.O. Box 692, SF-33101 Tampere, Finland b University of Jyudsky1ä, Department of Physics, P.O. Box 35, SF-40351 Jyudskyld, Finland
J. Gustafsson
a,
P . Kotilainen
a,
V. Nieminen
Received 18 March 1993 A phase measurement system for Jyviiskylii new K =130 heavy ion cyclotron has been designed and realized . The phase measurement is done using a set of capacitive probes to detect phase information from the internal ion beam . This data is vital for tuning purposes to obtain an isochronous magnetic field and to maximize the ion beam intensity.
1. Introduction A modern isochronous cyclotron requires a large number of control settings to accelerate efficiently a wide variety of ionic particles to different energies . In order to control the beam characteristics of the cyclotron, the phase information of the beam provides one of the most useful parameters . The on-line beam phase measurement can serve as an efficient way for tuning the precise isochronous magnetic field in a heavy ion cyclotron. The phase information of the internal ion beam is detected using a number of capacitive pick-up probes . This method makes it possible to tune systematically 15 trim coils, which control the radial magnetic field strength . This is necessary for achieving the isochronous magnetic field and maximizing the beam intensity at the output of the cyclotron. An automatic tuning of the magnetic field is also possible . For the measurement and tuning to be successful a careful study of RF disturbance in the cyclotron is necessary. Moreover, the experience gathered in other laboratories is most useful [3,5]. Using all this information a stable and sensitive phase measurements system has been developed. The accelerator in the Physics Department at University of Jyvdskyld is a K130 cyclotron equipped with an external ECR ion source . The extraction radius of the cyclotron is 96 cm . The phase measurement system was designed and developed to maximize the efficiency of the ion acceleration . Ten separate phase probes are installed radially inside the accelerator chamber. They are used as input sensors to the measurement system, which determines the beam phase related to RF . After precalculated trim coil current values final tuning for each individual magnetic coil setting can be done by monitoring the beam phase. Careful study of various
phase detecting methods [2-6] and numerous measurements at the cyclotron have been performed. The realised system is based on frequency domain analysis and implemented with a double conversion heterodyne principle. The pulse shaped beam signal is amplified and mixed with a local oscillator signal, which is dependent on the accelerator frequency. In order to achieve adequate signal purity a crystal filter operating at fixed intermediate frequency is used . This technique makes it possible to filter out harmful RF disturbance on the wide operating frequency range of 10 to 21 MHz of the cyclotron. The phase measurement system is controlled by an industrial ALTIM [10] control system being used with the K130 cyclotron. 2. Measurement principle The phase information of the beam is extracted from a frequency component, which is twice the accelerator frequency. This frequency is used because the very strong component of the accelerating voltage is coupled to phase probes at the fundamental frequency. Due to the pure sine shape of the accelerating voltage the undesirable signal at the phase probes contains only weak harmonic components (see fig. la). On the other hand the pulse shaped beam signal contains strong harmonic components of the circulating frequency . These components can be used in determining the phase of the beam relative to the accelerating voltage. The use of the third or higher harmonic component would actually yield even better signal to disturbance ratio (see fig. lb). We decided to use the second harmonic for the following reason: the higher the harmonic component of the beam signal, the narrower is the phase window that can be detected with-
0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B .V . All rights reserved
J. Gustafsson et al. / Beam phase measurement system
418
START 0 Hz RES BW 100 kHz
VBW 10 kHz
STOP 40.00 MHz SWP 120 msec
Fig. 1 . Frequency spectrum of accelerating voltage at
START 0 Hz RES BW 100 kHz
11 .45 MHz
out ambiguity. At the frequency of the second harmonic component the beam phase range of -90° < beam < 90° is converted to -180° < 0 < 180°, which
VBW 10kHz
STOP
40 00 MHz SWP 120 msec
without beam (a) and with He" beam of 700 nA (b).
can be detected without ambiguity. A small portion of the acceleration voltage is used as a phase reference when multiplied with the same frequency as the mea-
Fig. 2. Block diagram of the phase measurement system .
J. Gustafsson et al. / Beam phase measurement system sured signal . The multiplication is simple to realize for the second harmonic, but more complicated in the case of higher harmonics . The beam phase measurement is made between the doubled reference frequency . The reference signal is coupled inductively from one of the resonance cavities of the cyclotron and the beam signal component at the same frequency. In case of the first and second harmonic acceleration modes (w,f = nw, where w c is the frequency of the beam and n is the number of the harmonic mode) the second harmonic of the beam is used in the measurement . However, in the case of the third harmonic acceleration mode, the sixth harmonic component of the beam must be used . The phase difference of the reference and probe signals is transferred to the fixed 82 .2 MHz frequency, which is independent of the acceleration frequency . The 82 .2 MHz signal is generated using an external crystal oscillator . This frequency is used to convert the phase difference into two differential do voltages . The block diagram of the phase measurement system is represented in fig . 2 . One of the ten channels is measured at a time . The channel selection, probe signal summing and preamplifying is done immediately after the probe cabling coming out of the cyclotron . This is done in order to raise the beam signal to a level which eliminates the effects of the RF pickup to the cables between the cyclotron and the rest of the measurement electronics . That signal is transferred to a power supply room using about 30 m of low attenuation double shielded coaxial cable . Then a further amplification is done to raise the beam signal to about -30 dBm level . This is the minimum signal level needed to get an adequate frequency conversion . The local oscillator drive for conversion is generated by mixing the crystal oscillator and the reference signals . This technique is described later in this article . The doubled reference signal can also be used to simulate the beam. So the calibration of the system can be done and aging of the crystal oscillator compensated . A crystal filter is used at a fixed IF frequency to eliminate numerous spurious frequency components, mostly generated by non-linear signal processing . The phase of the filtered signal is compared with a signal taken directly from the crystal oscillator and two phase dependent do voltages are produced . In the I/Q demodulator the phase information is divided into two channels separated by a 90° phase difference . This way I and Q phase vectors are created and the beam phase is calculated as f/ I-Io `' /I +00 , _ QOl 1 Q
0 = arctan/1
where (I, Q) and (lo , Q o) are the probe signal components with beam and without beam, respectively . (Po = ¢1 + 9'2 is an error term, which is caused by the phase
419
shifts in electronic devices (ib,) and transmission cables (02) . In addition to this absolute measurement various relative measurements as a function of the accelerator radius, time or intensity of the beam are possible for analysing the beam profile . 3 . Hardware realization The hardware for the beam phase measurement has been designed and implemented to be modular . The circuit design of the modules was based on commercially available components . The RF part is implemented using 50 fl! impedance level throughout the system . The requirements of the phase linearity prevented us from using sharp filters prior to IF frequency. Only moderate filtering was used instead . In this way the phase shifts due to electronics were possible to define and compensate . Automatic gain control (AGO devices in signal paths were not used because a variable beam signal level would cause unpredictable phase changes due to level dependent phase inaccuracy of such components . Instead programmable step attenuators and amplifiers were used in order to control the level of the reference and beam signals . Next we will discuss the phase probe in more detail . This is followed by the discussion of the signal processing .
3.1 . Phase probes The phase probes were designed and implemented based on experiences of many other laboratories [2,3,5] . In these studies it seems that capacitive phase probing yields best performance . The capacitive pick-up probes were mounted at equal distances above and below the median plane of the cyclotron . Each probe contains a copper plate connected to the inner coaxial conductor . The outer shield prevents the plates to be charged by the beam hitting them (see fig . 3) . It also serves as the RF shield . The radial positioning of the probes is defined by the location of the trim-coils . The interesting features of the accelerator RF voltage cancellation [3,5,11] was also found partly in the K130 laboratory . From the probes at radius of 35-70 cm attenuation of about 25 dB was observed for the disturbing signal . At smaller distances from the center the signal of the upper and lower probes differ maximally 20 dB in amplitude and thus no signal attenuation occurs . On the other hand, wideband noise was found . It is probably caused by the outer wall of the cyclotron chamber . The noise level was about 20 dB above the most quiet channels, which is a matter of great significance . The electrical equivalent of the probes can be represented as a current source driving the resistive load and the probe capacitance in parallel (see fig . 4). The
42 0
J. Gustafsson et al. / Beam phase measurement system
48 mm
sin (2rzf_+ 27[fco)
Fig . 5. SSB generation . 20 mm
Fig. 3. The principal picture of the phase probes seen from side (A) and above (B). sensitivity of the probes was measured to be about 150 RV/wA with 50 iZ resistive load . The signal is carried out of the cyclotron using the double shielded coaxial cables . The vacuum shield is achieved using special SMA coaxial feedthroughs . One probe pair at a time is selected with 24-to-2 channel RF multiplexor. After the multiplexor the two selected signals are summed in a power combiner. In order to achieve the best possible reduction of disturbing RF voltage any two probe signal pairs have equal phase difference within 0.1°. Identical probe signals between different channels have been achieved by adjusting the electric length difference within 0.3°. The multiplexor has been implemented using magnetic latching RF relays to ensure the operation at the high magnetic field of the cyclotron and the electrical length repeatibility of each switch . 3 .2 . Signal processing
Unlike most other real time frequency analysis phase measurement systems [3,5,6,12], no front end filtering has been built to the system . In this way numerous advantages have been achieved . The most prominent
d
QE
(+)
R
I VW
Fig. 4. The electrical equivalent of the phase probe.
of them is the elimination of the complicated frequency dependent phase shift of the bandpass filters . Only phase linear amplifiers are inserted in the beam signal path before the mixer. The variable local oscillator signal is used to convert the beam signal to the fixed intermediate frequency. This signal is produced using an external crystal oscillator and the doubled reference signal coupled from the RF cavity . The doubled reference signal sin(2o) ...), where w DEE = 24Tf DEE is first mixed with the local oscillator signal sin co,o, where f,o = 82 .2 MHz. The upper sideband of the mixing result gives the frequency 102.2 MHz < (2f,,, +f,,) < 124.2 MHz. The amplitude of the lower sideband is reduced using phasing method sideband suppression [7]. Block diagram of the SSB generation is introduced in fig. 5. The reference signal is doubled with a passive frequency doubler and the wideband 90° phase shift is implemented using a quadrature hybrid . It gives the
REF Od Bm PEAK LOG 10dß/
WA SB FC CORR
ATTEN
10d6
EM nmiliilL iisnm.
rll~l~llirminiiirrnn i rIJW1i a Ê^IW~J aA a
START 1000 MHz RES BW 3MHz
VBW 1MHz
STOP 3000 MHz SWP 20 msec
Fig. 6. Frequency spectrum after the SSB stage.
J. Gustafsson et al. / Beam phase measurement system
wanted phase with maximum deviation of 1 .5° and amplitude balance of 0.5 dB . These error source cause the main imperfections in the SSB generation . The frequency spectrum at the output of the SSB generator is displayed in fig. 6. The spectral purity is acceptable to be used as the local oscillator signal for the mixer. There is a passive low pass filter stage after the SSB mixer for eliminating the 3 f LO + frf product, which is typically the most harmful [8] intermodulation product generated in the double balanced mixers . Thus a mixer type with high LO to IF port isolation was selected . This was done to eliminate of the crystal oscillator frequency leakage to the IQ-demodulator via the crystal filter . The spectrum displayed in fig. 6 is measured after the low pass filter . There is a 3 dB attenuator in front of the filter to prevent the mismatch of the SSB mixer output . An alternative to our approach would have been active signal regeneration using a phase locked loop . This was not used because of the potential lack of short-time stability due to the phase noise of the PLL . The fixed frequency, which is used to determine the phase information, is selected to be as high as possible . This is done in order to prevent the overlapping of the mixing components . The fixed IF frequency is the mixing result of the SSB generated local oscillator signal (2fDEE+fLO) and the beam signal component. This has frequency equal with the doubled DEE reference signal . The mixing results can be written as follows : fl F - (2fDEE +fLO)
t nfbeamI
where n is the harmonic number of the beam signal . In case of 2fDEE and nfbeam being equal, the lower sideband gives the fixed fLO component. The phase of
CH1 TRN
log MAG
10dB/REF OdB
4 : -14.073 dB 82.431 .273MHz
ai
1 : -3 d8 82 .2 MHz 2 : -59.313 dB 81 .573 MHz 3. -57719 dB
I
Avg 16
421
this signal carries the beam phase information . For the first and the second harmonic mode acceleration n = 2 gives the wanted result . In the third harmonic mode acceleration the sixth harmonic component n = 6 gives the wanted result . The numerous unwanted mixing results, as well as the wide beam signal spectrum are eliminated by means of a crystal filter . The nominal center frequency of the filter is equal to the crystal oscillator frequency. The bandwidth of crystal filter is 50 kHz. The stop band attenuation of the filter is 55 dB on average. An important parameter of the design is the phase response of the crystal filter . It sets the stability requirement of the crystal oscillator frequency. The amplitude and the phase responses of the crystal filter are introduced in fig. 7. The acceptable stability of the oscillator is achieved using a commercial oscillator module with the frequency stability [9] in the temperature range from 0 to 50°C,
f
Of
=±3x10-6 .
With the maximum frequency variation the phase shift caused by the crystal filter is 5°. Because of an almost constant environment the variations in the oscillator frequency are insignificant . According to the measurements this error is less than 0 .5°. 4. Sensitivity of the measurement The sensitivity of the probes is a function of the probe size, length of the beam pulse and the intensity of the beam . A complicated analysis of the probe
CH 1 TRN
phase
1 % REF 1521 ° 1J -
Cor Avg 16
2: 153 35° 82 .2 MHz
3: 152.04° 82 .200 MHz 4: 14805' 82 201 MHz
i
ï
1: 154 59 ° 82 .199 750 MHz
w
(b)
(a)
START
81 .500 000 MHz
STOP 83 .000 000 MHz
START 82 .199 OOOMHZ
Fig. 7. The amplitude response (a) and the phase response (b) of the crystal filter .
4
STOP 82 201000 MHz
422
J. Gustafsson et al. / Beam phase measurement system
crystal oscillator
level of the DEE voltage is not allowed to exceed -5 dBm at the input of the mixer. This restricts the minimum detectable beam current to be about 10 nA. The leakage of the crystal oscillator signal via mixers to the I/Q detector is another limiting factor for the minimum detectable beam signal. The leakage signal level at the IF port of the second mixing stage is about - 60 dBm . This signal is summed with the signal carrying the phase information (see fig. 8) .
Fig. S. Crystal oscillator signal leakage via SSB mixer. 5. Results and discussion sensitivity [3] is not considered here . A sensitivity of the phase measurement system is restricted by a couple of error sources. They are mostly due to the intensive signal caused by the DEE voltage to the probes . The nonlinearities at the preamplifier and mixer stages generate harmonic components [8] of the coupled DEE voltage . In the case of the first and the second harmonic acceleration modes the second harmonic component occurs at the frequency, which is used to detect beam phase. When a high DEE voltage level is picked by the probes, distortion of the DEE voltage limits the minimum detectable beam signal level. The amplitude of the disturbing voltage equals the beam signal at the beam current level of the order of 1 l.LA in the worst case . The minimum probe signal level for the conversion is of the order of -45 dBm . In order to prevent the disturbing harmonic distortion in the mixer, the
The phase measurement system has been in operation in K130 cyclotron since April 1992 . Some additional assemblies to achieve the maximum dynamic range for the beam intensity were done later on . The hardware has been reliable in operation and very useful in isochronisation of the magnetic field. Together with the beam intensity measurement and track separation information a good profile for some test beams has been developed. With the beam intensity larger than 100 nA the phase can be measured with an accuracy of better than 0.5°. At lower intensity numerous systematic errors make the absolute phase determination difficult . However, in that case the phase difference measurement between different probes is possible down to the range of 1-5 nA of the beam current. The software development for the phase information visualisation and beam intensity detection has
Fig. 9. Phase measurement control and readout on the monitor.
J. Gustafsson et al. / Beam phase measurement system
been done with ALTIM tools. The measurement control as well as all the other cyclotron controls can be done on any of the three touch monitors connected parallel, one in the ECR room and the others to the control room . The phase information is displayed both graphically and numerically . The beam phase information is normally defined by scanning all the probes . The phase history of the scan is displayed both graphically and numerically (see fig. 9). Any of the probes can also be selected individually and the beam phase of the selected probe is displayed continuously . So far the trim coils are tuned manually . Automatic tuning using neural network algorithms is currently being studied. Anyway, more experience and studies are necessary to achieve a still better understanding and control of the beam . This is necessary for finding a sophisticated automatic tuning method and achieving maximum intensity of the beam . A possible modification for the measurement hardware is the design of a prefilter for the disturbing DEE voltage pickup, which covers the accelerating frequency range but has acceptable phase response . One way to realise such a filter is to divide the beam signal to two channels, delay one of them on the accelerating frequency and then combine the channels . The problem of that approach is the large required bandwidth phase shift . Another modification for the system would be the elimination of the systematic error caused by the I/Q detector . The analog multiplier ICs, which generate the phase-dependent DC voltages of two RF input signals, can cause maximally ±3° error depending on the phase difference of the input signals .
423
Acknowledgement We would like to thank the AGOR group for discussion and some ideas during the development of our system . References
[1] M. Alonso, E.J . Finn, Fundamental University Physics, vol. 1 (Addison-Wesley, 1986) pp . 506-509. [2] B.F . Milton, R.F . Ronningen, J. Yurkon and M. Maier, Proc . 11th Int. Conf. on Cyclotrons and their Applications (Ionics, Tokyo, 1987) p. 183. [3] F. Loyer, Ganil 79R/168/CC/22 (1979). [4] M. Kase and I. Yokoyama, 8th Symp . on Accelerator Science and Technology, Saitama, Japan, 1991, p. 295. [5] W. Briiytigam et al., IEEE Trans. Nucl . Sci. NS-26 (2) (1979) 2375 . [6] Ch . Olivetto, Groupe Diagnostics Agor, Mesure de la Phase Centrale du Faisceau Interne 2, Chaine de mesure analogique, Principes, NT/DIA/14/09/90, (1990) . [71 R.S . Carson, Radio Communication Concepts : Analog (Wiley, 1990) pp . 240-244 . [8] MA COM Company, RF & Microwave Signal Processing Components, Data Book (1990) pp . 234-241 . [9] Quarzkeramik GMBH : Crystal oscillators, Data Book (1991) pp . 6-7. [101 V. Hdnninen, J. Lampinen and P. Taskinen, JYFL Annual Report (1989) p . 26. [11] E.A. Kowalski, D.W . Devins and A. Seidman, IEEE Trans. Nucl . Sci . NS-22 (3) (1975) 1505 . [12] S. Schneider and P.G . Molteno, The Beam Phase Measurement System for the Cyclotrons and Beamlines at the NAC, Int. Rep., National Accelerator Centre, Republic of South Africa .
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
Beam phase measurement system for the K130 cyclotron in Jyväskylä b and K. Kaski a b, E. Liukkonen Tampere University of Technology, Microelectronics Lab., P.O. Box 692, SF-33101 Tampere, Finland b University of Jyudsky1ä, Department of Physics, P.O. Box 35, SF-40351 Jyudskyld, Finland
J. Gustafsson
a,
P . Kotilainen
a,
V. Nieminen
Received 18 March 1993 A phase measurement system for Jyviiskylii new K =130 heavy ion cyclotron has been designed and realized . The phase measurement is done using a set of capacitive probes to detect phase information from the internal ion beam . This data is vital for tuning purposes to obtain an isochronous magnetic field and to maximize the ion beam intensity.
1. Introduction A modern isochronous cyclotron requires a large number of control settings to accelerate efficiently a wide variety of ionic particles to different energies . In order to control the beam characteristics of the cyclotron, the phase information of the beam provides one of the most useful parameters . The on-line beam phase measurement can serve as an efficient way for tuning the precise isochronous magnetic field in a heavy ion cyclotron. The phase information of the internal ion beam is detected using a number of capacitive pick-up probes . This method makes it possible to tune systematically 15 trim coils, which control the radial magnetic field strength . This is necessary for achieving the isochronous magnetic field and maximizing the beam intensity at the output of the cyclotron. An automatic tuning of the magnetic field is also possible . For the measurement and tuning to be successful a careful study of RF disturbance in the cyclotron is necessary. Moreover, the experience gathered in other laboratories is most useful [3,5]. Using all this information a stable and sensitive phase measurements system has been developed. The accelerator in the Physics Department at University of Jyvdskyld is a K130 cyclotron equipped with an external ECR ion source . The extraction radius of the cyclotron is 96 cm . The phase measurement system was designed and developed to maximize the efficiency of the ion acceleration . Ten separate phase probes are installed radially inside the accelerator chamber. They are used as input sensors to the measurement system, which determines the beam phase related to RF . After precalculated trim coil current values final tuning for each individual magnetic coil setting can be done by monitoring the beam phase. Careful study of various
phase detecting methods [2-6] and numerous measurements at the cyclotron have been performed. The realised system is based on frequency domain analysis and implemented with a double conversion heterodyne principle. The pulse shaped beam signal is amplified and mixed with a local oscillator signal, which is dependent on the accelerator frequency. In order to achieve adequate signal purity a crystal filter operating at fixed intermediate frequency is used . This technique makes it possible to filter out harmful RF disturbance on the wide operating frequency range of 10 to 21 MHz of the cyclotron. The phase measurement system is controlled by an industrial ALTIM [10] control system being used with the K130 cyclotron. 2. Measurement principle The phase information of the beam is extracted from a frequency component, which is twice the accelerator frequency. This frequency is used because the very strong component of the accelerating voltage is coupled to phase probes at the fundamental frequency. Due to the pure sine shape of the accelerating voltage the undesirable signal at the phase probes contains only weak harmonic components (see fig. la). On the other hand the pulse shaped beam signal contains strong harmonic components of the circulating frequency . These components can be used in determining the phase of the beam relative to the accelerating voltage. The use of the third or higher harmonic component would actually yield even better signal to disturbance ratio (see fig. lb). We decided to use the second harmonic for the following reason: the higher the harmonic component of the beam signal, the narrower is the phase window that can be detected with-
0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B .V . All rights reserved
J. Gustafsson et al. / Beam phase measurement system
418
START 0 Hz RES BW 100 kHz
VBW 10 kHz
STOP 40.00 MHz SWP 120 msec
Fig. 1 . Frequency spectrum of accelerating voltage at
START 0 Hz RES BW 100 kHz
11 .45 MHz
out ambiguity. At the frequency of the second harmonic component the beam phase range of -90° < beam < 90° is converted to -180° < 0 < 180°, which
VBW 10kHz
STOP
40 00 MHz SWP 120 msec
without beam (a) and with He" beam of 700 nA (b).
can be detected without ambiguity. A small portion of the acceleration voltage is used as a phase reference when multiplied with the same frequency as the mea-
Fig. 2. Block diagram of the phase measurement system .
J. Gustafsson et al. / Beam phase measurement system sured signal . The multiplication is simple to realize for the second harmonic, but more complicated in the case of higher harmonics . The beam phase measurement is made between the doubled reference frequency . The reference signal is coupled inductively from one of the resonance cavities of the cyclotron and the beam signal component at the same frequency. In case of the first and second harmonic acceleration modes (w,f = nw, where w c is the frequency of the beam and n is the number of the harmonic mode) the second harmonic of the beam is used in the measurement . However, in the case of the third harmonic acceleration mode, the sixth harmonic component of the beam must be used . The phase difference of the reference and probe signals is transferred to the fixed 82 .2 MHz frequency, which is independent of the acceleration frequency . The 82 .2 MHz signal is generated using an external crystal oscillator . This frequency is used to convert the phase difference into two differential do voltages . The block diagram of the phase measurement system is represented in fig . 2 . One of the ten channels is measured at a time . The channel selection, probe signal summing and preamplifying is done immediately after the probe cabling coming out of the cyclotron . This is done in order to raise the beam signal to a level which eliminates the effects of the RF pickup to the cables between the cyclotron and the rest of the measurement electronics . That signal is transferred to a power supply room using about 30 m of low attenuation double shielded coaxial cable . Then a further amplification is done to raise the beam signal to about -30 dBm level . This is the minimum signal level needed to get an adequate frequency conversion . The local oscillator drive for conversion is generated by mixing the crystal oscillator and the reference signals . This technique is described later in this article . The doubled reference signal can also be used to simulate the beam. So the calibration of the system can be done and aging of the crystal oscillator compensated . A crystal filter is used at a fixed IF frequency to eliminate numerous spurious frequency components, mostly generated by non-linear signal processing . The phase of the filtered signal is compared with a signal taken directly from the crystal oscillator and two phase dependent do voltages are produced . In the I/Q demodulator the phase information is divided into two channels separated by a 90° phase difference . This way I and Q phase vectors are created and the beam phase is calculated as f/ I-Io `' /I +00 , _ QOl 1 Q
0 = arctan/1
where (I, Q) and (lo , Q o) are the probe signal components with beam and without beam, respectively . (Po = ¢1 + 9'2 is an error term, which is caused by the phase
419
shifts in electronic devices (ib,) and transmission cables (02) . In addition to this absolute measurement various relative measurements as a function of the accelerator radius, time or intensity of the beam are possible for analysing the beam profile . 3 . Hardware realization The hardware for the beam phase measurement has been designed and implemented to be modular . The circuit design of the modules was based on commercially available components . The RF part is implemented using 50 fl! impedance level throughout the system . The requirements of the phase linearity prevented us from using sharp filters prior to IF frequency. Only moderate filtering was used instead . In this way the phase shifts due to electronics were possible to define and compensate . Automatic gain control (AGO devices in signal paths were not used because a variable beam signal level would cause unpredictable phase changes due to level dependent phase inaccuracy of such components . Instead programmable step attenuators and amplifiers were used in order to control the level of the reference and beam signals . Next we will discuss the phase probe in more detail . This is followed by the discussion of the signal processing .
3.1 . Phase probes The phase probes were designed and implemented based on experiences of many other laboratories [2,3,5] . In these studies it seems that capacitive phase probing yields best performance . The capacitive pick-up probes were mounted at equal distances above and below the median plane of the cyclotron . Each probe contains a copper plate connected to the inner coaxial conductor . The outer shield prevents the plates to be charged by the beam hitting them (see fig . 3) . It also serves as the RF shield . The radial positioning of the probes is defined by the location of the trim-coils . The interesting features of the accelerator RF voltage cancellation [3,5,11] was also found partly in the K130 laboratory . From the probes at radius of 35-70 cm attenuation of about 25 dB was observed for the disturbing signal . At smaller distances from the center the signal of the upper and lower probes differ maximally 20 dB in amplitude and thus no signal attenuation occurs . On the other hand, wideband noise was found . It is probably caused by the outer wall of the cyclotron chamber . The noise level was about 20 dB above the most quiet channels, which is a matter of great significance . The electrical equivalent of the probes can be represented as a current source driving the resistive load and the probe capacitance in parallel (see fig . 4). The
42 0
J. Gustafsson et al. / Beam phase measurement system
48 mm
sin (2rzf_+ 27[fco)
Fig . 5. SSB generation . 20 mm
Fig. 3. The principal picture of the phase probes seen from side (A) and above (B). sensitivity of the probes was measured to be about 150 RV/wA with 50 iZ resistive load . The signal is carried out of the cyclotron using the double shielded coaxial cables . The vacuum shield is achieved using special SMA coaxial feedthroughs . One probe pair at a time is selected with 24-to-2 channel RF multiplexor. After the multiplexor the two selected signals are summed in a power combiner. In order to achieve the best possible reduction of disturbing RF voltage any two probe signal pairs have equal phase difference within 0.1°. Identical probe signals between different channels have been achieved by adjusting the electric length difference within 0.3°. The multiplexor has been implemented using magnetic latching RF relays to ensure the operation at the high magnetic field of the cyclotron and the electrical length repeatibility of each switch . 3 .2 . Signal processing
Unlike most other real time frequency analysis phase measurement systems [3,5,6,12], no front end filtering has been built to the system . In this way numerous advantages have been achieved . The most prominent
d
QE
(+)
R
I VW
Fig. 4. The electrical equivalent of the phase probe.
of them is the elimination of the complicated frequency dependent phase shift of the bandpass filters . Only phase linear amplifiers are inserted in the beam signal path before the mixer. The variable local oscillator signal is used to convert the beam signal to the fixed intermediate frequency. This signal is produced using an external crystal oscillator and the doubled reference signal coupled from the RF cavity . The doubled reference signal sin(2o) ...), where w DEE = 24Tf DEE is first mixed with the local oscillator signal sin co,o, where f,o = 82 .2 MHz. The upper sideband of the mixing result gives the frequency 102.2 MHz < (2f,,, +f,,) < 124.2 MHz. The amplitude of the lower sideband is reduced using phasing method sideband suppression [7]. Block diagram of the SSB generation is introduced in fig. 5. The reference signal is doubled with a passive frequency doubler and the wideband 90° phase shift is implemented using a quadrature hybrid . It gives the
REF Od Bm PEAK LOG 10dß/
WA SB FC CORR
ATTEN
10d6
EM nmiliilL iisnm.
rll~l~llirminiiirrnn i rIJW1i a Ê^IW~J aA a
START 1000 MHz RES BW 3MHz
VBW 1MHz
STOP 3000 MHz SWP 20 msec
Fig. 6. Frequency spectrum after the SSB stage.
J. Gustafsson et al. / Beam phase measurement system
wanted phase with maximum deviation of 1 .5° and amplitude balance of 0.5 dB . These error source cause the main imperfections in the SSB generation . The frequency spectrum at the output of the SSB generator is displayed in fig. 6. The spectral purity is acceptable to be used as the local oscillator signal for the mixer. There is a passive low pass filter stage after the SSB mixer for eliminating the 3 f LO + frf product, which is typically the most harmful [8] intermodulation product generated in the double balanced mixers . Thus a mixer type with high LO to IF port isolation was selected . This was done to eliminate of the crystal oscillator frequency leakage to the IQ-demodulator via the crystal filter . The spectrum displayed in fig. 6 is measured after the low pass filter . There is a 3 dB attenuator in front of the filter to prevent the mismatch of the SSB mixer output . An alternative to our approach would have been active signal regeneration using a phase locked loop . This was not used because of the potential lack of short-time stability due to the phase noise of the PLL . The fixed frequency, which is used to determine the phase information, is selected to be as high as possible . This is done in order to prevent the overlapping of the mixing components . The fixed IF frequency is the mixing result of the SSB generated local oscillator signal (2fDEE+fLO) and the beam signal component. This has frequency equal with the doubled DEE reference signal . The mixing results can be written as follows : fl F - (2fDEE +fLO)
t nfbeamI
where n is the harmonic number of the beam signal . In case of 2fDEE and nfbeam being equal, the lower sideband gives the fixed fLO component. The phase of
CH1 TRN
log MAG
10dB/REF OdB
4 : -14.073 dB 82.431 .273MHz
ai
1 : -3 d8 82 .2 MHz 2 : -59.313 dB 81 .573 MHz 3. -57719 dB
I
Avg 16
421
this signal carries the beam phase information . For the first and the second harmonic mode acceleration n = 2 gives the wanted result . In the third harmonic mode acceleration the sixth harmonic component n = 6 gives the wanted result . The numerous unwanted mixing results, as well as the wide beam signal spectrum are eliminated by means of a crystal filter . The nominal center frequency of the filter is equal to the crystal oscillator frequency. The bandwidth of crystal filter is 50 kHz. The stop band attenuation of the filter is 55 dB on average. An important parameter of the design is the phase response of the crystal filter . It sets the stability requirement of the crystal oscillator frequency. The amplitude and the phase responses of the crystal filter are introduced in fig. 7. The acceptable stability of the oscillator is achieved using a commercial oscillator module with the frequency stability [9] in the temperature range from 0 to 50°C,
f
Of
=±3x10-6 .
With the maximum frequency variation the phase shift caused by the crystal filter is 5°. Because of an almost constant environment the variations in the oscillator frequency are insignificant . According to the measurements this error is less than 0 .5°. 4. Sensitivity of the measurement The sensitivity of the probes is a function of the probe size, length of the beam pulse and the intensity of the beam . A complicated analysis of the probe
CH 1 TRN
phase
1 % REF 1521 ° 1J -
Cor Avg 16
2: 153 35° 82 .2 MHz
3: 152.04° 82 .200 MHz 4: 14805' 82 201 MHz
i
ï
1: 154 59 ° 82 .199 750 MHz
w
(b)
(a)
START
81 .500 000 MHz
STOP 83 .000 000 MHz
START 82 .199 OOOMHZ
Fig. 7. The amplitude response (a) and the phase response (b) of the crystal filter .
4
STOP 82 201000 MHz
422
J. Gustafsson et al. / Beam phase measurement system
crystal oscillator
level of the DEE voltage is not allowed to exceed -5 dBm at the input of the mixer. This restricts the minimum detectable beam current to be about 10 nA. The leakage of the crystal oscillator signal via mixers to the I/Q detector is another limiting factor for the minimum detectable beam signal. The leakage signal level at the IF port of the second mixing stage is about - 60 dBm . This signal is summed with the signal carrying the phase information (see fig. 8) .
Fig. S. Crystal oscillator signal leakage via SSB mixer. 5. Results and discussion sensitivity [3] is not considered here . A sensitivity of the phase measurement system is restricted by a couple of error sources. They are mostly due to the intensive signal caused by the DEE voltage to the probes . The nonlinearities at the preamplifier and mixer stages generate harmonic components [8] of the coupled DEE voltage . In the case of the first and the second harmonic acceleration modes the second harmonic component occurs at the frequency, which is used to detect beam phase. When a high DEE voltage level is picked by the probes, distortion of the DEE voltage limits the minimum detectable beam signal level. The amplitude of the disturbing voltage equals the beam signal at the beam current level of the order of 1 l.LA in the worst case . The minimum probe signal level for the conversion is of the order of -45 dBm . In order to prevent the disturbing harmonic distortion in the mixer, the
The phase measurement system has been in operation in K130 cyclotron since April 1992 . Some additional assemblies to achieve the maximum dynamic range for the beam intensity were done later on . The hardware has been reliable in operation and very useful in isochronisation of the magnetic field. Together with the beam intensity measurement and track separation information a good profile for some test beams has been developed. With the beam intensity larger than 100 nA the phase can be measured with an accuracy of better than 0.5°. At lower intensity numerous systematic errors make the absolute phase determination difficult . However, in that case the phase difference measurement between different probes is possible down to the range of 1-5 nA of the beam current. The software development for the phase information visualisation and beam intensity detection has
Fig. 9. Phase measurement control and readout on the monitor.
J. Gustafsson et al. / Beam phase measurement system
been done with ALTIM tools. The measurement control as well as all the other cyclotron controls can be done on any of the three touch monitors connected parallel, one in the ECR room and the others to the control room . The phase information is displayed both graphically and numerically . The beam phase information is normally defined by scanning all the probes . The phase history of the scan is displayed both graphically and numerically (see fig. 9). Any of the probes can also be selected individually and the beam phase of the selected probe is displayed continuously . So far the trim coils are tuned manually . Automatic tuning using neural network algorithms is currently being studied. Anyway, more experience and studies are necessary to achieve a still better understanding and control of the beam . This is necessary for finding a sophisticated automatic tuning method and achieving maximum intensity of the beam . A possible modification for the measurement hardware is the design of a prefilter for the disturbing DEE voltage pickup, which covers the accelerating frequency range but has acceptable phase response . One way to realise such a filter is to divide the beam signal to two channels, delay one of them on the accelerating frequency and then combine the channels . The problem of that approach is the large required bandwidth phase shift . Another modification for the system would be the elimination of the systematic error caused by the I/Q detector . The analog multiplier ICs, which generate the phase-dependent DC voltages of two RF input signals, can cause maximally ±3° error depending on the phase difference of the input signals .
423
Acknowledgement We would like to thank the AGOR group for discussion and some ideas during the development of our system . References
[1] M. Alonso, E.J . Finn, Fundamental University Physics, vol. 1 (Addison-Wesley, 1986) pp . 506-509. [2] B.F . Milton, R.F . Ronningen, J. Yurkon and M. Maier, Proc . 11th Int. Conf. on Cyclotrons and their Applications (Ionics, Tokyo, 1987) p. 183. [3] F. Loyer, Ganil 79R/168/CC/22 (1979). [4] M. Kase and I. Yokoyama, 8th Symp . on Accelerator Science and Technology, Saitama, Japan, 1991, p. 295. [5] W. Briiytigam et al., IEEE Trans. Nucl . Sci. NS-26 (2) (1979) 2375 . [6] Ch . Olivetto, Groupe Diagnostics Agor, Mesure de la Phase Centrale du Faisceau Interne 2, Chaine de mesure analogique, Principes, NT/DIA/14/09/90, (1990) . [71 R.S . Carson, Radio Communication Concepts : Analog (Wiley, 1990) pp . 240-244 . [8] MA COM Company, RF & Microwave Signal Processing Components, Data Book (1990) pp . 234-241 . [9] Quarzkeramik GMBH : Crystal oscillators, Data Book (1991) pp . 6-7. [101 V. Hdnninen, J. Lampinen and P. Taskinen, JYFL Annual Report (1989) p . 26. [11] E.A. Kowalski, D.W . Devins and A. Seidman, IEEE Trans. Nucl . Sci . NS-22 (3) (1975) 1505 . [12] S. Schneider and P.G . Molteno, The Beam Phase Measurement System for the Cyclotrons and Beamlines at the NAC, Int. Rep., National Accelerator Centre, Republic of South Africa .