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A high sensitivity phase stabilizing system and beam current monitor
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
Nuclear Instruments and Methods in Physics Research A 337 (1994) 253-257 North-Holland
A high sensitivity phase stabilizing system and beam current monitor A. Facco
a,
K. Rudolph b , G. Bassato a, A. Battistella a INFN-Laboratori Nazionah di Legnaro, Via Romea 4, I-35020 Legnaro (Padova), Italy b Sektion Physik, University of Munich, Germany (Received 22 June 1993) A 5 MHz phase stabilizing system and beam current monitor for the superconducting linac ALPI at Laboratori Nazionali di Legnaro (LNL) was designed, constructed and tested . The system, based on a 5 MHz resonant phase detector, provides stabilization of the phase of the beam bunches produced by the LNL double drift buncher. In addition, the system gives a non-destructive reading of the pulsed beam current in the range between 0 .3 and 140 nA . The high impedance coupling of the preamplifier to the resonant phase detector allows for a sensitivity of 1 mV/nA, which is two orders of magnitude higher than previously reported for this kind of devices. The fast phase stabilization is performed by means of a voltage controlled phase shifter; the long term drifts are corrected using a digitally controlled delay. An automatic phase adjustment of the resonator amplifier output was included, in order to correct for any phase shift generated by the amplifiers due to changes of the input signal level. A general expression of the phase response in resonant phase detectors is evaluated and discussed. 1. Introduction The problem of eliminating phase fluctuations in ion beams bunched at low energy and then accelerated by a tandem has been discussed by many authors [1-31; these shifts are mainly due to changes in the field distribution inside tandem accelerators and to voltage fluctuations of injection platforms, causing variations in the time of flight of the ions along the low energy side of the acceleration line . In both cases, phase fluctuations appear mainly at frequencies below 1 kHz [31; they can reach amplitudes comparable or higher than the time width (typically 1-2 ns FWHM) of the beam pulses produced by low energy bunchers . In many laboratories [1,3-5,71 phase stabilization systems were installed in order to preserve the time resolution of the bunched beam ; such systems are based on phase detectors consisting of resonators excited by the travelling ion bunches. This technique is non-destructive for the beam and it can operate even when the beam current is as low as 1 nA or less; the phase of the output signal is averaged by the high Q resonator over a high number of pulses, useful for detecting low frequency fluctuations . 2. What happens in a resonant phase detector? The purpose of a resonant phase detector is to produce a low noise output signal with a constant phase delay with respect to the bunched beam. The
field is built up by many bunches: this improves the signal-to-noise ratio significantly, but introduces a delay in the phase detector response to changes of the beam phase, and also causes some attenuation in its amplitude. We shall assume that every bunch carries the same amount of charge . For sake of simplicity, we shall consider only one resonant frequency W: all other components are usually eliminated by filtering. A beam bunch, passing through the cavity at the time t' with a phase 0(t'), excites a signal which decays exponentially with the time : (1) AV(t', t, 0(t')) = uo e -(` -t')1T e 1w' e,
w 2arn
t
v o e -('-")/z e` e'O(t') dt' .
(2)
We can study the behaviour of the phase detector by solving the integral (2) with a beam phase 4~(t') which is changing in time at a frequency f2 : 0(t')=0o+t1Oo(f2) sin(f') . Since we are dealing with phase fluctuations of small amplitude compared with one rf period, i.e . Ao o ( .f2)
0168-9002/94/$07.00 O 1994 - Elsevier Science B.V . All rights reserved SSDI 0168-9002(93)EO628-6
25 4
A . Facco et al. /Nucl. Instr and Meth . to Phys . Res . A 337 (1994) 253-257
<< 2,Tr, we can use the approximation e '°~sin(Idr') - 1 + iA¢ o sin(f2t'). This means that the results are true
for small frequency modulation, which is our case . We obtain an expression of the signal in the resonator as a function of the phase and of the current of the beam : V(t)=V0 e` e"mv> (4) where
QL n , 7-r
Vo = - to
(5)
is a function of the beam current (see section 3), and
qb(r)=0 o +pA0 a (f2) sin(f2t+S,P(f2)),
where P= and
1
1
+n z r2
(6) I = losin(aot) Fig. 1. Equivalent circuit of the LNL phase detector .
(7)
SP(f) = arctg(- ƒ2,r) .
( 8) The response of the resonator to beam phase modulation contains a phase delay Sqr and an attenuation p; we can define a cutoff frequency at which Srf = -45° and p = I/ C: this is the inverse of the loaded resonator decay time, This value is not related at all with the frequency modulation associated to phase modulation, since our results are valid at any modulation amplitude O0o << 1. A resonant phase detector, then, is equivalent to an ideal phase detector in series with a low-pass filter . The amplitude of the signal in the resonator is Q L/n , rr times the amplitude of the signal produced by a single bunch. As a consequence, the resonator quality factor should be kept as high as possible in order to have a high signal output ; however, if the device is used to detect phase modulations, the desired cutoff frequency should be taken into account. 3. Sensitivity enhancement with high impedance coupling The power transferred to a normal conducting resonator by a 1 nA bunched beam is usually of the order of few picowatts; the main problem then is how to extract the signal and keep the signal-to-noise ratio as high as possible . The highest sensitivity, defined as the ratio between the rf voltage at the resonator pickup and the beam current [5], is obtained at critical coupling for a given input impedance of the pickup. In FET amplifiers the optimum input impedance for a low noise figure is
often of the same order of magnitude as the shunt impedance Rsh = Vó /2P of normal conducting high Q resonators, well above 50 S2 . In our phase detector, a two-gap LC resonator, we coupled the first stage of amplification, consisting of a low noise amplifier with 670 k1Z input impedance, directly to the highest voltage point in the resonator; this choice allowed us to satisfy both the requirement of nearly critical coupling and high impedance matching of the pickup . The equivalent circuit is shown in fig. 1 . A bunched beam passing through the two-gap resonator is well approximated by a current generator connected between ground and drift tube giving a current I sin(c)t) (all other components are filtered by the high Q resonator) ; assuming a bunch time width At « 2,rr/cw and a negligible gap length, the current amplitude becomes Io = 4Ib T(,6),
(10)
where I b is the average beam current, T(/3) = sin[(-rrd)/()3A)] is the transit time factor, /3 = u/c, A = 21rc/w and d is the distance between the two gaps . For any coupling, if Qo is the unloaded quality factor of the resonator and Q L the loaded one, the sensitivity becomes: S =
Vo Ib
= 4R5hT(P) QL Qo
and the power transmitted to the load is : z ( ) RLb . PL =8T2(P)Rsh R L + R,h
(11)
(12)
The maximum power is transmitted to the load for
R L = R sh , which is the condition for critical coupling .
A . Facco et al. I Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 253-257 4 . The LNL system
255
Beam
The system installed at LNL [6] is sketched in fig. 2 . It includes a 5 MHz resonator, a low noise linear preamplifier, a constant output amplifier with separate outputs for information on phase and amplitude of the resonator signal, and a rf controller. The computer control is done within the program which controls the whole LNL pulsing system. The fast phase stabilization is performed by a voltage controlled delay, driven by the phase error signal ; the slow stabilization is done via software changing a digitally controlled delay, whenever the phase error exceeds a threshold value of 500 ps . Both these delays are shifting the low energy buncher reference phase . 4.1 . The 5 MHz resonator Our phase detector (fig . 3) is a 2-gap, 5 MHz resonator consisting of an inductor and a drift tube 1 .6 m long in order to match the beam particles velocity; the optimum velocity is ß = 0.055, the same as of the first section of our linac . The drift tube is connected to the beam tube by means of two vacuum tight teflon holders ; only the inner part of the drift tube is in vacuum, all other parts of the resonator are in contact
5 MHz ref
Fig . 2. Layout of the LNL phase stabilization system. with air allowing for gas-flow temperature regulation Since the resonator performance depends only on
Fig . 3 . The LNL 5 MHz phase detector.
256
A. Facco et al . /Nucl. Instr. and Meth. in Phys. Res A 337 (1994) 253-257
Rsh and since we had no limitation in space, we could choose the frequency of the bunched beam for the resonator: this allows phase stabilization over the whole cycle, while the range covered by a cavity with a resonance frequency to = mm,, would be smaller by a factor I /m and the system, after a strong beam phase change, could jump in one of the m different stable positions. The resonator has an unloaded quality factor Qo = 1650 and the shunt impedance is Rsh = V0 /(2P) = 440 k S2 . The pickup is coupled directly to the drift tube . The input capacitance of our preamplifier at 5 MHz is 14 pF and the input impedance is 670 kn, slightly higher than the resonator one; the coupling, then, is slightly less than critical and Q,_ = 1000. The theoretical sensitivity of our resonator is then S = 1 mV/nA and its phase modulation cutoff frequency is ,2, = 2.5 kHz. 4.2 . Electronics
The resonator signal is extracted via a single cable connecting the input of a high impedance-low noise linear preamplifier BPMP [8] to the drift tube ; this device has an amplification of 13 .4 dB and an output impedance of 50 Q . A second stage of amplification is done by an AD640 demodulating logarithmic amplifier; the small signal gain is 40 dB and the rf output signal, always saturated in our case, consists of a 360 mV square wave which carries the phase information of the signal . The AD640 supplies also a do signal proportional to the logarithm of the input signal amplitude. Both outputs are sent to the rf controller . The rf signal, after filtering, is compared with the reference signal in a double balanced mixer; the resulting phase error signal is amplified and used to provide phase stabilization. The do signal is read by an ADC; with this information, by means of eq . (10) with a correction for transit time factor, the computer calculates and displays the value of the beam current. Moreover, the phase delays introduced by the amplification chain at different amplitudes of the input signal are corrected by software using a calibration table . 4.3. Measurements
The sensitivity was tested with a "Ni beam, bunched at 5 MHz, by measuring the beam current with a Faraday cup and comparing it with the resonator signal amplitude. The experimental sensitivity was found to be 1 mV/nA. The beam current monitor was then calibrated and tested in the range from 0.3 to 140 nA ; if necessary, the lower limit could be reduced to 0.1 nA by removing
a 10 dB attenuator after the first stage of amplification. This range fits our requirements ; however, the good quality of the signal even at the lowest level suggest that adding a second AD640 to the amplification chain could allow extending the range down to few picoamperes of beam current. We tested the effect of the system on the longitudinal characteristics of the beam : the time width of the bunched beam, stable at 2.8 ns when the phase stabilization system was active, increased to 3.5 ns during a 1 hour measurement after the phase stabilization was disabled . We measured also the long-term stability of the system by coupling the reference signal to the phase detector through a capacitive coupler; we recorded the phase error signal during 24 hours; we observed shifts of few nanoseconds related to changes in the environmental temperature . In order to eliminate this problem, a gas-flow temperature stabilizer will be installed. 5. Conclusions The system developed at LNL provides simultaneously phase stabilization and non-destructive beam current monitoring of the bunched beam . This double purpose is obtained by using an AD640 demodulating amplifier. The choice of a resonant phase detector with high impedance critical coupling of the low-noise preamplifier allowed a significant improvement of the phase detector sensitivity (two orders of magnitude higher than previously reported for such kind of devices). The loaded quality factor of the LNL phase detector was limited to Q,, = 1000 in order to match our need of a modulation cutoff frequency of 2.5 kHz for fast phase stabilization ; if this is not required, the sensitivity can be increased significantly by using high Q (e .g . superconductive) resonators . Further increase of the beam current monitor range toward low current intensities could be obtained by using two AD640 in cascade . Acknowledgements We acknowledge N. Dainese for the mechanical construction of the phase detector and S. Dong for his collaboration m mounting and testing the system . We are also grateful to A. Zanon and G. Tombola for their contribution in the construction of the electronics and to J.S . Sokolowski for his fruitful suggestions . We thank Marco Poggi and A. Dainelli for their help during the measurements performed with the beam. Our gratitude goes also to J. Simovic of the University of Munich for the layout of the demodulating amplifier, and to KFA Jülich for their courtesy in allowing us to use a BPMP low noise amplifier.
A. Facco et al. /Nucl. Instr. and Meth. i n Phys . Res. A 337 (1994) 253-257
References [1] F.J . Lynch, R.N . Lewis, L.M . Bollinger, W. Henning and O.D . Despe, Nucl . Instr. and Meth. 159 (1979) 245 . [2] S.J. Skorka, Proc . 3rd Int. Conf. on Electrostatic Accelerator Techn., IEEE (1981) 130. [3] R.N . Lewis, Nucl . Instr. and Meth . 151 (1978) 371. [4] T.W . Aitken and C.W . Horrabin, Phase stabilization system for bunched beams, in : Nuclear Physics - Appendix to the Daresbury Annual Report 1988/89, SERC Daresbury Laboratory, Warrington WA4 4AD, UK, p. 143. [5] S. Takeuchi and K.W . Shepard, Nucl. Instr. and Meth. 227 (1984) 217.
257
[6] A. Facco, K. Rudolph, A. Battistella, G. Tombola, F. Scarpa and A. Zanon, Proc. 3rd Europ. Particle Accelerator Conf ., Berlin, 1992, vol. 2, eds. H. Henke, H. Homemeyer and Ch . Petit-Jean-Genaz (Edition Frontières, Gifsur-Yvette, 1992) p. 1507 . [7] R.C . Connolly and D.D . Leach, Nucl . Instr. and Meth . A 273 (1988) 40 . [8] J. Dietrich et al., Proc . 3rd European Particle Accelerator Conf ., Berlin, 1992, vol. 1, eds. H. Henke, H. Homemeyer and Ch . Petit-Jean-Genaz (Edition Frontieres, Gif-surYvette, 1992) p. 1088 .
Nuclear Instruments and Methods in Physics Research A 337 (1994) 253-257 North-Holland
A high sensitivity phase stabilizing system and beam current monitor A. Facco
a,
K. Rudolph b , G. Bassato a, A. Battistella a INFN-Laboratori Nazionah di Legnaro, Via Romea 4, I-35020 Legnaro (Padova), Italy b Sektion Physik, University of Munich, Germany (Received 22 June 1993) A 5 MHz phase stabilizing system and beam current monitor for the superconducting linac ALPI at Laboratori Nazionali di Legnaro (LNL) was designed, constructed and tested . The system, based on a 5 MHz resonant phase detector, provides stabilization of the phase of the beam bunches produced by the LNL double drift buncher. In addition, the system gives a non-destructive reading of the pulsed beam current in the range between 0 .3 and 140 nA . The high impedance coupling of the preamplifier to the resonant phase detector allows for a sensitivity of 1 mV/nA, which is two orders of magnitude higher than previously reported for this kind of devices. The fast phase stabilization is performed by means of a voltage controlled phase shifter; the long term drifts are corrected using a digitally controlled delay. An automatic phase adjustment of the resonator amplifier output was included, in order to correct for any phase shift generated by the amplifiers due to changes of the input signal level. A general expression of the phase response in resonant phase detectors is evaluated and discussed. 1. Introduction The problem of eliminating phase fluctuations in ion beams bunched at low energy and then accelerated by a tandem has been discussed by many authors [1-31; these shifts are mainly due to changes in the field distribution inside tandem accelerators and to voltage fluctuations of injection platforms, causing variations in the time of flight of the ions along the low energy side of the acceleration line . In both cases, phase fluctuations appear mainly at frequencies below 1 kHz [31; they can reach amplitudes comparable or higher than the time width (typically 1-2 ns FWHM) of the beam pulses produced by low energy bunchers . In many laboratories [1,3-5,71 phase stabilization systems were installed in order to preserve the time resolution of the bunched beam ; such systems are based on phase detectors consisting of resonators excited by the travelling ion bunches. This technique is non-destructive for the beam and it can operate even when the beam current is as low as 1 nA or less; the phase of the output signal is averaged by the high Q resonator over a high number of pulses, useful for detecting low frequency fluctuations . 2. What happens in a resonant phase detector? The purpose of a resonant phase detector is to produce a low noise output signal with a constant phase delay with respect to the bunched beam. The
field is built up by many bunches: this improves the signal-to-noise ratio significantly, but introduces a delay in the phase detector response to changes of the beam phase, and also causes some attenuation in its amplitude. We shall assume that every bunch carries the same amount of charge . For sake of simplicity, we shall consider only one resonant frequency W: all other components are usually eliminated by filtering. A beam bunch, passing through the cavity at the time t' with a phase 0(t'), excites a signal which decays exponentially with the time : (1) AV(t', t, 0(t')) = uo e -(` -t')1T e 1w' e,
w 2arn
t
v o e -('-")/z e` e'O(t') dt' .
(2)
We can study the behaviour of the phase detector by solving the integral (2) with a beam phase 4~(t') which is changing in time at a frequency f2 : 0(t')=0o+t1Oo(f2) sin(f') . Since we are dealing with phase fluctuations of small amplitude compared with one rf period, i.e . Ao o ( .f2)
0168-9002/94/$07.00 O 1994 - Elsevier Science B.V . All rights reserved SSDI 0168-9002(93)EO628-6
25 4
A . Facco et al. /Nucl. Instr and Meth . to Phys . Res . A 337 (1994) 253-257
<< 2,Tr, we can use the approximation e '°~sin(Idr') - 1 + iA¢ o sin(f2t'). This means that the results are true
for small frequency modulation, which is our case . We obtain an expression of the signal in the resonator as a function of the phase and of the current of the beam : V(t)=V0 e` e"mv> (4) where
QL n , 7-r
Vo = - to
(5)
is a function of the beam current (see section 3), and
qb(r)=0 o +pA0 a (f2) sin(f2t+S,P(f2)),
where P= and
1
1
+n z r2
(6) I = losin(aot) Fig. 1. Equivalent circuit of the LNL phase detector .
(7)
SP(f) = arctg(- ƒ2,r) .
( 8) The response of the resonator to beam phase modulation contains a phase delay Sqr and an attenuation p; we can define a cutoff frequency at which Srf = -45° and p = I/ C: this is the inverse of the loaded resonator decay time, This value is not related at all with the frequency modulation associated to phase modulation, since our results are valid at any modulation amplitude O0o << 1. A resonant phase detector, then, is equivalent to an ideal phase detector in series with a low-pass filter . The amplitude of the signal in the resonator is Q L/n , rr times the amplitude of the signal produced by a single bunch. As a consequence, the resonator quality factor should be kept as high as possible in order to have a high signal output ; however, if the device is used to detect phase modulations, the desired cutoff frequency should be taken into account. 3. Sensitivity enhancement with high impedance coupling The power transferred to a normal conducting resonator by a 1 nA bunched beam is usually of the order of few picowatts; the main problem then is how to extract the signal and keep the signal-to-noise ratio as high as possible . The highest sensitivity, defined as the ratio between the rf voltage at the resonator pickup and the beam current [5], is obtained at critical coupling for a given input impedance of the pickup. In FET amplifiers the optimum input impedance for a low noise figure is
often of the same order of magnitude as the shunt impedance Rsh = Vó /2P of normal conducting high Q resonators, well above 50 S2 . In our phase detector, a two-gap LC resonator, we coupled the first stage of amplification, consisting of a low noise amplifier with 670 k1Z input impedance, directly to the highest voltage point in the resonator; this choice allowed us to satisfy both the requirement of nearly critical coupling and high impedance matching of the pickup . The equivalent circuit is shown in fig. 1 . A bunched beam passing through the two-gap resonator is well approximated by a current generator connected between ground and drift tube giving a current I sin(c)t) (all other components are filtered by the high Q resonator) ; assuming a bunch time width At « 2,rr/cw and a negligible gap length, the current amplitude becomes Io = 4Ib T(,6),
(10)
where I b is the average beam current, T(/3) = sin[(-rrd)/()3A)] is the transit time factor, /3 = u/c, A = 21rc/w and d is the distance between the two gaps . For any coupling, if Qo is the unloaded quality factor of the resonator and Q L the loaded one, the sensitivity becomes: S =
Vo Ib
= 4R5hT(P) QL Qo
and the power transmitted to the load is : z ( ) RLb . PL =8T2(P)Rsh R L + R,h
(11)
(12)
The maximum power is transmitted to the load for
R L = R sh , which is the condition for critical coupling .
A . Facco et al. I Nucl. Instr. and Meth . in Phys. Res. A 337 (1994) 253-257 4 . The LNL system
255
Beam
The system installed at LNL [6] is sketched in fig. 2 . It includes a 5 MHz resonator, a low noise linear preamplifier, a constant output amplifier with separate outputs for information on phase and amplitude of the resonator signal, and a rf controller. The computer control is done within the program which controls the whole LNL pulsing system. The fast phase stabilization is performed by a voltage controlled delay, driven by the phase error signal ; the slow stabilization is done via software changing a digitally controlled delay, whenever the phase error exceeds a threshold value of 500 ps . Both these delays are shifting the low energy buncher reference phase . 4.1 . The 5 MHz resonator Our phase detector (fig . 3) is a 2-gap, 5 MHz resonator consisting of an inductor and a drift tube 1 .6 m long in order to match the beam particles velocity; the optimum velocity is ß = 0.055, the same as of the first section of our linac . The drift tube is connected to the beam tube by means of two vacuum tight teflon holders ; only the inner part of the drift tube is in vacuum, all other parts of the resonator are in contact
5 MHz ref
Fig . 2. Layout of the LNL phase stabilization system. with air allowing for gas-flow temperature regulation Since the resonator performance depends only on
Fig . 3 . The LNL 5 MHz phase detector.
256
A. Facco et al . /Nucl. Instr. and Meth. in Phys. Res A 337 (1994) 253-257
Rsh and since we had no limitation in space, we could choose the frequency of the bunched beam for the resonator: this allows phase stabilization over the whole cycle, while the range covered by a cavity with a resonance frequency to = mm,, would be smaller by a factor I /m and the system, after a strong beam phase change, could jump in one of the m different stable positions. The resonator has an unloaded quality factor Qo = 1650 and the shunt impedance is Rsh = V0 /(2P) = 440 k S2 . The pickup is coupled directly to the drift tube . The input capacitance of our preamplifier at 5 MHz is 14 pF and the input impedance is 670 kn, slightly higher than the resonator one; the coupling, then, is slightly less than critical and Q,_ = 1000. The theoretical sensitivity of our resonator is then S = 1 mV/nA and its phase modulation cutoff frequency is ,2, = 2.5 kHz. 4.2 . Electronics
The resonator signal is extracted via a single cable connecting the input of a high impedance-low noise linear preamplifier BPMP [8] to the drift tube ; this device has an amplification of 13 .4 dB and an output impedance of 50 Q . A second stage of amplification is done by an AD640 demodulating logarithmic amplifier; the small signal gain is 40 dB and the rf output signal, always saturated in our case, consists of a 360 mV square wave which carries the phase information of the signal . The AD640 supplies also a do signal proportional to the logarithm of the input signal amplitude. Both outputs are sent to the rf controller . The rf signal, after filtering, is compared with the reference signal in a double balanced mixer; the resulting phase error signal is amplified and used to provide phase stabilization. The do signal is read by an ADC; with this information, by means of eq . (10) with a correction for transit time factor, the computer calculates and displays the value of the beam current. Moreover, the phase delays introduced by the amplification chain at different amplitudes of the input signal are corrected by software using a calibration table . 4.3. Measurements
The sensitivity was tested with a "Ni beam, bunched at 5 MHz, by measuring the beam current with a Faraday cup and comparing it with the resonator signal amplitude. The experimental sensitivity was found to be 1 mV/nA. The beam current monitor was then calibrated and tested in the range from 0.3 to 140 nA ; if necessary, the lower limit could be reduced to 0.1 nA by removing
a 10 dB attenuator after the first stage of amplification. This range fits our requirements ; however, the good quality of the signal even at the lowest level suggest that adding a second AD640 to the amplification chain could allow extending the range down to few picoamperes of beam current. We tested the effect of the system on the longitudinal characteristics of the beam : the time width of the bunched beam, stable at 2.8 ns when the phase stabilization system was active, increased to 3.5 ns during a 1 hour measurement after the phase stabilization was disabled . We measured also the long-term stability of the system by coupling the reference signal to the phase detector through a capacitive coupler; we recorded the phase error signal during 24 hours; we observed shifts of few nanoseconds related to changes in the environmental temperature . In order to eliminate this problem, a gas-flow temperature stabilizer will be installed. 5. Conclusions The system developed at LNL provides simultaneously phase stabilization and non-destructive beam current monitoring of the bunched beam . This double purpose is obtained by using an AD640 demodulating amplifier. The choice of a resonant phase detector with high impedance critical coupling of the low-noise preamplifier allowed a significant improvement of the phase detector sensitivity (two orders of magnitude higher than previously reported for such kind of devices). The loaded quality factor of the LNL phase detector was limited to Q,, = 1000 in order to match our need of a modulation cutoff frequency of 2.5 kHz for fast phase stabilization ; if this is not required, the sensitivity can be increased significantly by using high Q (e .g . superconductive) resonators . Further increase of the beam current monitor range toward low current intensities could be obtained by using two AD640 in cascade . Acknowledgements We acknowledge N. Dainese for the mechanical construction of the phase detector and S. Dong for his collaboration m mounting and testing the system . We are also grateful to A. Zanon and G. Tombola for their contribution in the construction of the electronics and to J.S . Sokolowski for his fruitful suggestions . We thank Marco Poggi and A. Dainelli for their help during the measurements performed with the beam. Our gratitude goes also to J. Simovic of the University of Munich for the layout of the demodulating amplifier, and to KFA Jülich for their courtesy in allowing us to use a BPMP low noise amplifier.
A. Facco et al. /Nucl. Instr. and Meth. i n Phys . Res. A 337 (1994) 253-257
References [1] F.J . Lynch, R.N . Lewis, L.M . Bollinger, W. Henning and O.D . Despe, Nucl . Instr. and Meth. 159 (1979) 245 . [2] S.J. Skorka, Proc . 3rd Int. Conf. on Electrostatic Accelerator Techn., IEEE (1981) 130. [3] R.N . Lewis, Nucl . Instr. and Meth . 151 (1978) 371. [4] T.W . Aitken and C.W . Horrabin, Phase stabilization system for bunched beams, in : Nuclear Physics - Appendix to the Daresbury Annual Report 1988/89, SERC Daresbury Laboratory, Warrington WA4 4AD, UK, p. 143. [5] S. Takeuchi and K.W . Shepard, Nucl. Instr. and Meth. 227 (1984) 217.
257
[6] A. Facco, K. Rudolph, A. Battistella, G. Tombola, F. Scarpa and A. Zanon, Proc. 3rd Europ. Particle Accelerator Conf ., Berlin, 1992, vol. 2, eds. H. Henke, H. Homemeyer and Ch . Petit-Jean-Genaz (Edition Frontières, Gifsur-Yvette, 1992) p. 1507 . [7] R.C . Connolly and D.D . Leach, Nucl . Instr. and Meth . A 273 (1988) 40 . [8] J. Dietrich et al., Proc . 3rd European Particle Accelerator Conf ., Berlin, 1992, vol. 1, eds. H. Henke, H. Homemeyer and Ch . Petit-Jean-Genaz (Edition Frontieres, Gif-surYvette, 1992) p. 1088 .