Method and means for measurement and control of pulsed charged beams

A beam of bunches of charged particles is controlled by generating a signal in response to the passage of a bunch and adding to that signal a phase-flipped reference signal. The sum is amplified, detected, and applied to a synchronous detector to obtain a comparison of the phase of the reference signal with the phase of the signal responsive to the bunch. The comparison provides an error signal to control bunching.

BACKGROUND OF THE INVENTION 
This invention relates to pulsed beams of charged particles. In particular, 
it is directed to measurement and control of the phase of bunches of 
charged particles in a beam. 
It is frequently important to have an accurate measurement of the phase of 
bunches of charged particles in a repetitive pulsed beam. It is also 
frequently of importance to control the bunching of such particles in 
response to the error from a fixed reference in order to maintain control 
of the phase. This is sometimes referred to as reducing pulse-separation 
jitter. It may also be desirable to control the signal input to a buncher 
to reduce the time spread in a bunch of charged partilces. In research 
accelerators precision control of bunching of a beam that is injected into 
an RF accelerator makes it possible to exercise more precise control on 
the output energy. Furthermore, the control of the phase is facilitated by 
the generation of an output signal marking the centroid of a pulse of 
charges and this information is useful as a time marker for time-of-flight 
experiments and the like. These considerations are not only of importance 
to the operation of a particle accelerator for research purposes but also 
have extensions to the use of precision particle beams in controlling 
diagnostic and therapeutic x-rays and timing pulses for injection in 
attempts to achieve thermonuclear fusion by beams of ions. 
A pulsed beam is commonly achieved by generating a dc beam and passing that 
beam through a buncher. The buncher accelerates some charged particles and 
decelerates others so that after a period of drift at varying speeds a 
portion of the beam is compacted into a bunch. This is similar to the 
bunching performed in a klystron, for example. However, as with a 
klystron, variations in the accelerating voltage, the initial entering 
velocity of the charged particles or voltages from any other power 
supplies in the system can cause spread in the width of the pulse of 
charged particles that has been bunched. If the bunch is to be subjected 
to further acceleration, then the accelerated bunch will probably be 
spread still further in energy. 
It is an object of the present invention to provide a method and means of 
measuring and controlling the phase of a bunch of charged particles in a 
particle accelerator. 
Other objects will become apparent in the course of a detailed description 
of the invention. 
SUMMARY OF THE INVENTION 
The phase of a bunched beam of charged particles in an accelerator is 
measured and controlled by passing the beam through a high-Q circuit to 
generate an electrical signal timed to the pulse. This signal is added to 
a reference ac signal that is reversed in phase at an audio rate. The sum 
of the reference signal and the signal induced by the beam is amplified 
and demodulated in a radio receiver and the demodulated signal is applied 
to a synchronous detector to provide a comparison of the phase of the 
separately added signals. Application of the output signal from the 
synchronous detector to a voltage-controlled phase shifter generates a 
signal for application to the buncher to optimize bunching of the charged 
particles.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a block diagram showing the apparatus of the present invention in 
operation on an accelerator. In FIG. 1, source 10 generates a beam 12 of 
ions that are to be accelerated. The beam 12 is bent in a bending magnet 
14 and directed through the grids 15 of a buncher 16. The buncher 16 
receives an alternating voltage that may be a sinusoid or a sawtooth at a 
high frequency. This voltage is applied to the beam 12 so as alternately 
to accelerate and decelerate the beam 12. The beam then proceedes to an 
accelerator 18 that is typically a Tandem Van de Graaff. From the 
accelerator 18 beam 12 is bent by another bending magnet 20 and directed 
through a helix 22. As a result of the time taken for the beam 12 to go 
from buncher 16 to helix 22 and the alternating components of acceleration 
and deceleration that have been applied to the beam at buncher 16, the 
beam 12 arrives at helix 22 in bunches. The purpose of the present 
invention is to detect the phase of those bunches nondestructively through 
the use of helix 22 and to generate and apply control signals to control 
the phase of the bunches. This is desirable because the beam 12 goes from 
helix 22 for further uses in accelerators or in units for radiography in 
which precision bunching and precision control of the phase may be 
important in improving the operation of the later equipment. For example, 
FIG. 1 shows as a possible application the delivery of the beam 12 to a 
superconducting buncher 24 in which the beam is further bunched for 
delivery to a linac 26. Operation of the buncher 24 and the linac 26 is 
facilitated by having narrow bunches arrive for acceleration in closely 
controlled phase. 
Control of the operation is timed from a master oscillator 28 which is 
connected to the superconducting buncher 24 and through a variable phase 
controller 30 to linac 26. The signal from the master oscillator is also 
taken to control the timing of buncher 16 through a voltage-controlled 
phase controller (VCP) 32. Voltage for the VCP 32 is derived by connecting 
the signal from the master oscillator through a variable phase controller 
34 to a phase flipper 36. The input to the phase flipper 36 comprises a 
reference voltage that is reversed in phase by phase flipper 36 under the 
control of a sampling signal from sampling signal generator 38. The 
reference voltage from phase flipper 36 in each of its phases is coupled 
capacitively to helix 22 where it is added to the voltage developed across 
helix 22 by the passage through helix 22 of a bunch of ions in beam 12. 
The signal comprising the sum of these two voltages is coupled 
capacitively to preamplifier 40 and thence to radio receiver 42 which is 
used as a high-gain tunable amplifier and detector. The amplitude-detected 
output from radio receiver 42 is passed through a gate that is controlled 
by a signal obtained by applying the output of sampling signal generator 
38 to a monostable element 44. The gate-off signal occurs twice per 
sampling cycle, i.e., at each phase flip of the reference signal. This is 
done to gate off undesirable signals caused during phase flip of the 
reference signal. The output of the gate is applied to a sync detector 46 
that is gated by the signal from sampling signal generator 38 and the 
output of sync detector 46 is applied to gain-bandwidth unit 48 to derive 
a voltage that is applied to control VCP 32 and hence the phase of the 
signal at buncher 16. 
Further details of the operation can be seen in FIG. 2 in which the 
corresponding elements of FIG. 1 are given the same numbers. In FIG. 2 
sampling signal generator 38 is a combination in which crystal-controlled 
oscillator 50 is connected to a transistor amplifier 52, thence to a pair 
of cascaded counters 54 and 56, which divide its input to produce a square 
wave at a frequency of 2 kilohertz. The output of counter 56 is coupled to 
monostable element 44 which is a flip-flop connected as a monostable 
multivibrator. The output of counter 56 is also connected through 
flip-flop 58 which produces a 1 kilohertz square wave to phase flipper 36 
and to sync detector 46. Phase flipper 36 is a double balanced mixer that 
also receives as an input a signal from master oscillator 28 that is 
shifted controllably in phase by variable phase controller 34. The output 
of phase flipper 36 is a sinusoidal voltage at the frequency of master 
oscillator 28 that is reversed in phase by 180.degree. at a rate of 1 
kilohertz. That signal is applied to helix 22 as the reference signal, to 
be added to the signal generated in helix 22 by the passage therethrough 
of a pulse of ions. Helix 22 is selected and tuned to be a half-wave 
resonant transmission line at the frequency of master oscillator 28. Such 
a line typically has a Q of at least 1000. Oscillations in helix 22 are 
excited by two signals. First, the passage through helix 22 of a bunch 
excites an oscillation at the resonant frequency that is timed to the 
passage of the bunch. Second, capacitive coupling to helix 22 excites 
helix 22 with the reference signal. The sum of these two signals is 
coupled capacitively to preamplifier 40 and is amplified there and in 
receiver 42. After passage through a peak limiter 60, the signal is 
applied to sync detector 46 through a switch 62 that is operated by the 
output of monostable element 44. Detector 46 produces an output that is 
proportional to the modulation of the combined signal from receiver 42, 
and the detected output is applied to gain-bandwidth unit 48. This is a 
pair of stages of controlled variable amplification and integration, the 
output of which is in turn applied to VCP 32. Individual integrated 
circuits and selected circuit elements are identified by number in FIG. 4 
and their function is identified by association with that number in Table 
I. 
TABLE I 
______________________________________ 
Element Number Function 
______________________________________ 
7400 Quad Position NAND gate 
2N2219 NPN Transistor 
4017 Counter Divider 
4013 Dual D Flip-Flop 
DBM Double Balanced Mixer 
1N4448 Diode 
1N446 Diode 
1N702A Zener Diode 
4016 Quad Bilateral Switch 
OP-05 Precision Operational 
Amplifier 
741 Operational Amplifier 
VCP Merrimac PSE-3 Series 
Voltage-Controlled 
Phase Shifter 
______________________________________ 
The various conditions for comparison of a received signal with a reference 
signal are shown in FIG. 3 both as phasor diagrams and as time plots. FIG. 
3A shows the condition when the phase error is zero. This is evidenced by 
the fact that phasor 70 representing the voltage induced by the passage of 
a pulse in the beam is 90.degree. behind phasor 72 and 90.degree. ahead of 
phasor 74. Phasors 72 and 74 are equal in magnitude and represent the 
reference voltage and its reversal in phase by 180.degree.. Phasor 76 
represents the sum of phasors 70 and 72 while phasor 78 represents the sum 
of phasors 70 and 74. Since phasors 72 and 74 represent the sinusoids that 
are applied alternately as reference signals to the helix to add to the 
signal represented by phasor 70, the result of the addition is a time 
signal that is switched at the same rate between the signals associated 
with phasors 76 and 78. These have the same amplitude so that the 
resultant output is the sinusoid 80 having a constant amplitude. Switching 
transients are ignored in this drawing because of the uncertainty as to 
when in time switching will take place. This is rendered academic in any 
event because the switching transients will be gated off. Phase flip of 
the reference signal represents phase modulation and gives rise to an rf 
spectrum that can easily exceed the bandwidth of the rf amplifiers. This 
causes spurious responses in the output of the radio receiver. A gating 
signal is thus required to gate off the false signal that occurs twice per 
sampling cycle. The action is accomplished by monostable element 44. 
FIG. 3B is a plot of the phasors and the associated time diagram when the 
beam is early in phase or advanced in time. Phasors 72 and 74 maintain the 
same relationship with respect to each other that they had before. Phasor 
70 is unchanged in amplitude but is advanced in phase so that the angle 
between phasor 70 and phasor 72 is less than 90.degree.. As a result 
phasor 76 representing the sum of phasors 70 and 72 is greater in 
amplitude than phasor 78 which represents the sum of phasors 70 and 74. 
The result is that switching from phasor 76 to 78 generates across helix 
22 the amplitude-modulated signal that is plotted as a function of time in 
FIG. 3B. The resulting curve 82 can be seen by reference to curve 84 to be 
larger during the time that phasor 76 is switched on and smaller in 
amplitude during the time phasor 78 is switched on. Those times are 
indicated respectively by the intervals that are noted as "76" and "78". 
The reverse of the situation just described is shown in FIG. 3C in which 
phasor 70 represents the result obtained when the beam phase is late. In 
FIG. 3C phasor 76 is smaller in amplitude than phasor 78 and curve 86 
illustrates the result in time of switching between phasors 76 and 78. 
FIGS. 4, 5, 6 and 7 are representations of plots from multichannel counters 
that represent counts of particle pulses as a function of time. Thus these 
pulses represent the spread of the phase of bunches with respect to a time 
measure that is taken most appropriately as the center of each of the 
bunches. FIGS. 4-7 were obtained on the bunched output of a Tandem Van de 
Graaff accelerator at the Argonne National Laboratory. This accelerator 
was used as an input device for the superconducting helix that is a 
further buncher for application to a linear accelerator. FIG. 4 represents 
the time spread observed in 15 minutes of normal operation of the 
accelerator on a beam of carbon ions without the phase controller of the 
present invention in operation. The detected bunches have a full-width, 
half-maximum (FWHM) spread of 1.2 nanoseconds. This is the result of 
uncompensated variations of power-supply voltages and the uncorrected 
minor drifts that occur for other reasons. FIG. 5 is a comparable plot 
obtained over a period of 90 minutes with the circuit of the present 
invention applied to control phase of the pulsed beam of carbon ions. It 
can be seen from FIG. 5 that even over the longer measuring interval, 
during which more variation might be expected to occur, the use of this 
circuit has reduced the FWHM value of the detected bunches to 0.885 
nanoseconds. This is a significant improvement in phase control of the 
pulses that will lead to more sharply defined injection time into any 
equipment that follows the buncher, first accelerator and detector. 
Similarly, FIG. 6 shows the time spread observed in 15 minutes of 
acceleration of a pulsed beam of nickel ions, which are more massive and 
hence more subject to transit time variations than the lighter carbon ions 
of FIGS. 4 and 5 due to their lower velocity. FIG. 7 shows the improvement 
obtained by use of the apparatus of the present invention.