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Patent No. 4687987 Beam current sensor (Kuchnir, et al., Aug 18, 1987)
ASSINGEE: The United States of America as represented by the United States (Washington, DC)
CONTRACTUAL ORIGIN OF THE INVENTION
The U.S. Government has rights in this invention under Contract No. DE-AC02-CH03000
between the U.S. Department of Energy and Universities Research Association,
Inc.
Abstract
A current sensor for measuring the DC component of a beam of charged particles employs a superconducting pick-up loop probe, with twisted superconducting leads in combination with a Superconducting Quantum Interference Device (SQUID) detector. The pick-up probe is in the form of a single-turn loop, or a cylindrical toroid, through which the beam is directed and within which a first magnetic flux is excluded by the Meisner effect. The SQUID detector acts as a flux-to-voltage converter in providing a current to the pick-up loop so as to establish a second magnetic flux within the electrode which nulls out the first magnetic flux. A feedback voltage within the SQUID detector represents the beam current of the particles which transit the pick-up loop. Meisner effect currents prevent changes in the magnetic field within the toroidal pick-up loop and produce a current signal independent of the beam's cross-section and its position within the toroid, while the combination of superconducting elements provides current measurement sensitivites in the nano-ampere range.
Notes:
BACKGROUND OF THE INVENTION
This invention relates generally to electric current measuring apparatus and
is particularly directed to the measurement of the current in a beam of charged
particles.
Charged particle beams are used in various fields ranging from high energy experimental
physics to medical applications. Beam currents for the various applications
of charged particle beams may vary from tens of amperes to pico-ampere current
levels. In the latter range of current values, the current may be difficult,
if not impossible, to accurately measure.
One approach currently in use for measuring particle beam currents involves
the use of a flux gate magnetometer. This zero flux current transformer includes
a supermalloy pick-up device which is positioned adjacent the particle beam
to be measured and is magnetized by the magnetic field of the beam. An AC signal
is applied to the thus magnetized supermalloy pick-up device and the extent
of magnetization, which corresponds to the particle beam current intensity,
is measured in terms of a second harmonic of the AC input signal. This approach,
however, is of limited use at extremely low currents, e.g., in the range of
nano-amperes. In addition, accurate current measurements require somewhat precise
alignment of the particle beam with the magnetized supermalloy material. However,
precisely locating and positioning the charged particle beam in many cases is
extremely difficult, particularly where very small beam currents are involved.
The present invention is intended to overcome the aforementioned limitations
of the prior art by providing a particle beam current sensor which is generally
insensitive to the exact location and cross-section of a beam of charged particles
in providing highly accurate beam current measurements for direct currents down
to the nano-ampere range.
OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide for the accurate
measurement of DC current in a beam of charged particles.
It is another object of the present invention to provide sensitivities in the
measurement of direct currents in the nano-ampere range.
Yet another object of the present invention is to provide an arrangement for
the measurement of current in a beam of charged particles which does not require
precise knowledge of the location of or alignment with the particle beam.
A further object of the present invention is to provide a low noise system for
measuring the current of a beam of charged particles employing a superconducting
detector.
A still further object of the present invention is to provide a beam position
and cross-section insensitive detector for measuring the current of a beam of
charged particles.
The present invention contemplates a system for measuring the direct current
of a beam of charged particles. The system includes a superconducting pick-up
loop in the form of a single turn toroidal electrode through which the particle
beam is directed. The pick-up loop is coupled by means of a superconducting
twisted pair to a detector including a Superconducting Quantum Interference
Device (SQUID) and its electronics which is rendered conductive in response
to a beam-generated magnetic flux within the axially symmetric pick-up loop.
The current thus provided from the SQUID and its electronics to the pick-up
loop establishes a magnetic field within the toroidal-shaped loop which nulls
out, or cancels, the beam-generated flux therein, with the corresponding voltage
representing the particle beam current. The entire system is maintained at liquid
helium temperatures with measurement sensitivities on the order of nano-amperes
attainable and is particularly adapted for use as a diagnostic tool for storage
of antiproton particle beams.
BRIEF DESCRIPTION OF THE DRAWINGS
The appended claims set forth those novel features which characterize the invention.
However, the invention itself, as well as further objects and advantages thereof,
will best be understood by reference to the following detailed description of
a preferred embodiment taken in conjunction with the accompanying drawings,
where like reference characters identify like elements throughout the various
figures, in which:
FIG. 1 is a simplified illustration in combined block and schematic diagram
form of a particle beam current sensor in accordance with the present invention;
FIG. 2 is a diagrammatic illustration of a current sensing pick-up loop having
an axially symmetric magnetic field in combination with a superconducting detector
for use in the particle beam current sensor of FIG. 1; and
FIG. 3 is a simplified schematic diagram of a beam current sensor arrangement
employing an evacuated cryostat chamber through which the particle beam is directed
in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENT
Referring to FIG. 1, there is shown in simplified block and schematic diagram
form a beam current sensor 10 in accordance with the present invention.
The beam current sensor 10 includes a generally cylindrical, superconducting
pick-up loop, or probe, 14 through which the particle beam 12 is directed generally
along the axis thereof. The superconducting pick-up loop probe 14 is in the
form of a hollow single loop toroid having interior and exterior surfaces and
a slot 15 (shown in dotted line form in FIG. 1) spaced generally equidistant
from the respective ends of the probe. The portions of the pick-up loop probe
14 separated by the median slot 15 are coupled to a superconducting inductor
20 within a SQUID detector 18 via a superconducting twisted pair 48. The self-inductance
of the single loop toroid and the self-inductance of the the twisted pair 48
coupling the pick-up loop probe 14 and the SQUID detector 18 are respectively
represented by inductors 16 and 17 and have values L.sub.P and L.sub.W, respectively.
Current is induced in the surface of the superconducting pick-up loop probe
14 by the Meisner effect in response to the particle beam 12 generated magnetic
flux therein and flows along the surface thereof so as to produce a donut-shaped
magnetic field-free region as shown in FIG. 2. The current thus induced in the
superconducting pick-up loop probe 14 is independent of the position of the
particle beam 12 transiting therethrough as well as the beam cross-section since
the current in the superconducting surface will distribute itself accordingly
to prevent field penetration therein. The current through the twisted pair 48
and input coil 20 opposes the magnetic field created by the particle beam 12
within the superconducting pick-up loop probe 14 in accordance with the Meisner
effect. This induced current prevents the magnetic flux arising from the particle
beam 12 from penetrating the toroidal shaped pick-up loop probe 14 and represents
a measure of the intensity, or direct current, of the particle beam 12. The
SQUID detector 18, which includes a Superconducting Quantum Interference Device
(SQUID) 22, is rendered conductive in generating a magnetic flux within the
pick-up loop probe 14 or the SQUID proper through inductor 46 which nulls out
the effect of the magnetic flux of the particle beam 12 therein, with an output
voltage V.sub.O from the SQUID detector 18 representing the particle beam current.
In the discussion which follows, the terms superconducting loop and SQUID are
used interchangeably in describing element 22.
The DC SQUID 22 used in the present invention is a device which includes two
Josephson junctions 26, 28 in a superconducting loop of inductance L. The first
and second Josephson junction 26, 28 operate as ideal tunnel junctions, each
with a critical current of I.sub.O and a self-capacitance of C. Each Josephson
junction is resistively shunted to eliminate hysteresis on the current-voltage
(I-V) characteristic. The I-V characteristics of the superconducting loop or
SQUID 22 varies with the applied flux .phi. threading the loop from .phi.=n.phi..sub.O
to .phi.=(n+1/2).phi..sub.O, where .phi..sub.O =h/2e (h=Planck's constant and
e=electron charge) is the flux quantum (2.07.times.10.sup.-15 Wb) and n is an
integer. With a constant current I.sub.B, this causes the voltage across the
superconducting loop or SQUID 22 to oscillate as a function of .phi.. The I-V
characteristic of the superconducting loop or SQUID 22 is a periodic function
of .phi., such that if the superconducting loop or SQUID 22 is biased with a
constant current I.sub.B from a bias current source 34 which is greater than
the maximum critical current of the Josephson junctions 26 and 28, the voltage
across the SQUID varies periodically. Thus, for a flux near (2n+1) .phi..sub.O
/4, the superconducting loop or squid 22 functions as a flux-to-voltage transducer
with a transfer function of V.sub..phi. =(.differential.V/.differential..phi.).
The equivalent flux sensitivity of the superconducting loop or SQUID 22 is determined
by dividing the RMS voltage noise across the device by V.sub..phi. to obtain
the equivalent RMS flux noise.
In the present invention the SQUID detector 18 is used as null detector and
operates as a flux-locked loop as described below. An AC flux (typically at
100 kHz) with a peak-to-peak amplitude of .phi..sub.O /2 is provided to inductor
46 within the SQUID detector 18 with the resultant 100 kHz voltage divided down
by the combination of inductor 30 and capacitor 32 and provided to an amplifier
36. If the average flux within the superconducting loop or SQUID 22 is n.phi..sub.O,
the voltage across the SQUID detector 18 is at 200 kHz. If the flux is increased
or decreased from this value, a 100 kHz component appears in the voltage across
the superconducting loop or SQUID 22, with a phase dependent upon the sign of
the flux change. The 100 kHz signal is amplified by amplifier 36 and provided
to a lock-in amplifier 38 at the modulation frequency. An oscillator 42 provides
this 100 kHz signal to the modulation/feedback coil 46 by means of AC-coupling
capacitor 43 and to the lock-in amplifier 38 in order to permit the SQUID detector
18 to lock onto the minimum output of the amplifier 36. Thus, the output of
the lock-in amplifier 38 is 0 at n.phi..sub.O, and may be, for example, positive
for .phi.=(n+.delta.).phi..sub.O and negative for .phi.=(n-.delta.).phi..sub.O,
where .delta. represents the change in flux and .delta.<<1. After further
amplification by means of amplifier 40, the voltage is coupled across a resistor
44 having a resistance R.sub.f in series with the modulation/feedback coil 46
in the SQUID detector 18 having an inductance L.sub.f. The feedback current
provided to the modulation/feedback coil 46 provides an opposing flux within
the superconducting loop 22 which tends to cancel .delta..phi..sub.O, with the
output voltage V.sub.O being proportional to .delta..phi..sub.O.
The SQUID detector used in a preferred embodiment of the present invention is
the DBS model SQUID available from the S.H.E. Corporation and exhibits a superconducting
input impedance of 2 microhenries which for a current of 200 nano-amperes through
its input generates a full scale output voltage of 10 V with a 200 ohm output
impedance. In its normal mode, the SQUID detector 18 responds from DC to 5 kHz
and in its fast mode from DC to 50 kHz. The different modes corresponding to
different time constants in the SQUID'S amplifiers provide a trade-off between
speed and stability. Because its output is a feedback to a very sensitive quantized
phenomena, i.e., the change in flux within the superconducting loop 22, its
linearity is determined by Ohm's Law in the feedback resistor 44. The linearity
can be further improved and the dynamic range greatly extended by automatically
resetting the lock and counting the number of resets with an up-down counter/voltmeter
52. In this manner, the feedback current may be kept small and the SQUID detector
18 operates in its most sensitive scale. The TTL compatible auto-reset and reset
sign outputs for the up-down counter/voltmeter 52 are included in the electronic
control unit (not shown) in the aforementioned commercially available SQUID
detector.
The RMS current noise of the SQUID detector 18 is 1.5 pA/.sqroot.Hz for frequencies
greater than 1 Hz, or 0.5/.sqroot.fpA for frequencies below 0.01 Hz. This means
that on observing slow particle accumulation rates with a reduced bandwidth
of 1 Hz, changes in current as low as 1.5 pA can be detected. In the case of
antiprotons, for a storage ring with a revolution period of 1.6 microseconds,
each antiproton contributes 0.1 pA and this detectable change corresponds to
15.times.155 antiprotons at any current level. The factor 155 arises from input
attenuation required for keeping lock under sudden 8 .mu.A beam steps.
Although the SQUID detector 18 itself is very fast, its feedback loop which
includes resistor 44 is not. In order to maintain the SQUID detector 18 locked
to the flux within the superconducting loop 22, sudden flux changes should be
kept less than .phi..sub.O /2. Thus, the impedance of the pick-up electrode
14 should be adjusted so that an 8 .mu.A signals result in a flux change of
less than .phi..sub.O /2. For sudden partial depletion of the accumulator beam,
a low pass filter inductor or eddy current shield, not shown, is needed at the
SQUID input since the automatic reset requires 35 microseconds. The commercially
available SQUID detector 18 offers four sensitivity ranges, where the least
sensitive range is capable of measuring current changes as small as 0.2 milliamps
corresponding to a beam of 0.2.times.155=31 milliamps. Whenn the SQUID detector
18 is cooled down, it traps the earth's magnetic field (10.sup.-4 Tesla) which
for a typical area of the superconducting loop 22 of 10 mm.sup.2 corresponds
to a trapped flux of 1.0 nWb or 5.0.times.10.sup.+5 .phi..sub.O. A doubling
of the number of fluxons .phi..sub.O corresponds to an input current of (0.1
microamps/.phi..sub.O).times.5.0.times.10.sup.+5 .phi..sub.O =50 mA, or a beam
current of 7.75 A.
The pick-up loop probe 14 in a preferred embodiment is in the form of a superconducting
flux transformer consisting of a single loop of cylindrical geometry around
the particle beam 12. The axial symmetry of the single loop, toroidal pick-up
loop probe 14 results in the particle beam-induced current therein being independent
of the particle beam cross-section or the particle beam position relative to
the single loop, toroidal pick-up loop probe 14. The flux, .phi., due to the
beam current I which is prevented from entering the area defined by and within
the single turn, toroidal pick-up loop probe 14 is .phi.=M.sub.p .times.I, where
M.sub.p .apprxeq.L.sub.p, which are respectively the beam-single loop pick-up
loop probe 14 mutual inductance and the single loop pick-up loop probe 14 self-inductance.
The current, i, thus provided to the inductor 20 at the input of the SQUID detector
18 is given by:
where L.sub.w is the self-inductance of the twisted leads coupling the SQUID
detector 18 to the pick-up loop probe 14 and L.sub.s is the self-inductance
of the SQUID detector input coil 20 (2 .mu.H). The flux actually detected by
the SQUID detector 18 and compensated for by means of the feedback signal provided
via resistor 44 to the modulation/feedback coil 46 is given by the expression:
where M is the mutual inductance between L.sub.s and the superconducting SQUID
loop 22 (20 .eta.H). Thus, the response of the beam current sensor 10 is proportional
to:
The magnetic field at a distance r from the beam is given by the expression:
and the energy excluded by the self-inductance of the single loop, toroidal
pick-up electrode 14 L.sub.p is given by:
Substituting and integrating Equation (5) from the inner radius a to the outer
radius b for a length c, we arrive at the following expression: ##EQU1## For
.mu.=1,
which expression for a=3 cm, b=6 cm, and c=10 cm yields L=0.014 .mu.H. The inductance
for a pair of wires of length s cm, diameter t cm, with the centers of the wire
separated by d cm is given by the expression:
which for s=10 cm, d=2t and t=0.02 cm yields a self-inductance of the twisted
leads L.sub.w =0.017 .mu.H. Using this value for L.sub.w and assuming M.sub.p
.apprxeq.L.sub.p and without a matching transformer, the current through the
input coil 20 of the SQUID detector 18 is given by the following expression
where the single loop, toroidal pick-up electrode 14 has the following dimensions--6
cm inner diameter, 12 cm outer diameter, and 10 cm long:
The condition for maintaining the SQUID detector 18 locked onto the magnetic
flux within the superconducting loop 22 under a sudden excursion of 8 .mu.A
is .phi..sub.s .ltoreq..phi..sub.O /2. Neglecting the effects of the mutual
inductance relative to the superconducting shield 80 as has been done thus far
in the calculations results in a self-inductance of the twisted leads 48 coupling
the SQUID detector 18 and the pick-up loop probe 14 given by the following expression:
where L.sub.w =14.0.times.10.sup.-9 .times.20.0.times.10.sup.-9 .times.8.0.times.10.sup.-6
/(0.5.times.2.07.times.10.sup.-15)-14.0.times.10.sup.-9 -2.0.times.10.sup.-6
H, or
and the ratio between the beam current I and the input current i is given by:
Referring to FIG. 3, there is shown a simplified schematic diagram of a beam
current sensor 10 in accordance with the present invention. The particle beam
12 is directed through the single loop, toroidal pick-up loop probe 14 which
is positioned within a closed container, or dewar, 66 filled with liquid helium
64 so as to maintain the superconducting pick-up loop probe 14 at a temperature
below its critical temperature. The liquid helium container 66 is positioned
within a vacuum chamber 60, with a super-insulation blanket 68 positioned between
the vacuum chamber 60 and the liquid helium dewar 66 for low temperature operation
of the pick-up loop probe 14. A SQUID probe connector and conduit 82 provides
access to the SQUID detector 18 and a means for obtaining SQUID measurement
readings from within the vacuum chamber 60. A helium transfer line 62 permits
liquid helium to be provided to the chamber 66, while the combination of a vacuum
duct 84 and valve 86 couples a vacuum pump 94 to the vacuum chamber 60 for effecting
the evacuation thereof.
The lower portions of the vacuum chamber 60 and the liquid helium dewar 66 are
removable to facilitate access to the SQUID detector 18 and the pick-up loop
probe 14. The detachable lower portions of the vacuum chamber 60 and the liquid
helium dewar 66 are respectively provided with a viton O-ring 76 and an indium
O-ring 74 to provide a vacuum seal therefor when in the closed position. A conventional
molecular sieve 78 is positioned adjacent a bottom portion of the vacuum chamber
60 in order to remove stray molecules therefrom. The superconducting twisted
pair of wires 48 coupling the superconducting pick-up loop probe 14 with the
SQUID detector 18 is enclosed within a lead shield 50 in order to eliminate
noise therefrom. A plurality of thermal shields 70 are positioned at respective
ends of the vacuum chamber 60 along the particle beam trajectory which allow
for the transit of the particle beam 12 therethrough, while maintaining the
interior of the vacuum chamber 60 at low temperature.
The superconducting pick-up loop probe 14 is shielded from extraneous magnetic
fields by means of a superconducting shield 80 in the form of a pipe aligned
along the axis of the particle beam 12 and through which the particle beam passes
along the length thereof. In the superconducting cylindrical shield 80 having
a radius a, the magnetic field H at the distance z>>a from the end of
the cylindrical shield falls off as follows: ##EQU2## Therefore, the magnetic
field at a distance z from the end of the superconducting shield 80 is given
by the following expression: ##EQU3## such that for a=0.03 m, 1.0.times.10.sup.-15
.apprxeq.exp (-61.33 z), or z.apprxeq.0.56 m. Thus, the length of the superconducting
shield 80 should be approximately 2 z.apprxeq.1.13 m.
The superconducting pick-up loop probe 14 will operate satisfactorily so long
as the level of radioactivity within the vacuum chamber 60 does not heat it
up above its critical temperature. In order to accommodate radioactivity in
the vacuum chamber 60, the superconducting pick-up loop probe 14 is preferably
comprised of a Type II superconductor, the critical current of which increases
with increasing irradiation. By protecting the SQUID detector 18 with suitable
lead shielding from the radiation of the particle beam, the SQUID detector 18
may be positioned within a few centimeters of the beam and will operate properly.
The materials incorporated in a typical SQUID probe (Nb,NbTi, BeCu, brass, Si,
SiO.sub.2, G-10, solder and some epoxy) are not particularly sensitive to irradiation
and thus would function properly within the beam current sensor 10 of the present
invention.
While the modulation/feedback coil 46 has thus far been described as incorporated
within the SQUID detector 18 and electromagnetically coupled to the superconducting
loop 22 for providing an opposing flux tending to cancel the flux therein arising
from the beam-generated signal from the pick-up loop probe 14, the modulation/feedback
coil may be electromagnetically coupled directly to the pick-up loop probe 14.
This arrangement is shown in FIGS. 2 and 3 where the modulation/feedback coil
is identified as element 79 and is coupled to the feedback resistor 44 within
the SQUID detector 18. In this arrangement, a feedback current within the modulation/feedback
coil 79 gives rise to a magnetic flux within the pick-up loop probe 14 tending
to cancel the beam-generated magnetic flux therein.
There has thus been shown a current sensor for measuring the DC component of
a beam of charged particles employing a superconducting single loop, toroidal
pick-up probe in combination with a SQUID detector which is capable of detecting
currents measured in nano-amperes. The low noise arrangement of the present
invention provides measurement sensitivities heretofore unavailable while the
natural distribution of the current on the superconducting surface of the toroidal
pick-up loop probe is capable of accurately sensing particle beam current without
precisely determining the location of the particle beam or compensating for
its cross-section.
While particular embodiments of the
present invention have been shown and described, it will be obvious to those
skilled in the art that changes and modifications may be made without departing
from the invention in its broader aspects. Therefore, the aim in the appended
claims is to cover all such changes and modifications as fall within the true
spirit and scope of the invention. The matter set forth in the foregoing description
and accompanying drawings is offered by way of illustration only and not as
a limitation. The actual scope of the invention is intended to be defined in
the following claims when viewed in their proper perspective based on the prior
art.