MAGNETIC FIELDS MEASUREMENTS UNDER EXTREME CONDITIONSIN CHARGED PARTICLE ACCELERATORS
Bolshakova I., Holyaka R. Magnetic Sensor Laboratory, Lviv Polytechnic National University,
1 Kotliarevsky Str., Lviv 79013, Ukraine

Abstract
Development results of magnetic monitoring system tobe operated under radiation conditions are presented. The
system meets the intelligent measuring device criteria byits functional parameters. Among them – self-diagnostics,
ability of choosing measurement algorithm and its adjustment to an experiment conditions. The selfdiagnostics is based on forming test magnetic field of theknown value around Hall generator (HG) and on measuring corresponding change of the Hall voltage.

1 INTRODUCTION
Magnetic field monitoring under radiation conditions
expects radiation resistant magnetometric devices to bedeveloped. Most often Hall generators (HG) are used as
the primary transducers of such devices. In order to reachgood HG radiation resistance special manufacturing technologies are developed [1]. Thus, optimizing doping
technology of discrete microcrystalline HG, one may
reach Hall voltage instability at the level of 0.15÷0.5 %when exposing to high-energy neutron fluence of
Φ=1014÷1015 n⋅cm-2 [2]. However, there is a restriction
concerning further improvement of measurement accuracy of magnetometric devices based on HG.
Results of magnetic monitoring system development
are presented in the paper. Self-diagnostics function andtransducing function correction under long-term radiation conditions of an experiment feature the system. The
developed system meets the intelligent measuring device
criteria by its functional parameters. Among them – selfdiagnostics, ability of choosing measurement algorithm
and its adjustment to an experiment conditions. It has aparticular importance for magnetic field monitoring incharged particle accelerators during long-term experiments, taking into account possible unpredictablechanges of HG parameters under radiation conditions.

2 BASIC SOLUTIONS
The magnetic field monitoring system developed isadapted to the number of specific requirements and to theoperating conditions, general for measuring devices of
the charged particle accelerators. Among them – radiation resistance, high measurement accuracy, noise immunity, long-term operation without periodical calibration et cetera. The requirements are met by means of appropriate intelligent measurement algorithms.

The main feature of the system developed is its selfdiagnostics based on forming test magnetic field of theknown value around the HG. Transducing function of themeasuring circuit is defined by means of measuring theHall voltage component caused by the test magnetic field.
The test magnetic field is formed by a miniature solenoid
which field covers the HG. High radiation stability ofsuch magnetometric devices may be explained by the factthat test field of the microsolenoid is not affected by irradiation. When operating under hard radiation conditions
change of the microsolenoid resistance made from copper
wire, is much lower in comparison with the semiconductor HG parameter drift. Besides, when supplying stablecurrent to the microsolenoid, its resistance change does
not lead to the test field magnitude change.
Another feature of the system is its improved measurement accuracy. Thus, at the multiple experimentswith the same magnetic field induction, which is generalfor charged particle accelerators, we assume carrying out
measurements of absolute magnetic induction value aswell as its non-reproducibility over the whole run to berational. Improvement of measurement accuracy in such
experiments is provided by means of analysis of the alldestabilizing factors, in particular, temperature instability, electromagnetic noise level, time drift of the signal.
Analysis of spread and dispersion of the measured quantities is of the particular importance.

3 CORRECTION PRINCIPLEIntelligent features of the magnetic measuring system
is based on the novel algorithm of the transducing function correction. The algorithm allows to avoid the problems of forming high test magnetic fields and is efficientfor measuring fields of any magnitudes. The algorithm
comprises three basic solutions:• simultaneous analysis of the transducing parameterby integral and differential components of the signal;• frequency separation of integral and differential
components of the signal;• special calculation method of values of transducing
function and measuring magnetic field.
Frequency separation of differential and integral
components of the signal which is the base of the given
algorithm, is caused by necessity of high measurement
accuracy of the signal change (differential component) at
high value of the signal formed by measuring field (inte-gral component). Thus when measuring change of test
field in actuating coil with the induction of ∆B = 1 mT atrelative measurement error of δB = 10-3 device resolution
should be not worse than ∆Bmin = 1 µT. Reaching such
resolution and appropriate stability at magnetic fieldmeasurements up to 10 T presupposes providing dynamic
range of the signal in measuring circuit not less than
140 dB. It is obvious that such parameters are practicallyunachievable. Solution of the problem means the independent processing of differential and integral components of the signal. The simplest method of such separation is the processing of integral component in the direct
current circuit, and test one – in frequency-selective circuit, set up at alternate current frequency in actuatingcoil. In each of the tracks dynamic range of 70÷80 dB isprovided, which permits to obtain ratio of maximum
measuring field value Bmax to the resolution by test field∆Bmin near 140÷160 dB.
Transducing function of the measuring circuit may be
expressed in a polynomial way

4 FUNCTIONAL PARAMETERS
OF THE SYSTEM
During complex analysis of the problem of developing radiation resistant magnetometric devices we havefound possible to unify the problem solutions for suchdevices on the base of specialized module – functionallyintegrated magnetometric transducer (FIMMT). FIMMT
combines two elements of the transduction – semiconductor HG and microsolenoid, which test field covers theHG (Fig.2).

nU Ho = Σ a j ⋅ B j ,j= 0

and its correction algorithm is based on solving the system of equations

 U He (BX ) = G ⋅ U Ho (BX ) dU He ( BX ) = G ⋅ dU Ho ( BX ) ,dBdB
where UHo, UHe – nominal and measured value of the Hall
voltage respectively, BX – measuring field induction
magnitude, aj – polynomial series coefficients, G – transducing function drift coefficient, which is determined
during the test measurements. Hall voltage derivatives by
the magnetic field induction dUH/dB are determined bymeans of measuring the differential test field of the actuating coil (Fig.1).

Fig.1. Correction algorithm

Fig.2. FIMMT structure
A considerable number of the system functions is realized by means of the FIMMT. Main functions are described below.
Direct measurement of magnetic field induction isperformed in the conventional way for high-precision
magnetometers, which expects alternative current to besupplied to the HG, and further synchronous signal detection. This provides high noise immunity, which isimportant, when HG in the magnetic measuring zone
with hard radiation is distanced from the signal processing instrumentation.
Dynamic measurement mode of the magnetic field
induction is based on direct supply current to the HG
without synchronous signal detection. Unlike the previous function, this mode provides high measurement performance and observation of transition process shapes.
Measurement accuracy of this mode is limited by its low
noise immunity. The dynamic mode shows the particular
efficiency at the periodical change of magnetic field,
which is specific for kicker electromagnets accelerators.
Measurement of weak magnetic field induction iscarried out by means of compensation method. During
the measurement process the field is formed in the microsolenoid which compensates the measuring field.
Measurement parameter is the current running throughthe microsolenoid, which provides zero value of the Hallvoltage. Such measurement mode does not have the disadvantages connected with the time and radiation instability of the HG. However, upper measurement limit is
the field induction of B = 1 mT.

Thermocompensation of transducing parameters is
performed by means of FIMMT temperature measurement and is realized by means of microsolenoid serving
as thermoresistor (temperature coefficient of the copper
wire resistance of the microsolenoid TKRCU ≈ 4⋅10-6 ).
Taking into account a large distance between FIMMTand signal processing circuit, measurements are carriedout by means of synchronous detection.
Measurements of electromagnetic fields forming
contaminating signals, are performed by means of microsolenoid as an antenna.
Magnetic flux change through the microsolenoid ismeasured by means of integrating the e.m.f. induced on
microsolenoid by the certain time interval.
Transducing parameter correction is performed bemeans of supplying alternative current to the microsolenoid, and direct current to the HG. During test field formation, operating current through the microsolenoid isequal to 30 mA. This results in its overheating by∆T ≈ 20 °C comparatively to the environment temperature. Measurements are carried out in several stages.
FIMMT temperature is measured (by means of measurement of the copper microsolenoid resistance change) on
the first stage after finishing transition heat mode. Second stage expects differential component of Hall voltage
to be measured, which is caused by the test magnetic
field of the microsolenoid. Integral component of theHall voltage is measured during the third stage, which
follows to determination of first approximation of thefield induction to be measured. Such measurements allowcalculating the HG transduction curvature, and, whencompared to the calibration value, appropriate correcting
the measurement results.
The monitoring system consists of the basic unit, setof probes on the base of the FIMMT, voltmeter and personal computer with the IEEE-488 interface bus. The
basic unit provides preliminary signal amplification,noise suppression, signal commutation, forming test signals etc.
The system includes three identical measurement
channels. It allows to perform three-dimensional magnetic field monitoring. Moreover, one may considerably
improve reliability of the measurement results under thelong-term operation conditions in charged particle accelerators, by means of directing these three channels by
one direction. It is one of the determinant functions of
the intelligent measuring devices.

Depending on the operation conditions and magnetic
field measurement range the actuating coil may be of thedifferent shapes, in particular cylindrical or toroidal ones
etc. View of one FIMMT modification is presented at theFig.2. HG location in the coil agrees with the maximum
actuating field criterion and its stability on the measurement plane. During the development of FIMMT construction the optimal ratio was determined between gap
width for HG, internal coil diameter D, wire diameter d,number of turns n in the each of m layers and operating
current Ic. In particular, actuating field with the magnitude of ∆B=5 mT is set up in an actuating coil with theparameters of D=1 mm, d=0.05 mm, N=n×m=500 at
Ic=30 mA. Depending on a temperature mode theFIMMT construction provides heat sink or heat insulation.
Corresponding software supports measurement algorithm, mathematical, logical and statistical data processing.

CONCLUSIONS ANDACKNOWLEDGEMENTS
Results are presented concerning the development of
the magnetic monitoring system to be operated underradiation conditions (fast neutron fluenc Φ = 1014 n⋅cm-2,neutron energy E>100 keV) with the measurement error
less than 0.1% in the fields of B=±5 T. The developed
system meets the intelligent measuring device criteria by
its functional parameters. Among them – self-diagnostics,
ability of choosing measurement algorithm and its adjustment to an experiment conditions.
The work is performed within the Project #1438 ofScience and Technology Center in Ukraine.

REFERENCES[1] I. Bolshakova, "Magnetic microsensors: technology,properties, applications", Sensors & Actuators: A.
Physical, Vol. 68(1998)/1-3, P.282-285.
[2] I. Bolshakova, "Radiation resistant microsensors ofmagnetic field", Proceedings of EPAC'98, Stockholm, Sweden, June 22-26, 1998, pp.1923-1924.