DEVICE FOR MEASURING THE MEDIAN SURFACE OF THE MAGNETIC FIELD IN AN ACCELERATOR

V. I. KUZ’MIN, Yu. K. PETROV

Principle of Operation of the Device

At present, methods for measuring the median surface of the magnetic field in accelerators are developing in three directions: electrodynamic methods [1, 2], methods with measuring coils [3, 4], and methods with permalloy sensors [5, 6, 7, 8]. Electrodynamic instruments, most often used in cyclotron- and phasotron-type accelerators, are not complicated in design; however, they give low accuracy and have bulky sensors. Methods using induction coils are applied in alternating fields and are suitable for synchrotron-type accelerators. These methods are usually based on measuring the radial component of the magnetic field. Their shortcomings include, first of all, the difficulty of eliminating errors caused by the vertical component of the magnetic field.

Recently, methods using permalloy sensors have become increasingly widespread. Permalloy sensors are small in size, highly sensitive, and make it possible to determine the detailed configuration of the median surface.

The device described below uses a permalloy sensor and is based on finding, in the gap, points at which the radial component of the magnetic field is equal to zero. The permalloy sensor is fastened to the rod of a displacement mechanism. The sensor displacement mechanism makes it possible to set the sensor in a horizontal plane and to change the height of the sensor position in the air gap of the electromagnet. The mechanism provides for the possibility of radial displacement of the sensor, which makes it possible to record the dependence \(z_m(r)\). To record the dependence \(z_m(\theta)\), the mechanism is moved together with the stand.

During measurements, the sensor is installed in the air gap in such a way that the axis of the sensor core is approximately parallel to the radius of curvature of the electromagnet. The axis of the tube is set horizontally by means of a liquid (reversing) level, and then the sensor is moved by the lifting mechanism in height to the point where the component of the magnetic field along the sensor core is equal to zero. The zero position is determined by an electronic circuit. The height of the sensor is marked by means of a mechanism built into the device.

liquid height indicator. In order to take into account the error caused by the vertical component of the magnetic field, the sensor is rotated by \(180^\circ\) about the axis of the tube, after which a new position of the sensor in height is found, where the component of the field along its axis is equal to zero. The arithmetic mean of the two height readings gives the value \(z_m\) with an accuracy of up to \(0.3\) mm.

Mechanism for Moving the Sensor

The general view of the moving mechanism is shown in Fig. 1. The main part of the mechanism is a tube. A counterweight with a level is screwed into the tube and locked with a screw. The tube can rotate through \(180^\circ\) in two plain bearings and is fixed by a latch. The plain bearings are made in a common brass sleeve, which is rigidly fastened to the inner tube of the vertical-displacement unit. The median-surface sensor is placed in the socket of a rod; the sensor cable is led out through the inner channel of the rod and is fastened at the rear opening of the rod by a lock. The rod can be moved along the axis of the tube and is fixed in the required position by a collet clamp. Marks are made on the rear end of the rod corresponding to the radial coordinates of the sensor position in the air gap. The front part of the tube is made of textolite so that the magnetic field being measured is not distorted.

Fig. 1.

The axis of the tube of the mechanism is set in the horizontal plane using a reversible level by means of an adjusting screw. The stability of the horizontal position of the tube is ensured by a spring and the tight seating of the axis of the examiner.

The vertical-displacement unit consists of inner and outer tubes. The outer tube is fastened in the examiner socket. The inner tube can move in the outer tube without play. Raising and lowering of the tube with the sensor is carried out by means of a screw with a handwheel. The invariability of the direction of the tube axis in the horizontal plane is ensured by a key.

Liquid Height Indicator

The liquid height indicator operates on the principle of communicating vessels \([9]\). It consists of two columns connected by hoses to a cylinder. A piston with a micrometer screw is inserted into the cylinder, and the liquid levels in the columns are read from the scales of the micrometer screw. During measurements of the median surface, one of the columns is installed on the moving mechanism, and the other is fixed rigidly at any point, and relative to it the position of the median surface is measured. The operating radius of the instrument is determined—

is determined by the length of the hoses, while the latter is limited by the transient processes of establishing the liquid level in the hoses. In the instrument described, vacuum hoses \(6 \times 6\) mm and 10 meters long were used. The diameter of the glass columns was 20 mm; the range of measured levels was 30 mm.

The moment when the points in the columns touch the water level is determined by the closing of an electrical circuit with an ammeter as an indicator. The points in the columns are made of silver wire 1 mm in diameter and are sharpened to a diameter of 0.3 mm. The work was carried out only on contact closure. This eliminates errors associated with the phenomenon of surface tension of the liquid and with backlash of the micrometer screw.

Magnetic-field zero indicator

To indicate the zero of the magnetic field, the method of a ferroprobe sensor with a large longitudinal exciting field (with saturation) is used. Such a sensor, together with the electrical circuit (Fig. 2), makes it possible to fix the zero of the magnetic field with an accuracy of up to \(\pm 0.003\) oersted [10].

The sensor consists of a strip of permalloy, grade 80 NKhS, with dimensions \(0.02 \times 0.1 \times 25\) mm. Directly on the strip a signal winding is wound with PEL-0.02 wire; the resistance of the winding is about 400 ohms. The permalloy strip, previously annealed, is clamped between two ground halves of the remagnetizing-winding frame, which ensures straightening of the strip in a definite plane. The remagnetizing winding, consisting of 700 turns of PEL-0.15 wire, is supplied with current at a frequency of 6 kHz from a ZG-10 generator.

Fig. 2.

Measurement errors

The total error in measuring the median surface is made up of the error in indicating the zero of the magnetic field, the error in indicating the height, and the error due to the nonhorizontalness of the permalloy strip.

As indicated above, the electrical circuit provides indication of the zero of the magnetic field with an accuracy of up to \(\pm 0.003\) oe, which corresponds to an error in determining the median surface of \(0.06\) mm (for a radius of curvature of the lines of force of 200 cm and a magnitude of the controlling field on the orbit of 100 oe).

The errors of the liquid height indicator are due to inaccuracy of reading from the micrometer-screw scale, the temperature difference of the liquid columns in the arms of the columns, the surface tension of the liquid, and backlash between the screw’s traveling nut and the associated ...

...with a piston. The height indicator used in the instrument was tested by means of calibrated feeler gauges; in this case the scatter of the indicator readings did not exceed $\pm 0.1$ mm.

The principal errors in measuring the median surface are usually caused by the inaccuracy of the horizontal positioning of the sensor in the gap. The latter is determined by the graduation value of the liquid level and by the rigidity of the connection between the level and the sensor.

The reversible level used has a graduation value of $27''$. The level can be set in the horizontal plane with an accuracy of up to half a division; in this case the error in measuring the median surface is $\pm 0.14$ mm.

For convenience of measurements, the tube’s liquid level is brought out of the air gap of the electromagnet and mounted on the counterweight. Under the action of its own weight, the heavy tube bends so that the tube axis forms an angle $\alpha$ with the horizontal plane. The calculated value of $\alpha$ for the instrument described is 0.00015 radian. The error in measuring the median surface is calculated from the formula $\delta z_m = R\alpha$ (where $R$ is the radius of curvature of the lines of force) and is 0.3 mm. This error can be allowed for when measuring the median surface.

When the tube is glued together from several elements, it is impossible to maintain absolute coincidence of the axes of all the elements. A change in the sensor height when the tube is rotated through $180^\circ$ creates an additional error. In the instrument described, this error was eliminated by choosing such a plane of rotation in which the entire curvature of the tube lies in the horizontal plane.

The instrument described was built and tested at the Tomsk Polytechnic Institute. The tests confirmed the high accuracy and reliability of the instrument. The root-mean-square error of 11 check measurements at one point was $\pm 0.14$ mm; the maximum error did not exceed 0.3 mm. With the instrument, a detailed configuration of the median surface of the magnetic field in the gap was determined easily and rapidly. Unlike all preceding domestic and foreign instruments, in the instrument described the attachment of the sensor is independent of the magnetic circuit. Thus, with the aid of this instrument, the height positioning of accelerator blocks relative to the median surface can be carried out. A drawback of this instrument is the impossibility of dynamic measurements of the median surface.

We shall briefly indicate possible ways of improving the characteristics of the instrument.

1. Use, for indication of the zero of the magnetic field, the ferroprobe method operating on the sum of the even harmonics [9, 10]. In this case the electrical circuit is simple, and the accuracy of indication can reach $1 \cdot 10^{-4}$ oersted.

2. Use, in the liquid height indicator, vessels of cylindrical form with a diameter on the order of 50 mm. The accuracy of measurements in this case can be as high as 0.02 mm.

3. Use a liquid level with a graduation value of 3--4 sec.

References

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