Device for Moving Sensors in the Magnetic Field of a Small-Sized Betatron

DEVICE FOR MOVING SENSORS IN THE MAGNETIC FIELD OF A COMPACT BETATRON

A. A. FILIMONOV, V. L. CHAKHLOV

(Submitted by the Scientific Research Institute of Nuclear Physics, Electronics, and Automation)

At present, a portable compact betatron PMB-6S has been developed and is being produced in series.

The design of this type of betatron has its own distinctive features:

a) a multistand structure;
b) small dimensions of the working region of the inter-pole space;
c) difficult access to the inter-pole space, since the return magnetic circuit is pressed into a protective housing having a small exit aperture measuring \(40 \times 50\ \mathrm{mm}^2\).

Along with serial production of the PMB-6 betatron, work is being carried out to improve the specific characteristics of compact betatrons. One promising way to improve the specific characteristics of a betatron is to improve its focusing properties.

This is achieved by forming a control field with azimuthal variation.

The formation of such a field is accompanied by a large amount of measurement work—approximately 1500–2000 points. It is impossible to perform such a volume of measurement work without automating the measurement process. In addition, the creation of a control field with azimuthal variation requires well-founded tolerances for its formation. This task requires the development of very precise measuring equipment that makes it possible to obtain objective measurement results.

As a rule, methods for measuring magnetic-field characteristics that were developed for large accelerators and that are free from the features described above presuppose the presence of some mechanism intended for moving sensors in the space under investigation. The most acceptable, in our view, are the mechanisms described in [1, 2]. However, use of the device [1] makes it possible to investigate the characteristics of the magnetic field only in the region limited by the size of the window, since the presence of a slide does not permit the sensor to be moved in the azimuthal direction.

The device [2] makes it possible to measure the characteristics of the magnetic field in the inter-pole gap at any azimuth and radius, but installation of the sensor in the investigated space is carried out with large errors both in the azimuthal and in the radial directions, since the sensor is moved in a closed space and manually. In connection with the above, it was necessary to develop

mechanism, to a greater extent free of the shortcomings inherent in the devices described above.

The mechanism for moving the sensor in the space under investigation must meet the following requirements.

1. Radial displacement. The range of displacement is from 3.5 to 0.11 cm, with an accuracy of fixing the sensor at the specified radius of ±0.2 mm. The radius is measured from the axis of the working gap of the electromagnet.

2. Azimuthal displacement. The range of displacement is \(n \cdot 2\pi\), where \(n < 0\). The accuracy of fixing the sensor at the azimuth, with a step of \(10^\circ\), must be no worse than ±\(2'\).

3. Vertical displacement of the sensor. The displacement range is from \(z = 0\) to \(z = 15\) mm, where \(z = 0\) is the median plane of the electromagnet. The accuracy of fixing by coordinate must be no worse than ±0.2 mm.

4. The speed of displacement of the sensor in azimuth must be no greater than 15 revolutions per minute.

5. The speed of displacement of the sensor along the radius must be no greater than 10 cm per minute.

Fig. 1. General view of the mechanism

1. The azimuthal and radial displacement of the sensors should, if possible, be carried out by a single actuator mechanism.

2. The misalignment of the sensor-displacement mechanism and the electromagnet must be no worse than ±0.3 mm.

The position of the sensor in the space under investigation is determined from the corresponding verniers of azimuth, height, and radius, brought outside the return magnetic circuit.

In addition, the mechanism must be set in motion both by some motor and manually.

The general view of the device is shown in Fig. 1.

It consists of two measuring disks, having along their outer diameter a gear transmission with the same number of teeth and the same module of engagement. In one of the disks (Fig. 2), at the zero azimuth, a radial groove is made, in which a carriage with the sensor moves. The movement of the carriage in the radial groove is effected by means of the carriage pin engaging with an Archimedean spiral made on the other measuring disk (Fig. 3).

Both measuring disks are placed in guides that serve as the housing. They have projections for fastening and adjusting the mechanism in the space under investigation. In addition to these projections,

there are also brackets for fastening the drive mechanism with limbs and handles.

The disks are set in rotation by means of the drive mechanism. Installation of the device for moving the sensor in the working gap of the accelerator, depending on the particular features of its design, may be provided either by fastening it on the central inserts or with the aid of special guides, which are attached to the housing of the measuring mechanism and to the electromagnet housing.

The assembled mechanism is positioned in the space under investigation so that the mean plane of the sensors coincides with the median plane of the electromagnet. Precise adjustment of the sensors is carried out by moving the sensors in the carriage with the aid of the lead screw. After the sensors have been adjusted in the magnetic field of the space under investigation, the betatron electromagnet is assembled into its working position.

Fig. 2. Measuring disk with carriage in the housing

We shall show the operation of the mechanism by the example of measuring the magnetic-field falloff index along the radius \(n=f(r)\) and the azimuthal nonuniformity \(\Delta H=f(\varphi)\).

To measure \(n\), it is first necessary to move the sensor to the angle at which one wishes to determine the law of variation \(n=f(r)\). The required azimuth is set (Fig. 1) by engaging transmission gear 7 with both disks 3 and 9 simultaneously and by moving carriage 5 to the specified angle; after this, gear 7, by means of which the rotational motion from actuator mechanism 11 is transmitted, is engaged with the toothed drive of disk 9; in this case the azimuthal displacement of disk 9 will cause a radial displacement of carriage 5 with sensor 4. Such displacement is possible owing to pin 8 of carriage 5, which engages with Archimedes spiral 6 of measuring disk 9. In this case the pitch of the Archimedes spiral determines the pitch of the sensor displacement.

To measure the azimuthal distribution of the field at some specified radius, it is necessary to move the sensor along the azimuth. In this case, by engaging gear 7 with disk 9, the carriage with the sensor is set to the required radius and, by means of

of the subsequent simultaneous engagement of gear 7 with disks 3 and 9, move them in azimuth.

For more detailed investigations of the structure of the magnetic field, it is sometimes necessary to have a smaller step than in measurements of a mass character. When using the mechanisms [1,2], a sensor displacement step of, say, 2–3 mm may turn out to be commensurate with the error in positioning the sensor.

For the mechanism developed, setting such a step is possible by variably bringing the transmission gear into engagement with two measuring disks or with one.

The output of information from the investigated space from the sensor is accomplished by means of annular buses laid in the groove of disk 3, to which the sensor leads and the brush-contact system are soldered. In order not to create short-circuited turns, the buses have a break.

Fig. 3. Measuring disk with an Archimedean spiral in its housing

The accuracy of moving the sensor in the working gap and of fixing it is determined by the kinematics of the device and by the special manufacturing technology of the main parts of the mechanism.

The developed mechanism was tested in operation in studying the topography of the magnetic field of small-sized accelerators of the PMB-6 type.

Advantages of the mechanism described:

1. It is possible to use a single motor to investigate radial and azimuthal characteristics.
2. It is possible to obtain any desired step for positioning the sensor in the magnetic field.
3. The operator occupied with moving the sensor in the magnetic field is freed.
4. Automation of the measurement process is possible.
5. The location of the sensor in the magnetic field is visually apparent.
6. Higher accuracy in fixing the sensor in the magnetic field.
7. The device makes it possible to measure magnetic-field characteristics in a working gap to which access is limited.

References

1. V. S. Grishin. A measuring device for measuring the magnetic field of small betatrons. Proceedings of the Fourth Interuniversity Conference on Electron Accelerators. Moscow, “Vysshaya shkola,” 1964.

2. L. M. Anan'ev, A. A. Vorob'ev, V. I. Gorbunov. The induction electron accelerator—a betatron. Moscow, Gosatomizdat, 1961.