A METHOD FOR MEASURING VIBRATIONSIN WIRE SCANNER BEAM PROFILE MONITORS
Naoko IIDA, Tsuyoshi SUWADA, Yoshihiro FUNAKOSHI, Takashi KAWAMOTOand Mitsuo KIKUCHI, KEK, 1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305-0801, Japan
Abstract

2

In the injector linac and the beam transport lines of theKEKB, a number of wire scanners will be installed fornon-destructive beam profile measurements. A drive system, which utilizes a stepping motor and linear guides, will
be used for them. Two prototypes have been evaluated interms of vibrations at a wire-mount frame in a vacuum environment using a laser displacement meter. Vibrations observed for the two prototypes were found to be as small as
20µm, which perfectly meets requirements from real beamoperations.
1INTRODUCTION

The KEKB accelerator is composed of 3 main components:
an injector linac, beam transport(BT) lines and main rings.
Electrons(positrons) are accelerated up to 8GeV(3.5GeV)
by the linac about 500m long and guided to the main rings
through the BT lines with the length of about 500m. Since
the KEKB is a factory machine, a well-controlled operation of its injector is required for minimizing tuning timeand a stable operation. From this viewpoint, beam diagnostic and monitoring tools will be essentially important. Of
those tools, wire scanners (WS's) will be installed in somecritical regions of the long linac and BT lines and used
to measure transverse beam distributions non-destructively.
In the machine tuning the wire scanners are used to determine beam emittances and Twiss parameters which areused in optics matching. In addition, they will be also used
to watch beam condition during the stationary operation.
In the measurement with the wire scanners, a thinwire (typically several hundred microns in diameter) is
moved across the beam transversely and γ-ray fromBremsstrahlung or secondary electrons, which are cased byan interaction between the beams and the wire materials,
are observed by a detector. Since a thin wire is used, themeasurement can be done almost non-destructively. On theother hand, we must be very careful with accurate positioning of wires. The positioning errors lead directly to systematic errors in results of measurements. The wire is driven bya pulse motor and linear guides. The positioning error canbe divided into two categories. One is a static displacement
mainly due to production errors of linear guides. The otheris a mechanical vibration of holders in which the wires are
stretched due to the wire-driving motion. In this report, weonly consider the displacement and vibration perpendicular to the beam direction and to the wire-driving direction.
Displacements or vibrations in other directions are negligibly small or not important.

TWO TYPES OF WIRE SCANNERS

We have been developed two different types of wire scanners as prototypes. Both are driven by a pulse motor witha maximum stroke of about 100mm. Type-A shown inFig. 1A has two linear guides in vacuum which can suppress wire vibrations directly. On the other hand, Type-B
in Fig. 1B has only one linear guide outside the vacuumchamber. For both types, a shaft about 200mm long supports the holder in which the wires are stretched. Fromthe viewpoints of minimizing the wire vibrations, Type-A
is better. However, Type-B has the advantages of a simpler structure and easier handling for maintenance: i.e. thestructure of Type A is more complicated in that the linear
guides and ball bearings are in a vacuum and so replacement of the wire is somewhat troublesome. For both types,
screw pitch is 2mm and the wire holders are driven by 4µm
with one pulse of the pulse motors. In the present measurement, the position of the wire holder is measured by using
Magnescale with the resolution of around 1µm. Note thatvibrations of the wires themselves can not be measured inthe present method.
3METHOD

We have employed a laser displacement meter(KEYENCELC2430) for the wire position measurement. A schematic
drawing of a system for the measurements is shown in Fig.
2. An object shaped like a flag was attached to the wireholder. The size of the flag is 40mm × 10mm with a shaft
whose cross section is a square 5mm on a side. A blockgauge with flatness of less than 50µm is pasted on the surface of the ”flag”. The displacement measurement is doneby emitting a laser beam at the flag and detecting the position of a reflected beam with a built-in PSD. With this system, a resolution of measurements is about 0.02µm. The
measurement was also done in the condition that the chamber was evacuated to less than 10−5 mbar. For this purpose
we installed an optical glass 5mm thick between the laser
displacement meter and the flag. Although the optical glass
5mm in thickness makes an offset of about +1.8mm, it has
nothing to do with results of the measurements.
The flag was installed at the wire holder as shown inFig. 2. Since the head of the flag is stuck out of a vacuum chamber hole, we can drive the wire holder only byabout 25mm-stroke with the flag attached. To enable themeasurements in a wider range of the stroke, the flag is attached to three different points of the holder shown in thefigure. This made it possible to measure the displacementin the range of about 50mm which is half of the full stroke.
We have done two different kinds of measurements; i.e.

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542Results are shown in Fig. 3 and Fig. 4. Fig. 3A and 4A
show results for Type-A and Fig. 3B and 4B for Type-B.
We only show results in the vacuum, since we did not observe any essential differences between the air and vacuumenvironments. The horizontal axis of Fig. 3 denotes theflag position (in units of mm) and the vertical axis shows
the displacements. The figures in Fig. 3 contain data ofthree holder positions in both forward and backward directions. The linear parts of the data were subtracted fromthe figures, since these may come from a mis-alignment of
the flag or others and has no essential importance. Sizesof static displacements in terms of the peak-to-peak values were about 10µm and 15µm for Type-A and Type-B,respectively. The displacement of Type-B shows a cyclicvariation with a cycle of 2mm. This cyclic variation canbe attributed to the pitch of a ball screw which converts therotary motion of the motor to the linear motion. We can expect to suppress the cyclic variation by enhancing rigidnessof a board which supports the linear guides.
The horizontal axis of Fig. 4 denotes pulse rates corresponding to the wire-driving-speed and the vertical axis
shows the maximum (peak-to-peak) displacements during
the continuous scans of the 25mm-stroke. Fig. 4A shows adata only in the forward direction. On the other hand, Fig.
4B contains results of the both directions.
As for the vibrations, all results of the maximum displacements for Type-A were less than 12µm except 21µm
in the case of 4kHz. For Type-B, the maximum displacement is about 18µm also at 4kHz which is larger than theothers in the range from 10 to 16µm. We have to noticethat the static displacements are included in the results ofthe vibration measurements. In the vibration measurementswith continuous scans, the static displacements dependent,,,,,,,,,

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4 RESULTS

7a static transverse displacement dependent on the wire position and vibrations during wire-driving. Each measurement was done both in the air and vacuum conditions. In
the measurement of the static displacement, we drove thewire in units of 0.7mm. We read a value on the laser displacement meter, which was an averaged value of 32768
data at each step after waiting for one second to ensure thatthe vibrations of the wire holder were damped. To see thereproducibility of measurements, we took data in the bothforward and backward directions. In the vibration measurement, the flag was installed at the position nearest to theshaft head. At this position we expected the largest amplitude of vibrations of the three. As a measure of a size of thevibration, we took peak-to-peak displacements during continuous scans of the 25mm-stroke in the both forward andbackward directions. In the measurement, analog signals
from the laser displacement meter, which is not an average
this time, were observed by using a digital oscilloscope.
The pulse rate of the pulse motor was changed from 1kHz
to 5kHz in units of 1kHz. At each step, measurements wererepeated 5 times and we took an average.

(b) Type-B
Figure 1: Schematic drawings of two wire scanners.
on the wire position were observed as vibrations. Besides
these vibrations, we observed resonant peaks around 65Hz
and 100 Hz. The 65Hz component is consistent with an
eigenvalue of the proper vibrations for the shaft which isestimated to around 70Hz, although we have not yet understood the origin of the other component. The eigen frequency of the flag is about 500Hz and was not observedin the present measurement. The origin of the resonancearound 4kHz is now under study.
With both types of prototypes, the oscillation amplitudes
are less than 16µm except for around 4kHz. Since the horizontal and vertical wires make an angle of 45 degree to thewire-driving direction, effective displacementsof the wires√
are these amplitudes divided by 2; i.e. less than 11µm.
This value is well below a requirement from the machine
operation for the linac and the BT lines. Type-A was installed in the Arc section of the linac after this vibration
measurement. We made some preliminary beam test withthe Air

Static Mode in Vacuum - Type A
6Vacuum
Optical Glass(t = 5 mm)
Laser Light,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,
Metal Flag,,,,,,,,,,,,,,,,,,,,,,,,,,,,*Laser Displacement Meter

Displacement [µm]Laser Displacement Meter

420-2-4Magnescale-6-50-40-30-100105060(3A)

Oscilloscope, Macintosh(via RS232C)

Static Mode in Vacuum - Type B
10Displacement [µm]KEYENCE LC2430( δ = 0.02 µm)

PSD
displacement

Figure 2: Schematic view of vibration measurement system.

50-5-10-10010203040

Wire position (mm)this prototype system [1].

(3B)

SUMMARY AND FUTURE PROSPECTS

We have developed two different kinds of wire scanners.
For both prototypes, we measured wire vibrations during
wire-driving motion in a vacuum environment. Measured
vibration amplitudes were ten-odd microns except for thepulse rate of 4kHz. These are well below the requirementfrom the viewpoint of beam operations in the linac and theBT lines. There is still some room for improvement forType-B. We can avoid the resonant pulse rate of 4kHz inthe actual operation.
Since it turned out that both prototypes are feasible, wewill adopt Type-B as an actual model. Type-B has the advantages of easier handling for maintenance and a low cost.

Figure 3: Results of static displacement measurements.

Dynamic Mode in Vacuum - Type A

Vibration Amplitude (p-p) [µm]5-20Wire position (mm)22201816141210860

REFERENCES[1] Y. Funakoshi et.al.:`BEAM TESTS OF A WIRE SCANNER
FOR THE KEKB INJECTOR LINAC AND BEAM TRANSPORT LINE' , in these proceedings.

23456

Stepping motor pulse rate [kHz](4A)
Dynamic Mode in Vaccum - Type B

Vibration Amplitude (p-p) [µm]

61

20
18
16
14
121080123456

Stepping motor pluse rate [kHz](4B)
Figure 4: Results of vibration measurements.