Applied Physics Researches Division (APRD)

Group 2014-2019 investigations

Posted on Mar 28, 2019

Group 2014-2019 investigations

Development of a new type of Vibrating Wire Monitor (VWM), which has two mechanically coupled

wires (vibrating and target), is presented. The new monitor has a much larger aperture size than the previous model of the VWM, and thus allows us to measure transverse beam halos more effectively. A prototype of such a large aperture VWM with a target wire length of 60 mm was designed, manufactured, and bench-tested. Initial beam measurements have been performed at the Fermilab High Intensity Neutrino Source facility, and key results are presented.

Fig. 1. (a) Photo of the Large Aperture Vibrating Wire Monitor (LA-VWM).
(b) Beam profile, measured by the LA-VWM in the horizontal direction at the Fermilab High Intensity Neutrino Source facility.

Measurements of secondary particles/photons reflected/generated from an oscillating wire in synchronism with the wire oscillation frequency are proposed. The differential signal on wire maximal deviations at oscillation process can provide a fast signal proportional to beam profile gradient. Idea of using such “Resonant Target” for beam transverse gradient profiling was tested with lightening the oscillating wire by a laser.

Fig. 2. (a). Layout of experiment: 1 – vibrating wire of VWM with wire length 80 mm, 2 – wire oscillation generating magnet poles, 3 – laser, 4 – photodiode.
(b) Forward and backward scan of the vibrating wire sensor. Red line – differential signal from photodiode at ultimate positions of wires during oscillations. Green line – averaged signal of 1000 measurements of photodiode at ultimate positions of wires, Blue line – vibrating wire oscillation frequency also depends on reflecting on wire photon numbers.

Proton/ion beams of multiple charge/mass ratios can be very complex. Orthogonal X-Y projections are often inappropriate to represent these profiles. An array of vibrating wires, rotating around the beam axis is under development. The mechanical implementation is described. An algorithm to reconstruct the profile is proposed. The tradeoffs between the number of wires, the rotation angles, the response time and the profile resolution are discussed.

Fig. 3. (a). Numerals indicate: (1) – measured beam, (2) –frame of monitor, (3) – vibrating wires, (4) –support flange with gear teeth on the flange rim, (5) – loading gear with shaft translated rotation, (6) inclination axis supporting lips, (7) – gearbox-transmitter translate inclination axis rotation to the shaft (8).
(b). Test distribution of the beam to be measured.

Two types of neutron monitors with fine spatial resolutions are proposed based on vibrating wires. In the first type, neutrons interact with a vibrating wire, heat it, and lead to the change of its natural frequency, which can be precisely measured. To increase the heat deposition during the neutron scattering, the use of gadolinium layer that has the highest thermal neutron capture cross-section among all elements is proposed. The second type uses the vibrating wire as a “resonant target.” Besides the measurement of beam profile according to the average signal, the differential signal synchronized with the wire oscillations defines the beam profile gradient. The monitor’s spatial resolution is defined by the wire’s diameter.

Fig. 4. Schematic sketches of (a) middle-scaleVWNM:1 – vibrating wire,2 –magnets, 3 – magnet poles,4 –clips, 5 – base, and(b)small-scaleVWNM:1 – vibrating wire,2 – support base,3 – wire contactingwasher,4 – magnet, 5 – two thin washers from ceramics, washer with or without hole,6 – elastic washer,7 – clamping washer from magnetic material.

We propose a new type of wire scanner for beam profile measurements, based on the use of a vibrating wire as a scattering target. Synchronous measurements with the wire oscillation allow to detect only the signal coming from the scattering of the beam on the wire. This resonant method enables fast beam profiling in the presence of a high level of background. The developed wire scanner, called resonant target vibrating wire scanner, is applied to photon beam profiling, in which the photons reflected on the wire are measured by a fast photodiode. In addition, the proposed measurement principle is expected to monitor other types of beams as well, such as neutrons, protons, electrons, and ions.

Fig. 5. (a). Main view of RT-VWS (1) mounted on a pendulum (2), (3)—wire, (4)—laser, (5)—photodiode, and (6)—photodiode circuit. The swinging motion of the pendulum with RT-VWS is presented by arrows. The laser and photodiode remain fixed.
(b) The main signal (1, blue curve). differential signal (2, magenta curve). (b) The algorithm recovers the laser beam profile (3, brown curve).

As an instrument for Korea Multi-Purpose Accelerator Complex (KOMAC) facility proton beam profiling, a vibrating wire monitor (VWM) has been installed and tested at TR23 target room. Experiments were done at very low (100 nA) beam current conditions. At the number of particles about 1011 proton/train and trains repetition rate of 0.1 Hz we have measured the beam profile by a few scanning steps. The experience accum1ulated in these experiments turned out to be useful for the VWM upgrades (e. g. understanding interactions of protons with wire materials and heat transfer processes) and will be particularly helpful for the KOMAC beam halo measurements in the future high-current operation.

Fig. 6. (a) Main view of the monitor with an aperture of 40 mm and a wire length of 80 mm: 1 – vibrating wire, 2 − magnets, 3 − magnet poles, 4 − clamps, 5 − basis.
(b) Beam profile, reconstructed by procedure of estimation of frequency drop for each train. Diamonds are the averaging of experimental data at fixed positions of VWM (circles correspond to squares standard deviation by the set of measurements). Solid line is fit of experimental points by the Gaussian curve.

For measuring laser beam profiles, a vibrating wire monitor (VWM) has been introduced. The measurements were carried out at different speeds of scan. Preliminary estimates were made for the calculation of the VWM response times with respect to the thermal losses along the wire, and radiative and convective losses. These estimates, however, do not determine the difference between the beam profile and the frequency response of the VWM for a given scan rate. To evaluate the reliability of the frequency response of the VWM, comparisons between forward and reverse beam scans at different speeds have been used. The results of these scans are used to correct the thermal inertia in the frequency response of the VWM.

Fig. 7. Scanning with a of max. (10 steps / s (0.0166 mm / s)) and min. (200 steps/s (0.3322 mm/s) speeds: (a) – primary data. Scan graphics are shifted to each other by 10 and 84 steps. Curves 1 and 2 are forward and backward scans.

The possibility of using a vibrating wire as a target in the method of scanning transverse profiles of the thin beams in accelerators has been studied. In the case where the transverse dimensions of the beam are comparable to the amplitude of the wire oscillations, the vibrating wire can be used for a fast measurement of the transverse beam profile without moving the sensor. The scanning procedure is replaced by the use of the movement of the wire during its mechanical oscillations. The method is tested on a focused beam of a semiconductor laser with a spot size at the focus of ~ 0.1 mm. The reconstruction of the transverse profile is performed on the basis of the measurements of the photons reflected from the vibrating wire by using fast photodiodes.

Fig. 8. (a). Photo of experiment. 1 – laser, 2 – collimator, 3 – short-focus lens, 4 – fast photodiode, 5 – vibrating wire, 6 – turntable with microscrew.
(b). Dependence of the number of photons on the coordinate – profile