Performance Studies of the Vibration Wire Monitor on the Test Stand with Low Energy Electron Beam

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1 Proc. 2nd Int. Symp. Science at J-PARC Unlocking the Mysteries of Life, Matter and the Universe Performance Studies of the Vibration Wire Monitor on the Test Stand with Low Energy Electron Beam Kota OKABE, Masahiro YOSHIMOTO and Michikazu KINSHO J-PARC Center, Tokai, Ibaraki , Japan (Received October 13, 2014) In the high intensity proton accelerator as the Japan Proton Accelerator Research Complex (J-PARC) accelerators, serious radiation and residual dose is induced by a small beam loss such a beam halo. Therefore, diagnostics of the beam halo formation is one of the most important issues to control the beam loss. For the beam halo monitor, the vibration wire monitor (VWM) has a potential for investigating the beam halo and weak beam scanning. The VWM has a wide dynamic range, high resolution and the VWM is not susceptible to secondary electrons and electric noises. We have studied the VWM features as a new beam-halo monitor on the test stand with low energy electron gun. The frequency shift of the irradiated vibration wire was confirmed about wire material and the electron beam profile measured by using the VWM was consistent with the results of the Faraday cup measurement. Also we calculated a temperature distribution on the vibration wire which is irradiated by the electron beam with the numerical simulation. The simulations have been fairly successful in reproducing the transient of the irradiated vibration wire frequency measured by test stand experiments. In this paper, we will report a result of performance evaluation for the VWM on the test stands and discuss the VWM for beam halo diagnostic KEYWORDS: J-PARC, beam monitor, beam halo 1. Introduction The J-PARC is a high intensity proton accelerator facility aiming to realize 1 MW class beam power [1]. A negative hydrogen ion beam from the linac is injected into the 3 GeV rapid cycle synchrotron (RCS) through stripping to a proton beam by a charge strip foil placed in the RCS injection point. In 2013 summer-autumn period, the injection beam energy has been increased to 400 MeV with the upgrade of the linac [2]. After that, the RCS will aim at final goal of the 1 MW output. If such a high-intensity beam is to be provided for routine user program in the J-PARC, the quality of the high intensity beam must be improved. A space-charge-dominated high-intensity proton beam has various beam instabilities and the beam halo is one of the most important behaviors. In the high power proton accelerator as the J-PARC, even small ratio of the beam loss such a beam halo causes serious radiation dose. Especially, huge radio-activation around injection section in the RCS and the beam collimate section in the 3-50 GeV beam transport line (3-50BT) are quite serious. In order to control the beam loss, diagnostics of the beam halo formation is one of the most important issues in The Physical Society of Japan

2 the J-PARC accelerators. In general, the beam-halo quantity is less than 10-4 of the beam core. Therefore, beam profile monitors for halo measurements require a very wide dynamic range, high sensitivity and resolution, and noise rejection techniques for instruments and methods. In the case of the RCS, beam-halo formations are determined by a combination various devices and methods [3]. However, some technical issues such as signal noise by secondary electrons or AC power supplies for beam halo measurements exist, so we developed a new beam-halo diagnostic system for the high-intensity proton beam. For the new beam halo monitor, we focused on development of the vibration wire monitor (VWM) which has a potential for investigating the beam halo and weak beam scanning [4]. For the feasibility study of the VWM feature as a new beam-halo monitor, the VWM have been studied on the test stands with low energy electron gun. Also a temperature distribution on the vibration wire which is irradiated by the electron beam was calculated by the numerical simulation. 2. Vibration Wire Monitor for the Beam Halo Monitor 2.1 Feature of the vibration wire monitor The VWM for beam diagnostic was developed at the Yerevan Physics Institute, and first experimental results were obtained at the Yerevan synchrotron injector electron beam [5]. The principle of the VWM is to pick up the temperature-rising-induced frequency shift by using a vibration wire irradiated by the beam. The novelty of the method is that temperature shift of the wire provides information about the number of particles that interact with the wire. Figure 1 shows a schematic view of the VWM developed by Bergoz Instrumentation for beam profile measurements [6]. Fig. 1. The VWM for beam halo measurements developed by Bergoz Corporation. About the VWM for beam halo monitor, both ends of the conductor wire are fixed at wire clips and between two pairs of permanent magnets. The wire length L is twice as the length between the permanent magnets because second harmonic oscillations of the

3 wire are excited efficiently. The connector for the amplifier circuit is the lower arm of the wire clips which are isolated from monitor support. As a result of the Lorentz-force interaction between the oscillating current through the wire and the magnetic field, wire oscillations of the VWM arise. When the wire of the VWM is connected to a positive feedback circuit, the wire oscillation is excited by an amplifier to support the oscillations [7]. Thus, information of the wire s tension is provided by the frequency of natural oscillations of the wire. 2.2 Frequency shift of the wire under beam irradiation The wire oscillation of the VWM can be described as that of a support with a strained vibrating wire with rigidly-fixed wire ends. The frequency F 0 of the second harmonic of the natural wire oscillation is 1 F 0 = σ0 / ρ. (1) L The distance between the fixed wire ends is L, σ 0 is the wire's initial tension, and ρ is the wire's material density. Due to temperature stress, the oscillation frequency of a vibrating wire has a strong temperature dependence. The relative oscillation frequency shift is defined by the expression, ΔF Eα = ΔT. (2) F0 2σ 0 Where α is the wire's material coefficient of thermal expansion, and E is its module of elasticity. The wire's temperature shift ΔT is an average value along the wire. The wire's heating is caused by interactions of the beam with the wire. Thus, the frequency of natural oscillations of the wire provides information on its temperature. In principle, the VWM is not susceptible to secondary electrons, which are ones of the major noise sources for beam monitoring in high-intensity accelerators. Also, we assume that a VWM potential dynamic range of 10 5 will be achieved because the VWM can measure the temperature change from 0.01 to several hundred K. Therefore, we focus on this novel beam monitor for the beam halo diagnostics, and research and development of the VWM are carried out. 3. Experiment of the VWM by Low Energy Electron Beam 3.1 Experimental set-up In order to perform the feasibility test of the VWM as a beam-halo monitor, we developed low electron beam test stand for the VWM. Figure 2 shows the test stand for feasibility test of the VWM. Low energy electron beam gun to irradiate the VWM is installed at left hand side of the test stand in the Fig. 2. Extracted beam energy of the electron gun is about 4 kev and average beam current of continuous beam operation is about 160 na. This electron gun has an electrostatic lens for controlling beam spot size. And the Faraday cups with beam slit for electron beam profile measurements are installed to compare with the VWM measurements. The wire was made of a SUS316L, whose diameter is 0.1 mm and whose length between the wire ends is 120 mm. The distance between permanent magnets is 60 mm. The VWM is attached to a movable rod with a stepping motor and is driven in the horizontal direction toward the

4 vacuum-chamber center. The Faraday cups for beam profile measurement are two types. One has no slit to measure total current of electron beam, the other one has narrow slit whose width is about 0.3 mm. Fig. 2. The electron beam test stand for performance study of the VWM. 3.2 Equilibrium Temperature and Frequency Shift of Irradiated Wire The equilibrium temperature of an irradiated wire in vacuum is determined from the balance between the heating from the electron beam and two heat-transfer effects of the wire. One of sources of heat transfer is thermal radiation through the wire surface, and the other is thermal conduction of the wire's material through the ends of the wire. One-dimensional heat conduction equation of irradiated wire is C p T( x) qhi T T ( x) T ( x) 2 s 0. (3) 2 2 t r x 4 r Where x is position along the wire from the VWM center, T is wire temperature, C p is the heat capacity per unit volume, r is a wire radius, ε is a radiation efficiency of the wire surface, σ s is the constant of Stefan-Bolzmann, T 0 is a temperature of external environment, λ is the thermal conductivity of wire material, I is irradiating beam current at the wire. The quantity of heat transfer caused by the interactions of the beam with the wire depends on the particles species, its energy, the material of the wire and the wire's geometry. The average heating q h with a low energy electron beam passing though the wire is q h = ke. (4) Where the heat transfer coefficient k is the amount of energy loss converted into wire heat, E is a kinetic energy of the electron beam. From equations (3)-(4), thermal distribution of irradiated wire was calculated numerically. Figure 3 shows results of numerical calculation for thermal distribution along the wire and time variation of mean temperature ΔT. Any parameters of the VWM and wire material was assumed installed VWM in the test stand. Energy of electron beam from

5 T[K] Freq. of the VWM [Hz] Mean temperature [K] Frequency shift [Hz] the electron gun is 4 kev, transverse beam size (sigma) is 0.5 mm and beam current is 45.6 na. We assume that material of the vibrating wire is SUS 316L, C p = 0.59 J/(gK), σ s = 5.67e-8 W/(m 2 K 4 ), T 0 = 300 K, λ = 16.7 W/(mK), k = 0.58, ε = After long-term irradiation by the beam, the wire's temperature will reach a condition of thermal equilibrium. From figure 3, wire temperature become thermal equilibrium 120 seconds after irradiation sec 20sec 40sec 60sec 80sec 100sec mean temp.[k] freq. of the VWM[Hz] Distance along the wire from the VWM center [m] Time [sec] Fig. 3. Results of the numerical calculation for thermal distribution (left) and mean temperature shift of the irradiated wire (right). We assume that electron beam hit around the middle of the VWM. Figure 4 presents the typical experimental result of the VWM frequency shift at the test stand. The wire hitting current and the VWM are same parameter with Fig.3. In Fig. 4, the experimental results of the frequency shift of the VWM is compared with the simulation results. The natural frequency of the VWM was about Hz. The electron beam hit the wire only in the period between 0 sec and 180 sec. Maximum phase shift of 2.14 Hz was measured and the length of time before temperature equilibrium was about 130 sec. From Fig. 4, the frequency shift data of the VWM is consistent with that from numerical calculation Experimental results Numerical calculation Time [sec] Fig. 4. Results of the numerical calculation for thermal distribution (left) and mean temperature shift of the irradiated wire (right). We assume that electron beam hit around the middle of the VWM.

6 Measured current [ua] Comparison of the VWM with the faraday cup beam profile measurement The VWM position is scanned and we take maximum frequency shift of the wire which is thermal equilibrium. Then, the beam profile data are obtained by these differentiating data at each VWM position. From the wire frequency under a thermal equilibrium condition, we can estimate the mean temperature of the wire and the number of irradiating particles per unit time Faraday Cup VWM Position [mm] Fig. 5. The result of the beam profile measurements with the VWM and Faraday cup. Figure 5 shows the result of the beam profile measurements for the scanning at the horizontal axis. The measured beam current of the VWM reproduced from frequency shift data by eq. (2), (4) and mean temperature from numerical calculation results. The qualitative trend of the profile measurement by using the Faraday cup is in good agreement with the VWM results around an electron beam core. However, measurement current of the VWM around lower and upper position from a beam core had any significant value and it was not consistent with measurement results by the Faraday cup. We assume that this significant frequency shift was caused by environment temperature shift or heat load effect of passing electron beam. 4. Summary The VWM has potential for beam halo measurement because its dynamic range is wide and the VWM is not susceptible to secondary electrons. We have studied the VWM features as a new beam-halo monitor on the test stands with low energy electron gun. The frequency shift of the irradiated vibration wire by the electron beam was confirmed. Also we calculated a temperature distribution on the vibration wire with the numerical simulation. The simulations have been fairly successful in reproducing the transient of the irradiated vibration wire frequency measured by test stands experiments. An electron beam profile measured by using the VWM was consistent with the results of the Faraday cup measurement. On the basis of these results, we will test the feature of the VWM with different wire materials, make the improvements in the structure of the VWM and develop a new measurement circuit for highly accurate frequency measurement of the VWM.

7 References [1] S. Nagamiya: Nucl. Phys. A, 774 (2006) 895. [2] H. Hotchi et al.: PRST-AB, 15 (2012) [3] M. Yoshimoto, et al.: Proc. IPAC 12, New Orleans, USA, May (2012) [4] S. G. Arutunian, et al.: PRST-AB, 2 (1999) [5] S. G. Arutunian, et. al.: PRST-AB, 6 (2003) [6] Begoz Instrumentation: home page [7] S. G. Arutunian, et al.: Nucl. Ins. Meth. A 572 (2007) 1022.