Letter to the Editor
Simple, high-sensitive, and non-destructive beam monitor for RI beam facilities

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Abstract

A non-destructive beam current monitor for measuring the AVF cyclotron beam was designed, fabricated, tested and installed into a beam line to evaluate cross-sections rate in nuclear experiment at the CRIB. The monitor comprises a metal-alloy toroidal core, a pick-up coil, a metal box, and a low noise amplifier, and has a high dynamic range of 10nA100μA.

Introduction

Non-destructive beam current monitoring is very important in nuclear physics for radioactive isotope (RI) beam facilities, especially during RI beam production. Non-destructive beam monitors enable to measure the beam current during RI beam production and also enhance a good operation of the accelerator and the beam transport lines. These monitors, such as current transformers (CT), have been developed using different methods, and new developments are also underway. For measuring a bunched beam with a chain of short pulses, Paal studied a CT with an rms resolution of 100 pA [1]. For measuring a circulating ion current, Peters studied a cryogenic current comparator in a high-energy heavy-ion ring at GSI [2]. The circulating ion current was measured by comparing it to a known DC current when both passed through a cryogenic cylinder. The resulting currents generated on the surface of the cryogenic cylinder could be measured using a low-Tc SQUID. Watanabe is investigating a cryogenic current comparator by using a high-Tc SQUID [3]. Unlike the conventional beam monitoring system, both cases referred need cryogenic system and wide space for installation of the equipment.

The CT using a magnetic alloy (MA) core has been developed at several institutions. The MA core is made of thin magnetic alloy ribbon tape. An important feature of the MA is a large magnetic permeability, which increases the inductance of the pick-up coil. A fast CT enables accurate observation of a bunched beam of 53 mA with a frequency range of several Hz to 1 MHz [4]. To measure the beam current up to 15 A, all toroidal cores, except a DCCT, are made of FINEMET [16] because of its large saturation magnetic field [5]. FINEMET is also applied to an untuned RF cavity with a TM01 mode resonance for a large current synchrotron [6], [7]. A low quality factor (Q) and a low RF loss due to a small imaginary permeability are the characteristic features of this cavity.

A low-energy in-flight type RI beam separator, called CRIB [8], [9], was constructed by the Center for Nuclear Study, University of Tokyo (CNS) at the RIKEN Nishina Center [10]. Non-destructive beam current monitor is really needed to monitor the primary beam condition as well as to determine the beam intensities during the RI beam production for experiments. A new and simple core monitor has been developed and successfully installed in the CRIB beam transport line. This monitor has excellent features of non-destructive, high sensitivity, wide band, and has little RF noises from the RIKEN AVF cyclotron. The dynamic range of beam intensity measurements was designed to be 0.00110eμA. To design the non-destructive monitor we introduced a synchronous detection method for the bunched beam current, and TM01 mode resonance of the metal cavity using the FINEMET type MA. It is important to measure single pulses with a repetition rate of 12–24 MHz, which is corresponding to the 2nd harmonics of isochronous frequency of the AVF cyclotron.

This paper reports the design and performance of the MA-loaded CT, called the E7 core monitor (E7CM). The principles, design and electromagnetic properties of the monitor are explained in Section 2. Performance of the beam experiments is described in Section 3. Discussion of the noise reduction is provided in Section 4 and Section 5 summarizes the study.

Section snippets

Principle of the current detection by the E7CM monitor

A bunched ion beam with a current Ibeam passing through the center of the monitor induces a magnetic field in the core; and then induces a current I2nd in the pick-up coil which has a number of turns N, according to Faraday's law and Lentz's law. The most important property of the CT ratio, I2nd/Ibeam, depends on the magnetic permeability and the dimensions of the core. The theoretical CT ratio is then expressed asI2nd/Ibeam=1N(1+Y0(Z2l+Zb))where Z2l=ZsZmN, Zb=Zin , and Y0=1/NZm.

The term Y0(Z2l

Gain evaluation by using a wire current system

Experimental studies using the bunched ion beams, called online tests, will be discussed in 3.2 Performance at the low-energy beam transport line, 3.3 Performance at the CRIB beam transport line. Before the experimental study, we performed some basic examinations using a wire current system, which comprises a metal pipe, two metal flanges and the E7CM monitor. The system was designed to have semi-coaxial structure with a characteristic impedance of 50Ω in the frequency of interest. The metal

RF noise reduction

Generally, the high power RF system of the AVF cyclotron is the major part of the external noise sources. The TEM modes noise from the high-power RF system will travel along the beam transport line. The CRIB is located downstream of the beam transport line. In this configuration, the earth's potential of the high-power RF system could be higher than that of the CRIB, because the large ground current flows into the earth. In order to suppress the noise invasion from the upstream devices, an

Summary

A non-destructive ion beam current monitor E7CM, which has a simple structure but has a good signal to noise ratio, was installed in the CRIB to measure the primary beam intensity without interrupting the primary beams. This monitor has a metal alloy core and a pick-up coil. It detects the magnetic flux induced by bunched ion beams. The detected beam signal with a duty cycle of about 1100 was amplified and observed by using an oscilloscope or a spectrum analyzer. The frequency spectrum of the

Acknowledgments

Sincere gratitude is extended to the staff members of the RIKEN AVF Cyclotron for their operation of the HyperECR ion source and the accelerator during the experiment. We are indebted to A. Goto at RIKEN and K. Hatanaka at RCNP and T. Mitsumoto at SHI for the valuable discussion. We are also grateful to T. Watanabe for the comments.

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