Development of new HTS-SQUID and HTS current sensor for HTS-SQUID beam current monitor

Abstract

Two years ago, a prototype of a highly sensitive beam current monitor with a high-temperature superconducting (HTS) SQUID, an HTS current sensor and an HTS magnetic shield, that is, an HTS-SQUID monitor, was installed in the beam transport line of the RIKEN ring cyclotron (RRC). As a result, the beam intensity of a sub-μA beam was successfully measured by the prototype HTS-SQUID monitor. In fact, the intensity of a sub-μA 40Ar15+ (63 MeV/u) beam was successfully measured with a 500 nA resolution. However, the current resolution of the prototype HTS-SQUID monitor is not sufficient to measure the current of a uranium beam, which is accelerated in a new radioactive isotope (RI) beam facility called "RI Beam Factory" (RIBF). A minimum current resolution of 1 nA is required for the measurement of the uranium beam. Therefore, we are developing a new HTS-SQUID monitor so as to improve the current resolution. This new monitor consists of three parts, the HTS SQUID, an HTS current sensor and an HTS magnetic shield, and these parts have been separately developed this year. The high-permeability core that is installed in the two input coils of the HTS-SQUID is an extremely important part in this new HTS-SQUID monitor. A 50-fold improvement in gain was successfully realized using the high-permeability core compared with that obtained without the high-permeability core. Another key factor is the substrate of the HTS current sensor. A MgO ceramic tube was used for the substrate of the HTS current sensor in the prototype HTS-SQUID monitor. However, it was difficult to form the bridge circuit using the MgO ceramic substrate in the new HTS-SQUID monitor, because the bridge circuit that magnetically connects the HTS current sensor and the HTS-SQUID has to be three-dimensional. To solve this problem, silver (Ag) of 99.9% purity was adopted for the substrates of the HTS current sensor in the new HTS-SQUID monitor. Then the surfaces of the substrates were coated by a thin layer (70μm) of Bi2-Sr2-Ca1-Cu2-Ox (Bi 2212), which is an HTS material. We report the results of this development.