## 1 Introduction  
As superconducting wires allow the flow of very high current density levels compared to copper wires, superconducting magnets generate a high magnetic field with compact and light windings. There are many industrial and scientific devices employing superconducting magnets to generate a high magnetic field, and several devices have been commercialized given their high performance and quality levels [1; 2; 3; 4; 5]. Superconducting magnets are classified as low-temperature superconducting (LTS) magnets wound with lower temperature superconducting wires, such as NbTi and Nb\({}_{5}\)Sn [6; 7], and high-temperature superconducting (HTS) magnets fabricated with high-temperature superconducting tapes such as Rare-earth barium copper oxide (REBCO) and Bismuth strontium calcium copper oxide (BSCCO) [8; 9]. LTS wires, which were commercialized much earlier than HTS tapes, are much less costly than HTS tapes but require liquid helium or massive and expensive cryo-coolers to keep the magnet temperature under 4.2 K due to the low critical temperature required (\(<\) 10 K). Recently commercialized HTS tapes, on the other hand, have a high critical temperature (\(>\) 90 K) and do not need liquid helium to cool the magnets [8]. This means that it is possible to operate a superconducting magnet with an inexpensive and compact liquid-helium-free system. Therefore, the handling and operation of the HTS magnet system would be very safe and comfortable for both users and vendors [10]. For these reasons, research on HTS magnets has been active recently. Many research institutes adopt or consider the use of HTS tapes to develop nuclear magnetic resonance (NMR) systems that exceed 1 GHz [11; 12; 13; 14; 15]. Magnetic resonance imaging (MRI) researchers have also been developing liquid-helium-free HTS MRI systems to manufacture robust, safe and compact MRI system [16; 17; 18; 19; 20]. HTS magnets have also been applied to dipole magnets in relation to research on accelerators [21; 22; 23; 24] and rotating machines for power generation and ship propulsion [25; 26; 27; 28]. In addition, many studies of HTS magnet applications are underway in many other fields. The Korea Basic Science Institute (KBSI) also commenced a research project to develop a conduction-cooled HTS magnet for an electromagnetic property measurement system (EMPS) in 2020. The goal of the project is to develop a liquid-helium-free 3 T/25 mm REBCO magnet for various physical property measurements and optical experiments. The operating temperature of the magnet will exceed 20 K, and a cost-effective small cryo-cooler can sufficiently cool the magnet. Because the total system volume including the HTS magnet, cryostat, and cryo-cooler is smaller than those of other LTS magnet systems or a conventional electromagnet, users can transport our system easily and safely. The low magnetic field error is another merit of our system. In general, the center magnetic field at room-temperature (RT) bore is approximately calculated by the pre-acquired magnet constant (center field per the operating current). The center field, however, can vary slightly under identical operating conditions due to many reasons, such as the characteristics of the magnetic components of the cryostat, the screening current used, and others [29; 30; 31; 32; 33]. To compensate for these types of field errors, we installed a cryogenic Hall sensor at the top of the REBCO magnet. The ratio of the center field and the magnetic field measured by the Hall sensor is pre-calculated, and users can determine the center field value easily with the Hall sensor. A drawing of the 3 T HTS EMPS system is shown in Figure 1. The system mainly consists of a cryostat, a REBCO magnet, a Gifford-McMahon (GM) 20 K cryo-cooler [34], thermal shields, HTS current leads, and thermal links for conduction-cooling, among other components. The 3 T REBCO magnet was designed using a genetic algorithm, often used in relation to the optimization theories, to minimize the consumption of the HTS tape [35; 36].