A THIN SUPERCONDUCTING SOLENOID MAGNET FOR BALLON EXPERIMENTS IN SPACE SCIENCE Akira Yamamoto KEK, High Energy Accelerator Research Organization, Tsukuba, 305-0801, Japan Abstract A very thin and transparent superconducting solenoid magnet is being developed to investigate cosmic-ray antiparticles in the Universe. The uniform magnetic field is provided in a cosmic-ray particle detector system, BESS-Polar, for high-altitude long duration ballooning at Antarctica. The coil is wound with high-strength aluminum stabilized superconductor which has been recently much advanced in mechanical strength while keeping electrical resistivity low enough for the coil stability. The progress of the high-strength aluminum stabilized superconductor and the development of the thin solenoid magnet are described. 1. INTRODUCTION The Balloon-borne Experiment with a Superconducting Solenoid Magnet Spectrometer, BESS, has been carried out, as a US-Japan space science cooperation program, since 1993 [1-2]. Figure 1 shows a picture just before launching of the BESS spectrometer, in northern Canada, 2002. It aims at highly sensitive search for cosmic-ray antiparticle and for the novel primary origins such as evaporation of Primordial Black Holes (PBH) initiated in the early history of the Universe [3.4]. The search for antihelium to investigate the asymmetry of particle/antiparticle composition in the Universe is also a major fundamental question in the history of the Universe [5]. Fig. 1 Balloon launching of the BESS superconducting magnet spectrometer. It provides a central magnetic field of 1T maintained constant in persistent mode. TOF Counters Solenoid Jet chamber Middle TOF Inner DC Silica Aerogel 00.51m Cherenkov TOF Counters Fig. 2 Cross section of the BESS-Polar spectrometer with a thin superconducting solenoid magnet. The BESS spectrometer features a cylindrical detector configuration with a thin and transparent superconducting solenoid magnet. Most cosmic ray particles can freely traverse the spectrometer [6,7]. The unique cylindrical configuration enables the spectrometer to have a large geometrical acceptance of ~ 0.3 m2.sr efficiently in a compact spectrometer volume. The BESS spectrometer with a total weight of 2 ton has been launched by using a large balloon, and flown at high altitudes above 36 km, in northern Canada, as an ideal location to measure low energy cosmic rays with the minimum effect from the earth’s magnetic field which comes in/out in the vertical direction. Seven flights have been successfully carried out so far, measuring, for the first time, the low energy spectrum of antiprotons, and searching for antihelium at the highest sensitivity ever achieved [8.9,10]. To extend the scientific objectives in this ideal experimental environment, the “BESS-Polar program” is being prepared to realize long duration balloon flights around the south pole in Antarctica, where one can expect the maximum flux of low-energy cosmic-ray charged particles coming down to the earth, and continuous day-light helps guarantee very stable floating of the balloon [11]. The BESS- Polar spectrometer is being developed to achieve an ultimately transparent spectrometer to maximize the sensitivity and the resolution of the low energy particles and a very compact spectrometer to be launched in Antarctica. The BESS-Polar thin superconducting solenoid is designed to provide a central magnetic field of 1 T in a cylindrical volume of 0.9 m in diameter and 1 m in length with a half wall material density of < 1 g/cm2 for most of cosmic-ray particle to pass through without interactions in the coil wall, and to allow a continuous operational time period of ≥ 2 weeks in floating. An acceleration load (shock) of 10 G must be taken into account in the design against dynamic ballooning conditions. Figure 2 shows a cross section of the general BESS-Polar spectrometer. The thin solenoid coil is being developed as a key component in the spectrometer. This seminar gives a brief introduction for the thin solenoid and fundamental technology such as high strength aluminum stabilizer, as well as progress in development of the extremely thin superconducting solenoid to be launched for long duration flights at Antarctica. 2. THIN SOLENOID MAGNETS 2.1 Fundamentals The concept of the thin superconducting solenoid magnet is to provide maximize magnetic field with minimum amount of material in the coil and cryostat. The radiation thickness, X, is a useful scale the thinness and transparency of the solenoid coil and it is determined with an exponential decay (1/e) of electron energy loss in its passing through the material [12]. In large-scale solenoid magnets, it is primarily determined by the thickness of material for the conductor stabilizer and the coil support cylinder. As a simple example, the mechanical hoop stress and necessary thickness for an ideal solenoid coil (with no radial field component) are given by -1 -1 2 σh = t • r • p = t • r • B /2µ0 2 -1 -1 X = t / X0 = {r • B /2µ0}σ h • X0 where p is magnetic pressure, t is coil thickness, r is coil radius, B is magnetic field, σh, is hoop stress, and X0 is one radiation length of the material, respectively. In case of the air-core solenoid having the radial field component, additional axial stress, σz (~ 1/2σh), needs to be taken into account. In a simply conservative approach, a stress intensity, σSI = σh-σz ~1.5σh, should be taken into account in the necessary coil thickness. The strength of the aluminum stabilizer and support cylinder is required to be sufficiently stronger than σh or σSI. In an ultimately thin solenoid design as described below, an approach can be made by using the so-called “equivalent stress”, given by σeq = [{(σh- 2 2 2 1/2 σz) +σh +σz }/2] , which should be typically 10~20 % lower than σSI. The thickness and the stress of the coil are related with a ratio of the electromagnetic stored energy, E, to the effective coil mass, M, defined by E/M ={ ∫ B/2µ0 dv} /{ d•Vcoil} ~ σh / 2d where d is effective density of the coil to be able to absorb the stored energy. The E/M ratio in the coil approximately equivalent to “enthalpy” of the coil, H, and it determines the average coil temperature after the stored energy absorption during a quench as follows E/M = H (T2) - H (T1) ~ H (T2). where T2 is average coil temperature after the full energy absorption in the quench, and T1 is the initial temperature. The parameter of E/M ratios of 5, 10, and 20 kJ/kg approximately correspond to ~ 65, 80 K, and 100 K, respectively, as shown in Fig. 3. From a viewpoint of superconducting stability, the small resistivity, ρ, of pure-aluminum at 4.2 K is a very important advantage. A useful parameter is the so-called “minimum quench energy” (MQE) of the superconducting coil. In a simplified one-D model this may be characterized by -1 1/2 MQE = γ C • ∆T • A (Jnρ) • {2 L0 T •∆T} where γC is the specific heat of the stabilizer, ∆T is temperature difference between normal state and superconducting state, A is cross sectional area of stabilizer, Jn is the current density in the -8 -2 stabilizer after normal transition, Lo is the Lorentz number (=2.4x10 ΩWK ), and T is the average temperature during the transition. The lower resistivity contributes to increase the stability and reduces the amount of stabilizer needed in the coil. (QWKDOS\ > MNJ @ 7HPSHUDWXUH>.@ Fig. 3 Enthalpy of aluminum as a function of temperature. 2 2 In summary, the coil thickness scales with r•B /σSI or r•B /(E/M), and the aluminum stabilizer is required to be mechanically strong but with low electrical resistivity at 4.2 K (i.e. having large residual resistivity ratio, or RRR). 2.2 Aluminum stabilized superconductor Aluminum stabilized superconductor has been developed since the1970s, and widely used in large- scale detector magnets for high energy physics [12, 13]. Since1980s, a special effort has been devoted to the development of high strength aluminum stabilized superconductor having low resistivity at 4.2 K, in a series of R&D programs for ASTROMAG [14], SDC [15], ATLAS Central Solenoid [16, 17], and BESS-Polar [18,19]. The development was focused on achieving uniform reinforcement of the conductor/stabilizer in order to minimize the strain in the coil and maintain mechanically fully elastic conditions. Ideally it should be as strong as the support cylinder. The high strength aluminum stabilizer has been developed using a combined technology of “micro-alloying” with “cold-work hardening” [12, 18]. The micro-alloying contributes to mechanical reinforcement and its long-term stability against annealing effects in subsequent processes such as coil curing. A list of possible additives for this micro-alloying is summarized in Table 1, in comparison with impurities to be eliminated. Some additive metals such as Mg, Si, Cu, and Zn have a common feature of contributing little to electrical resistivity under the so-called “solid-solution” condition. The additive provides internal stress, and weakly traps dislocations in the base metal. It contributes to the mechanical reinforcement with smaller contribution to the resistivity. On the other hand, a metal such as Ni has a different feature of much lower solubility. Beyond this threshold, the additive is crystallized and/or precipitated according to the process in the base metal as shown in Fig. 4 [18]. Table 1. Possible additives into pure Al in micro-alloying process Additive A d Solubility Resistivity contribution (in solution / crystallization) [g/cm3] [% wt] [10-12 Ω.m/wppm] • solid solution: Mg 24 2.4 17.4 0.54 0.22 Si 28 2.6 1.65 0.7 0.088 Cu 64 8.9 6 0.35 0.03 Zn 65 7.1 83@ 400C 0.10 0.023 • crystallization/precipitation Ni 59 8.8 0.05 @640C 0.81 0.061 <0.006 @<500C •major impurities to be eliminated Cr 52 6.9 0.72 4.4 0.18 Mn 55 7.2 1.5 3.5 0.34 Fe 56 7.9 0.05 2.6 0.058 The crystallized/precipitated additive creates barriers against dislocation with little increase of resistivity.