Design of the precompression mechanism and gravity support for the central solenoid assembly in the JT-60SA tokamak

JT-60SAトカマク装置におけるセントラルソレノイドの予荷重機構と重力支持脚の機械設計

土屋 勝彦; 木津 要; 村上 陽之; 吉澤 憲生; 小出 芳彦; 吉田 清

Tsuchiya, Katsuhiko; Kizu, Kaname; Murakami, Haruyuki; Yoshizawa, Norio; Koide, Yoshihiko; Yoshida, Kiyoshi

JT-60SA装置の超伝導セントラルソレノイド(CS)は、4つのソレノイドモジュールから構成され、それぞれ独立した電流制御が可能である。そのため、各モジュールの間には反発力,引力が非一様に生じるため、トカマク本体に設置するにあたっては、このような電磁力にたえる支持構造が必要である。通常用いられるステンレス鋼で製作した場合、コイル運転条件である4K冷却時には、モジュールの収縮の方がステンレス鋼のそれよりも大きいため、支持構造物とモジュールの間にギャップを生じ、支持が不安定となる恐れがある。これを防ぐには、熱収縮や電磁力によるモジュールの変位によって生じうるギャップを見積り、これを打ち消すための予荷重を室温時に与えてやればよい。CSの支持構造物には、この予荷重を与えられる構造と、運転時の電磁力だけでなく、室温時の予荷重負荷時にも耐える機械的強度を担保することが求められる。これに基づき、プラズマ運転シナリオデータベースから最も大きな影響を受ける条件でも耐える構造設計とした。予荷重は一時的に設置する油圧ラムを使って印加するが、必ずしも電流中心を貫く場所に油圧ラムを設置してもモジュール面に一様に圧力がかからないことがわかり、均一性を高めるよう設置位置の最適化を行った。

JT-60SA is full superconducting tokamak that was constructed in JAEA Naka site in corporation with JAEA and F4E. The central solenoid (CS) assembly in JT-60SA consists of 4 modules of superconducting solenoid which has outer diameter of 2m and height of 1.6m. The currents for each module were independently controlled. CS was designed to produce enough flux to control the plasmas with 5.5 MA during 100 sec. Superconducting conductor for CS consists of NbSn strands. The support structure for CS assembly consists of the tie-plates (inner and outer), buffer zones and key-blocks. CS must be cooled down to 4K before charging, and modules will be shrunk during this process. The support structure made of stainless steel was also shrunk at 4K. Thermal expansion ratio of stainless steel, however, is different from that of modules, which would result in the gap between modules and supports. In order to cancel this gap, pre-compress mechanism needs to be introduced in the support structure for CS assembly. Mechanical pressure for the pre-compress will be controlled by hydraulic rams that are set at the top of each support. During the pre-compress process in which both key-blocks clamp the modules, tension works at the tie plates. The support structure for CS assembly, especially tie plates, should have sufficient mechanical strength to withstand the stress induced by the pre-compress at room temperature, not only to withstand the electro-magnetic force which was produced during the plasma operation. Space for installation of CS assembly is limited by TF coils, so that cross section of tie-plate is also limited. Final structure was successfully designed to adopt the stainless steel with 0.120.17 wt% of nitrogen content (SS316LN) for the material of the main parts of support structure.