西门子MRI磁体中持续电流开关工作状态的外部识别与电热建模
Identification and electrothermal modeling of persistent current switch operating regimes in a Siemens MRI magnet using external electrical measurements
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中文总结 AI 辅助
该研究识别出西门子MRI磁体PCS存在自维持电阻态，建立其电热模型，将电流RMSE降低9.4倍，明确了持续电流精度的控制因素及自维持边界。
中文摘要 AI 辅助
持续电流开关（PCS）用于闭合MRI磁体的超导回路；加热器使其变为电阻态，以便外部电源改变磁体电流。PCS控制通常被视为二元状态：加热器开启时为电阻态，加热器关闭时为超导态。对西门子MAGNETOM Avanto磁体的MPS2服务电源（Digit Systems）的外部电流和电压日志显示，指令无法唯一确定PCS状态：加热器关闭后，焦耳自热可维持电阻态。设计专用操作将两种电阻态区分开：加热器通电时，支路电阻在0.5V以下几乎恒定，为1.86至2.19Ω；加热器关闭后，自维持态遵循关系R_sw=0.65|V_node|^1.5（R_sw单位为Ω，V_node单位为V），在0.4V时比加热器开启时的电阻低12倍。两种状态均满足同一电热关系R_sw=C(P_h h + V_node²/R_sw)^a，其中h为二元加热器指令，电阻范围达690倍。研究发现两点：其一，支路电阻与线圈电感决定重分布时间常数τ=L/R_sw；因此最终保持时长控制持续电流精度。对其他西门子磁体的18次服务会话分析（未采用Avanto的系数）显示，中位保持时长为13τ，中位剩余支路电流为线圈电流的8ppm。其二，PCS特性与外电路负载线的交点设定自维持边界，跨会话估计值为0.062V，介于观测到的重捕获电压0.06V与保持电压0.07V之间。动态仿真中，两个固定系数特性R(V,h)将电流均方根误差（RMSE）从1.449A降至0.154A（降低9.4倍）；省略显式加热器输入的模型相比仅含电感的模型无改进。
英文摘要
The persistent current switch (PCS) closes the superconducting loop of an MRI magnet. During ramping it is commonly treated as a binary element: with the heater on, the PCS branch is assumed effectively open; after heater turn-off, it is assumed superconducting. Field measurements with an MPS2 service power supply (Digit Systems) on a Siemens MAGNETOM Avanto show that the PCS remains electrically active and that its resistive state can persist after heater turn-off through Joule self-heating. Dedicated maneuvers identified two resistive regimes. With the heater on, the low-voltage resistance is nearly constant; after heater turn-off, self-heating produces a power-law voltage dependence. Both are described by a single effective electrothermal relation, $R_{sw}=C\,(P_h h+V_{node}^2/R_{sw})^a$, where $h$ is the recorded heater state, spanning a 690-fold resistance range. Final-hold data from 17 Siemens magnet sessions confirmed the voltage dependence of effective PCS resistance. Since branch-current relaxation follows $τ=L/R_{sw}$, the required hold time depends on switch state rather than a fixed delay; the median margin after source settling was $3.6\,τ$. The intersection of the PCS characteristic with the external-circuit load line defines the boundary of self-sustained resistive operation. For Avanto, the predicted 0.062 V threshold lay between superconducting recovery at 0.06 V and resistive-state persistence at 0.07 V, and retrospectively classified all 24 archived segments correctly. Including both PCS resistive regimes in the dynamic model reproduced the observed current redistribution: RMSE decreased from 1.449 to 0.154 A, a factor of 9.4. This improvement required explicit representation of heater state.
