霍爾效應推進器之毫牛頓推力測試平台

Abstract

霍爾推進器的發展源於1960年代，是離子推進器的一種，可作為太空船和人造衛星在太空中航行或調整軌道的動力來源。由於具高比值的霍爾推進器可攜帶酬載的質量較電網式離子推進器高而受到歐美國家的關注與發展。 其工作原理是推進氣體從陽極噴入環形艙體，中性氣體被離子化產生出電子和離子，從陰極噴出的電子受軸向電場作用向陽極漂移，漂移的過程中受到艙體外電磁線圈提供的徑向磁場影響而產生的E×B力量所束縛進而產生霍爾電流，電子在軸向的運動受到限制，增加了在艙體內滯留的時間，增加中性氣體離子化的效率。由於離子質量遠大於電子，受到磁場的影響很小，在準中性電漿中主要受靜電力向艙體外加速飛出，產生推力。其推力約在20毫牛頓，約為霍爾效應推進器質量的千分之一倍。 本實驗研究使用扭轉平衡儀，利用兩撓性樞軸支撐旋轉樑的中心，將推進器安裝在旋轉樑一端使其推力產生旋轉。感測器量測其旋轉產生之位移變化，進而用回授控制來控制馬達輸出推力，平衡旋轉樑，使旋轉樑回到零點。另一端則安裝平衡重物，平衡推進器本身的重量，使推力量測不會受到環境的影響。 實驗推力測試平台在成功大學太空與電漿科學研究所中進行組裝與測試，結果顯示量測的精度可到0.2毫牛頓，量測範圍在0毫牛頓至100毫牛頓之間。

Hall thruster, which is a type of the ion thruster, has been developed since 1960s. Since Hall thrusters have higher specific impulse (Isp) than any traditional propulsion systems, it has been used as the propulsion source of spacecraft and satellites for deep space missions and orbit transfers. Due to its high efficiency, a lot of space related researchers such as Europe and America pay much attention and spend a lot of efforts in its development. A typical Hall thruster consists of a chamber, magnetic coils, an anode and a cathode neutralizer. The working gas is introduced into the circular chamber from the anode and is ionized in the chamber. Electrons emitted from the cathode drift to anode by the axial electric field. Electrons are then confined by the E x B drift mechanism. The constraint produces a current called Hall current. Since electrons are confined in the chamber, the residence time of electrons in the chamber increases and ionization efficiency also improves. Since the mass of ions is much greater than electrons, it is hardly affected by the magnetic field. Ions are accelerated by the electrostatic force in the quasi-neutral plasma and exit the chamber to produce the thrust. The thrust level studied in this work is on the order of 20 mN which is about one-thousandth of the Hall thruster’s mass. In the present thesis, to measure the thrust produced by the Hall thruster, a torsional balance stand is designed, fabricated and tested. The balance arm is supported by two flexural pivots which are located at the center. The thruster and a counter weight is mounted at opposite ends respectively. As the thruster produces thrust, torque is produced which causes the arm to rotate. A linear displacement sensor is used to sense the displacement caused by the rotation. Then, a voice coil motor provides a force to compensate the rotation to move the arm back to the balance point. This is generally referred to as null balance operation. The stand is assembled and tested in the vacuum chamber of Plasma and Space Science Center (PSSC), National Cheng Kung University (NCKU). The test results show that the measurement range of the in-house stand is from 0 to 100 mN with a resolution of 0.2 mN.