quantitatively the ion current characteristics of the experiment. The ion current ratio is enhancement of the ion current collected by the HVSA in biasing condition with respect to that in the ion saturation condition. In other words, it can be considered to be the area ratio of ram sheath surface with respect to the array surface since the ions are collected through the sheath surface. This understanding agrees with the physical meaning of Eq. (7). The ion current dependences on the CL parameter are different in the ram and in the airplane modes as shown in Fig. 14, and are explained as follows. In the case of the unbiased array, namely infinite CL parameter, the ion current ratio is equal to unity for the ram mode, whereas it equals zero for the airplane mode because of the vanishing frontal area of the model in the ion flow direction. The increment of the array potential or decrement of the array size decreases the CL parameter and thickens the ion sheath covering the model so that both curves of ion current converge gradually since the ions are collected through the virtual surface area of the ion sheath instead of the physical surface of the HVSA. The ion current ratio is also dependent significantly on whether or not the insulated surface looks toward the ram direction as seen in Fig. 5. Estimation for 2D/HV Experiment Figure 15 shows the CL dependence of the ion current ratio for the various solar array models at the ram mode. All data except for that of the baseline model converge to 0.36 at high CL value because of the electrode occupation rate. The 2D/HV system has a 6.4 m length array and can produce 570 V as maximum voltage in LEO. Using
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