However the experimental and numerical model [2] treats the solar array as a conductive plate completely covered with a sheath. The ions then interact with the whole plate and the electrons are completely repelled for large potentials. For real solar arrays where the sheath may not completely 'cover the array or where some parts of the array are positive, these assumptions on the ion and electron interaction with the array will not be valid. This motivates us to use a PIC (particle in cell) code to consider the problem of the ion drag on a model solar array consisting of a conductor surrounded by two dielectrics. The sheath from the conductor does not completely cover the array and the electrons can participate in the charging of the dielectrics. In this paper we discuss ion drag, its simulation by PIC codes and compare it to the results of Kuninaka & Kuriki [2]. Ion Drag The ion drag is considered in a control volume fixed on the charged body. At steadystate there shall be three components of the momentum flux. First, there will be the momentum flux of ions entering the control volume. These ions have momentum defined by the orbital velocity of the charged body. Second, there will be the momentum flux of ions going out of the control volume due to Coulomb scattering with the charged body. These ions have their trajectories modified by the electrostatic potential of the body but do not directly strike the body. Third, there will be the momentum flux of ions which are reflected from the body surface as neutrals and go out of the control volume. We assume all the ions which strike the surface are neutralized and are reflected after some momentum accommodation. Since the first and second components depend on the control volume, we combine these components into one and call them the ram-scattering component. We call the third component the reflection component. Interacting Limit A solar array is made up of interconnected solar cells with the interconnectors being biased and exposed to space. The model of Kuninaka & Kuriki [2] assumes that the electrostatic sheaths from all charged interconnectors overlap. Hence the entire array can be treated (as far as the incoming ions are concerned) as one charged body with a potential drop across the body equal to the potential difference across the array. We shall call this model the interacting limit since the sheaths from the connectors all overlap and interact with each other. In their analysis they neglected the electrons (except that they kept quasineutrality in the far field), took the ions as cold so that the ion flow is supersonic and took the plasma as collisionless. They showed that the force on the array (modelled as a charged plate) depends only on the parameters O = eA0/((l/2)m,72orbltal), ^=Larray/(Forbiul/wPi), P, (7n and a, where A0 is the potential drop across the array, Larray is the length of the array, Mpt is the ion plasma frequency, Forbita) is the orbital velocity of the array, p is the angle of attack measured from the uncharged side of the array and crn(t) are the normal and tangential momentum accommodation coefficients. The dimensionless potential <I> measures the energetic effect of the potential of the array on the ions relative to their incoming kinetic energy. The dimensionless length £ measures the spreading of the sheath relative to the conductor size. When then the sheath does not spread much beyond the array relative to the array length. For the experimental results of Kuninaka & Kuriki showed that the force became independent of This is a
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