This happens at current levels in the micro-to milliampere, and the complete V-I curve of an arc then appears as in Fig. 3 Slope Ballast Resistance Battery Line(No Ballast) P Arc Line Battery ballast Line Figure 3. Complete V-I Arc Characteristic Assuming we wish to operate at point P on this line, if we simply connect the arc to a constant-voltage source, such as a battery, we obtain an unstable arrangement. This is because, if current, say, increases slightly above Ip, the arc would now demand less than the equilibirum voltage Vp which the source delivers, and so I would increae even (to the supply limit, or to current fluctuation would cause a rapid snap back to the stable operating point Q(at ver One solution is to insert a series resistance RB(ballast )and increase th open-circuit voltage to, say VB (Fig 3). Since the voltage available to the arc is now VR-RBI, the supply line cuts the arc line form above(Fig 3), and repeating the argument we notice stable operation at P. But, of course, we dissipate in the ballast the L (a-v)which leads to inefficient operation. Notice, however, that AC arcs can be efficiently ballasted by a series inductor, the familiar"choke in fluorescent fixtures Alternatively, one can use a current-regulated power supply, provided its regulating speed and authority are high enough. For space applications, this takes the form of high frequency solid state switching regulators, capable of stabilizing the arc with minimal losses(efficiency>90%). A series inductor can help with the high frequency part of the fluctuation spectrum 16.522, Space Propulsion Lecture 11-12 Prof. Manuel Martinez-SanchezThis happens at current levels in the micro- to milliampere, and the complete V-I curve of an arc then appears as in Fig. 3: Figure 3. Complete V-I Arc Characteristic Assuming we wish to operate at point P on this line, if we simply connect the arc to a constant-voltage source, such as a battery, we obtain an unstable arrangement. This is because, if current, say, increases slightly above Ip, the arc would now demand less than the equilibirum voltage Vp which the source delivers, and so I would increae even further, and run away (to the supply limit, or to destruction). Conversely, any negative current fluctuation would cause a rapid snap back to the stable operating point Q (at very low current). One solution is to insert a series resistance RB (“ballast”) and increase the source open-circuit voltage to, say VB (Fig. 3). Since the voltage available to the arc is now VB − RBI , the supply line cuts the arc line form above (Fig. 3), and repeating the argument we notice stable operation at P. But, of course, we dissipate in the ballast the power I p(VB − Vp ), which leads to inefficient operation. Notice, however, that AC arcs can be efficiently ballasted by a series inductor, the familiar “choke” in fluorescent fixtures. Alternatively, one can use a current-regulated power supply, provided its regulating speed and authority are high enough. For space applications, this takes the form of high frequency solid state switching regulators, capable of stabilizing the arc with minimal losses (efficiency>90%). A series inductor can help with the high frequency part of the fluctuation spectrum. 16.522, Space Propulsion Lecture 11-12 Prof. Manuel Martinez-Sanchez Page 7 of 18