Another observation is that one would like the , ratio to approach 1 by either going to an accelerator only device or to have a relatively large payload mass for a closed loop accelerator. This would decrease and thereby increase at the expense of going to a lower duty cycle and larger peak accelerator power which may not be desirable. SCR's have been the switching element used in mass drivers since their inception due to their very long lifetimes. Since SCR's are power limited compared with triggered vacuum gap switches and others, this has forced mass drivers to be a relatively low acceleration (lOOOg's) device. Most mass driver designs of the past have been 0.5 to 5.0 Km in length due to this limitation by SCR's. Because the early mass driver designs were for missions requiring very long life, high throughput rates, and no wear, SCR's seemed the logical choice. For missions that are less demanding in throughput rates and which permit some wear and periodic repair and replacement, triggered vacuum gap switches and fiber brushes can be used instead of SCR's. Other considerations besides specific mass, voltage and current are required to determine the suitability of an SCR to a particular drive circuit . These include: di/dt at turn on (typically 8001200A/us maximum), dV/dt (typically 100,000 - 400,000 A2 sec maximum), peak surge forward current (typically 5000-10,000 A maximum) and finally turn off time (typically 20 Us for inverter SCR's and up to 120 us for phase control SCR's). Voltages for SCR's are typically 1200 V max with some going to 2.2 KV and 3.3 KV, but these are phase control SCR's and have slower turn off times. Data used in the preceeding specs, as well as a source of other specs, comes from data sheets for the IR 420PB, 470PB, 55OPB, 161RM, 161RL, 250PAM, and 250 PAL series SCR's;
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