The history of the power applied to the target during a typical PSII
implant consists of millions of cycles, each of which can be
represented by the dashed line in Figure 1. (An
idealized power cycle distribution, used for subsequent analysis, is
shown by the solid line in this figure.) This power history, which is
not shown to scale, shows a finite current rise time of about 1
s, a flat-top current with a duration of about 8
s, a
current fall time of about 1
s, and then a zero-current period of
approximately 10 ms. Hence, there is power to the target during only
about one one-thousandth of the total process time. These are typical
numbers for a PSII implant, but they will vary by a factor of 2-5,
depending on the hardware and on the desired implant conditions.
Based on this curve, the PSII process can be thought of as producing a
very high power density during a short power pulse, and then no power
during the rest of the cycle.
This power history complicates temperature prediction for PSII targets. The target power is cycled about 100 times per second for periods ranging from a few minutes to many hours. Hence a numerical code for predicting target temperatures must follow many time steps for a complete implant. Fortunately, the temperature increase during a single pulse is often quite small, so the simulation can average the power over a cycle and consider the process as one of constant power. The limitations of this assumption will be considered in a later section.