*SNAPIN* reads the magnetics waveform MV-VS-SL to obtain the
surface voltage *V*_{sur}. The surface voltage is comprised of two
components: a resistive part, required to sustain the toroidal current
against plasma resistivity, and an inductive part, which produces a
time rate of change of magnetic stored energy. Because high electron
temperatures are achieved in TFTR, the resistive equilibration
time is rather long--of order seconds. As a consequence, the current
profile, the inductance parameter *l* _{i}, and the magnetic stored
energy can have significant time derivatives long after the plasma
current has reached a flattop.

*SNAP* uses the voltage for two calculations: to calculate the ohmic
power input, and to calculate the plasma resistance. The appropriate
input for both calculations is *V*_{res}, the resistive component of
*V*_{sur}.

If no special action is taken, *SNAPIN* will simply pass along to
*SNAP* the voltage it reads from MV-VS-SL as the resistive
voltage, *V*_{sur}. This will cause *SNAP* to miscalculate the ohmic
input power and the plasma resistance. In beam heated plasmas this is
not a significant problem since typically , but
it can be important for ohmically-heated plasmas.

If you manually enter zero as the surface voltage in the magnetics
menu of *SNAPIN*, a special subroutine VSURLAM will be
activated to correct the *V*_{sur} value to obtain a better estimate of
the resistive component. VSURLAM reads in the time history of
*V*_{sur}, the Shafranov Lambda (), *I*_{p}, *R*, *a*,
the toroidal field current, and . It calculates
*l*_{i} from and . Then it
corrects *V*_{sur} to get *V*_{res} by subtracting the component associated
with *d*/*dt* of the magnetic stored energy. Also, an attempt is made
to correct for the fact that the surface voltage as reported by
waveform MV-VS-SL is D/Dt (poloidal flux between magnetic axis
and *a*), while the true surface voltage should be evaluated at fixed
toroidal flux.

The correction is:

where is the total toroidal flux in the plasma,
. The
VSURLAM routine allows you to use a smoothing time for the
waveforms which enter this calculation different from the averaging
time used for all other waveform reads in *SNAPIN*. This can be useful
because the surface voltage is quite noisy, and may require a longer
averaging time than most other diagnostic inputs (be careful to avoid
including the beam turn-on transient and periods when the VC
current is ramped. In large plasmas [ cm] which might
deposit heat on the outer RF limiter, the VC current is
often ramped up a few hundred msec before the start of auxiliary
heating).

Fri Jul 11 15:18:44 EDT 1997