  Synchrotron radiation observed in a quiescent TCV runaway discharge is
studied using filtered camera images targeting three distinct wavelength
intervals. Through the tomographic SART procedure the high momentum, high pitch
angle part of the spatial and momentum distribution of these relativistic
particles is reconstructed. Experimental estimates of the distribution are
important for verification and refinement of formation-, decay- and
transport-models underlying runaway avoidance and mitigation strategy design.
Using a test distribution it is demonstrated that the inversion procedure
provides estimates accurate to within a few tens of percent in the region of
phase-space contributing most to the synchrotron image. We find that combining
images filtered around different parts of the emission spectrum widens the
probed part of momentum-space and reduces reconstruction errors. Next, the SART
algorithm is used to obtain information on the spatiotemporal runaway momentum
distribution in a selected TCV discharge. The momentum distribution is found to
relax towards an avalanche-like exponentially decaying profile. Anomalously
high pitch angles and a radial profile increasing towards the edge are found
for the most strongly emitting particles in the distribution. Pitch angle
scattering by toroidal magnetic field ripple is consistent with this picture.
An alternative explanation is the presence of high frequency instabilities in
combination with the formation of a runaway shell at the edge of the plasma.
