  The Terahertz Intensity Mapper (TIM) is designed to probe the star formation
history in dust-obscured star-forming galaxies around the peak of cosmic star
formation. This will be done via measurements of the redshifted 157.7 um line
of singly ionized carbon ([CII]). TIM employs two R $\sim 250$ long-slit
grating spectrometers covering 240-420 um. Each is equipped with a focal plane
unit containing 4 wafer-sized subarrays of horn-coupled aluminum kinetic
inductance detectors (KIDs). We present the design and performance of a
prototype focal plane assembly for one of TIM's KID-based subarrays. Our design
strictly maintain high optical efficiency and a suitable electromagnetic
environment for the KIDs. The prototype detector housing in combination with
the first flight-like quadrant are tested at 250 mK. Initial frequency scan
shows that many resonances are affected by collisions and/or very shallow
transmission dips as a result of a degraded internal quality factor (Q factor).
This is attributed to the presence of an external magnetic field during
cooldown. We report on a study of magnetic field dependence of the Q factor of
our quadrant array. We implement a Helmholtz coil to vary the magnetic field at
the detectors by (partially) nulling earth's. Our investigation shows that the
earth magnetic field can significantly affect our KIDs' performance by
degrading the Q factor by a factor of 2-5, well below those expected from the
operational temperature or optical loading. We find that we can sufficiently
recover our detectors' quality factor by tuning the current in the coils to
generate a field that matches earth's magnetic field in magnitude to within a
few uT. Therefore, it is necessary to employ a properly designed magnetic
shield enclosing the TIM focal plane unit. Based on the results presented in
this paper, we set a shielding requirement of |B| < 3 uT.
