  Motional Stark effect (MSE) spectroscopy represents a unique diagnostic tool
capable of determining the magnitude of the magnetic field and its direction in
the core of fusion plasmas. The primary excitation channel for fast hydrogen
atoms in injected neutral beams, with energy in the range of 25-1000 keV, is
due to collisions with protons and impurity ions (e.g., He$^{2+}$ and heavier
impurities). As a result of such excitation, at the particle density of
10$^{13}$-10$^{14}$ cm$^{-3}$, the line intensities of the Stark multiplets do
not follow statistical expectations (i.e., the populations of fine-structure
levels within the same principal quantum number $n$ are not proportional to
their statistical weights). Hence, any realistic modeling of MSE spectra has to
include the relevant collisional atomic data. In this paper we provide a
general expression for the excitation cross sections in parabolic states within
$n$=3 for an arbitrary orientation between the direction of the motion-induced
electric field and the proton-atom collisional axis. The calculations make use
of the density matrix obtained with the atomic orbital close coupling method
and the method can be applied to other collisional systems (e.g., He$^{2+}$,
Be$^{4+}$, C$^{6+}$, etc.). The resulting cross sections are given as simple
fits that can be directly applied to spectral modeling. For illustration we
note that the asymmetry detected in the first classical cathode ray experiments
between the red- and blue-shifted spectral components can be quantitatively
studied using the proposed approach.
