  It has been theoretically predicted that the \ExB drift caused by the
spontaneously generated potential in scrape-off-layers (SOLs) and divertors in
tokamaks is of a similar size to the poloidal component of the parallel flow
and turbulent flow, thereby it significantly impacts on the plasma transport
there. Many experiments indeed have implied the role of the electric potential,
however, its direct observation through its \ExB flow measurement has never
been realized because the drift velocity ($10^2$--$10^3$ m/s) is significantly
below the detection limit of existing diagnostics. To realize a cross-field ion
flow measurement, variety of systematic uncertainties of the system must be
narrowed down. Here, we develop a conceptual design of the Doppler spectrometry
that enables to measure the impurity flows with $10^2$-m/s accuracy, based on
an in-situ wavelength-calibration techniques developed in astrophysics field,
the iodine-cell method. We discuss its properties and applicability. In
particular, the scaling relation of the wavelength accuracy and various
spectroscopic parameters is newly presented, which suggests the high importance
of the wavelength resolution of the system. Based on transport simulations for
the JT-60SA divertor, the feasibility of the system is assessed.
