  We discuss the diagnostics available to study the 5-10 MK plasma in the solar
corona, which is key to understanding the heating in the cores of solar active
regions. We present several simulated spectra, and show that excellent
diagnostics are available in the soft X-rays, around 100 Angstroms, as six
ionisation stages of Fe can simultaneously be observed, and electron densities
derived, within a narrow spectral region. As this spectral range is almost
unexplored, we present an analysis of available and simulated spectra, to
compare the hot emission with the cooler component. We adopt recently designed
multilayers to present estimates of count rates in the hot lines, with a
baseline spectrometer design. Excellent count rates are found, opening up the
exciting opportunity to obtain high-resolution spectroscopy of hot plasma.
