  Highly charged ions (HCIs) are promising candidates for the next generation
of atomic clocks, owing to their tightly bound electron cloud, which
significantly suppresses the common environmental disturbances to the quantum
oscillator. Here we propose and pursue an experimental strategy that, while
focusing on various HCIs of a single atomic element, keeps the number of
candidate clock transitions as large as possible. Following this strategy, we
identify four adjacent charge states of nickel HCIs that offer as many as six
optical transitions. Experimentally, we demonstrated the essential capability
of producing these ions in the low-energy compact Shanghai-Wuhan Electron Beam
Ion Trap. We measured the wavelengths of four magnetic-dipole ($M$1) and one
electric-quadrupole ($E$2) clock transitions with an accuracy of several ppm
with a novel calibration method; two of these lines were observed and
characterized for the first time in controlled laboratory settings. Compared to
the earlier determinations, our measurements improved wavelength accuracy by an
order of magnitude. Such measurements are crucial for constraining the range of
laser wavelengths for finding the "needle in a haystack" narrow lines. In
addition, we calculated frequencies and quality factors, evaluated sensitivity
of these six transitions to the hypothetical variation of the electromagnetic
fine structure constant $\alpha$ needed for fundamental physics applications.
We argue that all the six transitions in nickel HCIs offer intrinsic immunity
to all common perturbations of quantum oscillators, and one of them has the
projected fractional frequency uncertainty down to the remarkable level of
10$^{-19}$.
