  Charge density wave (CDW) correlations have recently been shown to
universally exist in cuprate superconductors. However, their nature at high
fields inferred from nuclear magnetic resonance is distinct from that measured
by x-ray scattering at zero and low fields. Here we combine a pulsed magnet
with an x-ray free electron laser to characterize the CDW in YBa2Cu3O6.67 via
x-ray scattering in fields up to 28 Tesla. While the zero-field CDW order,
which develops below T ~ 150 K, is essentially two-dimensional, at lower
temperature and beyond 15 Tesla, another three-dimensionally ordered CDW
emerges. The field-induced CDW onsets around the zero-field superconducting
transition temperature, yet the incommensurate in-plane ordering vector is
field-independent. This implies that the two forms of CDW and high-temperature
superconductivity are intimately linked.
