A generator rotor assembly (42) comprises a cylindrical ring structure (46) defining a central hollow portion and arranged to rotate around a rotational axis. The cylindrical ring structure (46) comprises a plurality of permanent magnet packages (48) arranged coaxially around the rotational axis, the permanent magnet packages (48) comprising a plurality of coaxially stacked ring-shaped segmented layers (80), a plurality of tie rod holes (86) and a plurality of tie rods (54). The coaxially stacked ring-shaped segmented layers (80) comprise a plurality of contiguous segment sheets (82) arranged around the rotational axis to form the ring-shaped layer, the stacked layers (80) being staggered such that segment breaks between two contiguous segment sheets (82) in one of the layers are angularly offset with respect to segment breaks between two contiguous segment sheets (82) in an adjacent layer. The tie rod holes (86) extend axially through the layers of the permanent magnet packages (48), wherein the plurality of tie rod holes (86) of adjacent permanent magnet packages (48) are complementary in size and position, such that a plurality of tie rod bores is defined. The tie rods (54) extend through respective ones of the plurality of tie rod bores.