A method for supplying a jet of abrasive fluid for the purpose of providing a borehole by removing earth formation material through abrasion comprises a drill string and a drilling assembly connected to the drill string. The drilling assembly comprises a jetting device with a mixing space, a drilling fluid, a particle inlet, an abrasive fluid outlet for discharging a mixture of drilling fluid and magnetic particles, and a magnetic particle circulation system comprising a supporting surface which is exposed to a return stream along the drilling assembly. The method includes fixing a magnetic device with respect to the supporting surface, selecting a magnetic field density that which increases along the sloping supporting surface, attracting magnetic particles onto the supporting surface, and making the magnetic particles move over the sloping supporting surface (under the influence of the magnetic field of the magnetic device.