Magnetic field inside a torroid of n no. of turns having current I and radius r is B.What is the %change in magnetic field if a metal core of relative permeability 6.8×10-5 (5 is the -ve power of 10) is placed inside the torroid?

The measurement ofpeaklaserintensitiesexceeding 10(20) W/cm(2) is in general a very challenging task. We suggest a simple method to accurately measure such highintensitiesup to about 10(23) W/cm(2), by colliding a beam of ultrarelativistic electrons with thelaserpulse. The method exploits the high directionality of the radiation emitted by ultrarelativistic electrons via nonlinear Thomson scattering. Initial electron energies well within the reach oflaserwake-field accelerators are required, allowing in principle for an all-optical setup. Accuracies of the order of 10% are theoretically envisaged