  Ultra-intense ultra-short laser is firstly used to irradiate the
capacity-coil target to generate magnetic field. The spatial structure and
temporal evolution of huge magnetic fields were studied with time-gated proton
radiography method. A magnetic flux density of 40T was measured by comparing
the proton deflection and particle track simulations. Although the laser pulse
duration is only 30fs, the generated magnetic field can last for over 100
picoseconds. The energy conversion efficiency from laser to magnetic field can
reach as high as ~20%. The results indicate that tens of tesla (T) magnetic
field could be produced in many ultra intense laser facilities around the
world, and higher magnetic field could be produced by picosecond lasers.
