The figure shows a region of length ‘l’ with a uniform magnetic filed of 0.3T in it and a proton entering the region with velocity $4 \times {10}^5 m{s}^{-1}$ making an angle ${60}^0$ with the field. If the proton completes 10 revolution by the time it cross the region shown, ‘l’ is close to (mass of proton = $1.67 \times {10}^{-27}$ kg, charge on the proton = $1.6 \times {10}^{-19}$C)