A magnetic lens, comprising: a magnetic yoke; and an exciting coil made up of litz wires, wherein the magnetic yoke is at outside of the exciting coil and surrounds the exciting coil; characterized in that the magnetic lens further comprises: a power supply controlling system, arranged to supply power to the exciting coil and control flow directions and magnitudes of currents of the exciting coil, wherein each of the litz wires comprises a first group of wires and a second group of wires; and wherein the power supply controlling system comprises: a first power supply controller, arranged to: supply power to the first group of wires, control currents in the wires in the first group to have the same flow direction, and adjust magnitudes of the currents in the wires in the first group, to vary the magnetic field intensity of the magnetic lens to achieve a required magnetic field intensity; and a second power supply controller, arranged to: supply power to the second group of wires and adjust magnitudes of currents in the wires in the second group to keep the total thermal power dissipation of the first group of wires and the second group of wires within the litz wire unchanged even when the magnetic field intensity of the magnetic lens varies; wherein the second power supply controller is further arranged to: control the currents in the wires in the second group to make a sum of magnitudes of currents with a positive flow direction equal to a sum of magnitudes of currents with a negative flow direction, such that a magnetic field generated by the second group of wires is zero. a magnetic yoke; and an exciting coil made up of litz wires, wherein the magnetic yoke is at outside of the exciting coil and surrounds the exciting coil; characterized in that the magnetic lens further comprises: a power supply controlling system, arranged to supply power to the exciting coil and control flow directions and magnitudes of currents of the exciting coil, wherein each of the litz wires comprises a first group of wires and a second group of wires; and wherein the power supply controlling system comprises: a first power supply controller, arranged to: supply power to the first group of wires, control currents in the wires in the first group to have the same flow direction, and adjust magnitudes of the currents in the wires in the first group, to vary the magnetic field intensity of the magnetic lens to achieve a required magnetic field intensity; and a second power supply controller, arranged to: supply power to the second group of wires and adjust magnitudes of currents in the wires in the second group to keep the total thermal power dissipation of the first group of wires and the second group of wires within the litz wire unchanged even when the magnetic field intensity of the magnetic lens varies; wherein the second power supply controller is further arranged to: control the currents in the wires in the second group to make a sum of magnitudes of currents with a positive flow direction equal to a sum of magnitudes of currents with a negative flow direction, such that a magnetic field generated by the second group of wires is zero. a power supply controlling system, arranged to supply power to the exciting coil and control flow directions and magnitudes of currents of the exciting coil, wherein each of the litz wires comprises a first group of wires and a second group of wires; and wherein the power supply controlling system comprises: a first power supply controller, arranged to: supply power to the first group of wires, control currents in the wires in the first group to have the same flow direction, and adjust magnitudes of the currents in the wires in the first group, to vary the magnetic field intensity of the magnetic lens to achieve a required magnetic field intensity; and a second power supply controller, arranged to: supply power to the second group of wires and adjust magnitudes of currents in the wires in the second group to keep the total thermal power dissipation of the first group of wires and the second group of wires within the litz wire unchanged even when the magnetic field intensity of the magnetic lens varies; wherein the second power supply controller is further arranged to: control the currents in the wires in the second group to make a sum of magnitudes of currents with a positive flow direction equal to a sum of magnitudes of currents with a negative flow direction, such that a magnetic field generated by the second group of wires is zero. a first power supply controller, arranged to: supply power to the first group of wires, control currents in the wires in the first group to have the same flow direction, and adjust magnitudes of the currents in the wires in the first group, to vary the magnetic field intensity of the magnetic lens to achieve a required magnetic field intensity; and a second power supply controller, arranged to: supply power to the second group of wires and adjust magnitudes of currents in the wires in the second group to keep the total thermal power dissipation of the first group of wires and the second group of wires within the litz wire unchanged even when the magnetic field intensity of the magnetic lens varies; wherein the second power supply controller is further arranged to: control the currents in the wires in the second group to make a sum of magnitudes of currents with a positive flow direction equal to a sum of magnitudes of currents with a negative flow direction, such that a magnetic field generated by the second group of wires is zero.