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The free electron laser (FEL) at Osaka University can be continuously varied over a range from 5.0 to 20.0 um when using the 30 MeV electron beam. The FEL has a double pulse structure. The structure consists of a train of macropulses with a pulse width of 15 us, and each macropulse contains a train of 330 micropulses with a pulse width of 5 ps. The FEL's tunability and short pulse make possible new medical applications, such as investigating protein dynamics and ablating soft tissues. Precise control of the micropulse train is essential for FEL medical applications because macropulses of long pulse duration lead to undesirable thermal effects. An FEL pulse control system, using an acousto-optic modulator (AOM), was developed to investigate the non-thermal effects of FEL on living tissues. This system provides efficiency (~ 70%) and a fast switching speed (> 200 ns), and we predict that FEL will serve as a novel tool in many new applications.
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