상대론적 초고속전자회절 장치의 극초단 전자빔 길이 측정 기술 개발, Development of S-band RF Deflector to Measure Ultra-Short Electron Bunch Duration

by Sunjeong Park (Ph.D Candidate)

Thursday Nov 9, 2017, 2:00 PM → 4:00 PM Asia/Seoul

210-4호 (제1과학관)

Recently, direct observation of ultrafast dynamics in atomic scale has attracted a lot of interest for the possibility of a new scientific discovery. To investigate these ultrafast dynamics directly, mostly precise X-ray or electron diffraction patterns are used. X-FELs (X-ray Free Electron Lasers) or ultrafast electron diffraction (UED) systems have been developed to provide such an ultrashort photon or electron pulse with a high temporal and spatial resolution. Especially considering space charge effect, UED systems generating a few MeV energy of electron beam can operate at the femtosecond temporal resolution and atomic dimension of spatial resolution, even though they are very compact in size compared with X-FELs. In KAERI, a RF photocathode gun based UED system has been developed to conduct THz pump/e-beam prove experiments. The system is so designed as the electron beam emitted from the photocathode by an UV light of 100 fs pulse length is able to be accelerated up to 3 MeV in the gun. And the accelerated electron bunch passes through several magnetic lenses to be compressed that the time duration of electron bunch should be less than 100 fs at the sample position to observe as short dynamics as possible. In this thesis, the measurement process of ultrashort electron bunch duration in a relativistic electron diffraction system is discussed. In particular, among several methods of measurement, a RF deflector working on TM120 mode is considered due to its high performance in measuring not only longitudinal bunch duration but also timing jitter between electron bunches. When an electron bunch passes through the RF deflector, it would deflect to the vertical direction by the strong transverse magnetic field confined in the resonator. Then the vertically deflected electron bunch makes its image on the screen with its longitudinal length information. Making the best use of the RF deflector, the measured 2 pC electron bunch duration of the KAERI UED system was 69 fs and the timing jitter between electron bunch was 62 fs. To conclude, the contribution of the RF deflector to ultrafast electron bunch measurement is discussed and an outlook is given for the more shorter electron bunch length measurement in the future.

Thesis Advisor: Prof. Hongjoo Kim