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Ultrafast Electron Diffraction (UED)
The UED employs a 1.6-cell photocathode RF gun operating at a resonant frequency of 2856 MHz to produce 2-5 MeV electrons. A frequency-tripled Ti:Sapphire laser (266 nm) generates the femtosecond electron pulses through the photoelectric effect off a copper cathode. The facility RF system operates at a maximum repetition rate of 48 Hz and experiments sensitive to sample heating or other average power effects may operate at arbitrarily lower rates. While high charge per pulse of several picocoulombs is possible, charge must usually be lower to avoid sample damage and to achieve lower emittance and better angular resolution of the diffraction pattern.
The drive laser presently produces NIR pulses that are split for UED sample pumping and for frequency-tripling to produce the probe pulses for the photocathode. The drive laser amplifier stretcher and compressor optics are designed for bandwidth supporting down to 50 fs pulses, and seed pulses from the current 160-fs oscillator should undergo negligible broadening. Spectral and temporal measurements indicate that further optimization of the dispersion of the pulse stretcher and compressor will reduce the NIR pulsewidth from the present 245 fs FWHM down to 160 fs. A further improvement in instrument temporal resolution will be achieved in upcoming maintenance periods when a newer broadband seed oscillator will be installed to enable pulses down to 100 fs to be amplified. The e-probe is typically shortened via compression through the acceleration process.
The electron beam diagnostics consist of a movable Faraday cup and a beam profile monitor. The sample chamber has 10 sample slots with cryo-capability and is equipped with a x-y-Φ motorized manipulator. The sample requirement is the same as conventional TEM with a 3 mm sample-diameter.
Four meters downstream of the diffraction chamber is the high efficiency detector, designed for high energy electrons. Cryogenic capability with 10 sample slots. The diffraction pattern is imaged from the phosphor screen to a downstream 45 degree in-vacuum mirror through a viewport by a 50 mm focal length f/#=0.95 lens onto a video camera (Andor iXon 897). The camera utilizes a back-illuminated electron multiplication
CCD (EMCCD) array composed of 512 x 512 pixels each of 16 x 16 micron dimensions. Total dark current plus readout noise is effectively less than one electron, and quantum efficiency is greater than 90%. The EMCCD technology allows near single-photon sensitivity to be achieved with the electron count per pixel digitized to 16 bits of resolution and dynamic range. The camera maximum readout rate is specified as 56 frames per second, which should allow recording of every electron pulse diffraction pattern at the 48-Hz maximum repetition rate of the MUED facility RF system.
Downstream of the Andor detector, which transmits the undiffracted electrons through a central hole to reduce camera saturation (see figure below). there is a diagnostic port with Faraday cup to permit measurement of transmitted charge and beam loss, which is normally very small.
BNL ATF UED Source Parameters (Typical Operation)
|Beam energy, MeV||3|
|N e- per pulse||1.25 E+6|
|Temporal resolution, fs||180|
|Beam size diameter, µm||300 (100 best)|
|Max repetition rate, Hz||5-48|
|N e- per sec per µm2||88-880|