Ultrafast Electron Sources for Diffraction and Microscopy Workshop December 12 th - 14 th 2012, California NanoScience Institute at UCLA Novel Ultrafast Electron Diffraction System (“streaked”-UED) Luigi Faillace

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Ultrafast Electron Diffraction Ultrafast electron diffraction (UED) has the potential for real-time imaging of structural changes on atomic length scales, thus promising to make a profound impact on a large area of science including biology, chemistry, nano and material sciences [*] *P. Musumeci et al., Relativistic electron diffraction at the UCLA Pegasus photoinjector laboratory, Ultramicroscopy 108 (2008) 1450– 1453 Probing electron beam ~200 fs rms long 1 pC 3.5 MeV IR laser pulse 1-10  J 40 fs rms Pump pulse 0.5 mJ 800 nm 0.1 mJ 400 nm 40 fs rms 12 bit camera f/0.95 lens coupling Lanex screen or MCP detector Collimating hole 1mm diameter Two axes x-y sample-holder movement RF gun UCLA/BNL/SLAC 6 MW GHz Pegasus pump and probe setup e- beam

Conventional vs. Relativistic Electron Diffraction Low-energy electrons (conventional electron gun) Compact system due to larger diffraction angle Low SNR (few electrons per bunch in order to reduce space charge >>> pulse broadening) Low SNR (few electrons per bunch in order to reduce space charge >>> pulse broadening) Thousands of pulses to obtain a good diffraction pattern Thousands of pulses to obtain a good diffraction pattern Need of a relativistic electron Gun Need of a relativistic electron Gun Longer diffraction camera length Longer diffraction camera length More intense electron bunches possible due to weaker space charge effects Single-shot measurement Conventional Relativistic Ultrafast Electron Sources for Diffraction and Microscopy Workshop

Streaked UED System Electron Energy100 keV Number of electrons/pulse10 7 Pulse Length (at the sample)20 ps Deflector RF power1 kW Deflector Nominal Voltage20 kV Temporal Resolution of System30 fs Innovations  Compromise between Conventional and Relativistic UED systems  Same physics of current UED systems but cheaper and more compact  UED measurements in small laboratories Pump laser pulse Diffracted e - bunch Ultrafast Electron Sources for Diffraction and Microscopy Workshop Main Parameters Originally proposed by P. Musumeci et al., P. Musumeci et al. RF streak camera based ultrafast relativistic electron diffraction. Review of Scientific Instruments (2009) vol. 80 pp

SUED System Block Diagram Ultrafast Electron Sources for Diffraction and Microscopy Workshop

Photoelectron gun (electrostatic design) SuperFish E (V/cm) 9.8 MV/m Breakdown risks are present inside the gun itself, but 100 kV is considered a safe value below the 250kV threshold (empirically determined*) that is actually the limit inside a gap (cathode- anode) of about 1cm. -2D simulations with code (approach from Eindhoven University of Technology**) -The replaceable cathode sample is held by a hollow cylindrical body made out of aluminum while the vacuum vessel, that the anode electrode is attached to, is stainless steel. -The isolation between cathode anode is realized by using an insulating cone Surface electric field distribution along gun surfaces (start: from and back to cathode center in a counterclockwise path) 3D model rendering *L. L. Alston, High Voltage Technology, Oxford University Press, **T. van Oudheusden. Electron source for sub-relativistic single-shot femtosecond diffraction. Ph.D. Thesis Dissertation (2010), Eindhoven University of Technology. Ultrafast Electron Sources for Diffraction and Microscopy Workshop

ASTRA (10 ps beam)EGUN (DC beam) (100μm input beam) 96 nm56 nm (50μm input beam) 50 nm7.1 nm Photoelectron gun (beam dynamics simulations) EGUN e- beam Emittance evolution Transverse size Ultrafast Electron Sources for Diffraction and Microscopy Workshop

Feed-through NEG pump TMP pump Window for back-illumination Stainless steel Vacuum vessel cathode anode insulator Aluminum holder 3D model of the 100 kV photo-gun, from SolidWorks Ultrafast Electron Sources for Diffraction and Microscopy Workshop Photoelectron gun (initial engineering)

Deflecting Cavity proportionality factor K between the temporal and transverse coordinate on the screen located at distance L from the cavity center. σ t is the rms pulse length and σ 0 is the spot size with the cavity voltage off minimum attainable temporal resolution σ0σ0 100μm Deflecting Voltage20 kV Drift length L50 cm K1 mm/ps Resolution30 fs Ultrafast Electron Sources for Diffraction and Microscopy Workshop

Deflecting Cavity (RF design) Optimized cell geometry. The use of nose cones allows the concentration of the field toward the center of the deflecting gap, which creates a stronger field and better deflection, especially in our case of slow electrons (β=0.54). Ultrafast Electron Sources for Diffraction and Microscopy Workshop

Input RF coupler Nose cone type cells Side cell Inter-cell coupling slot 3D model of the X-Band Deflector, from SolidWorks Ultrafast Electron Sources for Diffraction and Microscopy Workshop Deflecting Cavity (initial enginering)

Detector System Pictures of the the diffraction patterns will be taking by using a micro channel plate (MCP) detector that basically works as an electron amplifier in which the incoming electrons generate secondary electrons. In this case, the incoming electrons enter channels in which they are accelerated by an electric field and generate the secondary electrons. The size of the channels is on the order of 12 μm, which is the highest attainable spatial resolution. There are four plates of which three are connected to high voltage supplies and one is grounded. The four plates are respectively at -1 kV, 0 V, +1 kV and +3 kV. The accelerated electrons hit a phosphorous screen in which photons are produced due to the electrons from the channels. This image is recorded by a CCD camera. Ultrafast Electron Sources for Diffraction and Microscopy Workshop Diffraction pattern from a gold foil at Pegasus Lab

Complete System Evolution of the beam size and the horizontal emittance are shown (from GPT). Emittance compensation is obtained after the solenoid (0.3μm from z=0.45m to z=1m).. beam images at the screen Ultrafast Electron Sources for Diffraction and Microscopy Workshop

Synchronization Ultrafast Electron Sources for Diffraction and Microscopy Workshop Electron beam, laser and RF cavity synchronized by locking 80 MHz laser oscillator cavity with sub-harmonic of the RF frequency by commercial active phase lock loop.

Photo-gun NEG pump Solenoid Laser-sample Interaction chamber RF deflector MCP CCD camera HV feed-through Steering magnets 3D model of the SUED system, from SolidWorks Tasks to be performed at UCLA Ultrafast Electron Sources for Diffraction and Microscopy Workshop Complete System (initial engineering)

The SUED system will fabricated and tested at the Pegasus Laboratory at UCLA. The major components of the system are: ①100 kV photo-gun for 20 ps, 80 mA electron bunch generation; ②Sample holder ③RF deflector (20 kV voltage) for electron bunch streaking. Allowing ; ④MCP detector ⑤Synchronization System ⑥Data acquisition/analysis system We would love to hear feedback about what you like (or don’t like) about this system and anything we can do to improve it. You will hopefully be our customers! Conclusions Ultrafast Electron Sources for Diffraction and Microscopy Workshop

Thank you !