Ultrafast Electron Sources for Diffraction and Microscopy Workshop

Report
Novel Ultrafast Electron Diffraction
System (“streaked”-UED)
Luigi Faillace
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
December 12th - 14th 2012, California NanoScience Institute at UCLA
ABOUT RADIABEAM
 RadiaBeam Technologies, LLC. is a small business with
core expertise in accelerator physics.
 Spin-off from UCLA (2004)
 Extensive R&D Program (DOE, DOD, DHS, NSF)
 Growing products line for research laboratories and
industrial customers (magnets, diagnostics, RF structures,
complete systems)
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Customers
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
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 [*]
RF gun UCLA/BNL/SLAC
6 MW 2.856 GHz
e- beam
Pegasus pump and probe setup
Two axes x-y
sample-holder
movement
12 bit camera
f/0.95 lens coupling
Lanex screen or
MCP detector
Probing electron beam
~200 fs rms long
1 pC 3.5 MeV
IR laser pulse
1-10 mJ 40 fs rms
Collimating
hole
1mm diameter
Pump pulse
0.5 mJ 800 nm
0.1 mJ 400 nm 40 fs rms
*P. Musumeci et al., Relativistic electron diffraction at the UCLA Pegasus photoinjector laboratory,
Ultramicroscopy 108 (2008) 1450– 1453
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Conventional vs. Relativistic Electron
Diffraction
Conventional
• 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)
• Thousands of pulses to obtain a
good diffraction pattern
Relativistic
• Need of a relativistic electron Gun
• Longer diffraction camera length
• More intense electron bunches
possible due to weaker space
charge effects
• Single-shot measurement
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Streaked UED System
Cathode
RF Deflector
Anode
Detector
Screen
Sample
Photocathode
laser
pulse
e- bunch
Diffracted
- bunch
eDiffracted
Deflected
e- bunch
e- bunch
time
Pump
Pump laser
laser
pulse
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. 013302
Main Parameters
Innovations
 Compromise between Conventional and Relativistic UED
systems
 Same physics of current UED systems but cheaper and more
compact
Electron Energy
100 keV
Number of electrons/pulse
107
Pulse Length (at the sample)
20 ps
Deflector RF power
1 kW
Deflector Nominal Voltage
20 kV
Temporal Resolution of System
30 fs
 UED measurements in small laboratories
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
SUED System Block Diagram
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Photoelectron gun
(electrostatic design)
SuperFish
-
-
E (V/cm)
9.8 MV/m
-
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)
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 (cathodeanode) of about 1cm .
*L. L. Alston, High Voltage Technology, Oxford University Press, 1968.
**T. van Oudheusden. Electron source for sub-relativistic single-shot femtosecond diffraction.
Ph.D. Thesis Dissertation (2010), Eindhoven University of Technology.
3D model rendering
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Photoelectron gun
(beam dynamics simulations)
Emittance evolution
EGUN
Transverse size
e- beam
Emittance evolution
ASTRA (10 ps beam)
EGUN (DC beam)
Emittance @exit
(100μm input beam)
96 nm
56 nm
Emittance @exit
(50μm input beam)
50 nm
7.1 nm
Transverse size
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Photoelectron gun
(initial engineering)
NEG pump
Stainless steel
Vacuum vessel
anode
insulator
Window for
back-illumination
Aluminum
holder
Feed-through
TMP pump
cathode
3D model of the 100 kV photo-gun, from SolidWorks
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Deflecting Cavity
K 

2
0  t
t   0 /K
2
σt is the rms pulse length and σ0
is the spot size with the cavity
voltage off
minimum attainable
temporal resolution
K (m m /p s )
proportionality factor K
between the temporal and
transverse coordinate on
the screen located at
distance L from the cavity
center.
2f
m 0 c 
c
20 kV
10 kV
5 kV
3 kV
1 .0
0 .8
eV 0

0 .6
0 .4
0 .2
0 .0
0 .0
0 .1
0 .2
0 .3
0 .4
D rift le n g th L (m )
σ0
100μm
Deflecting Voltage
20 kV
Drift length L
50 cm
K
1 mm/ps
Resolution
30 fs
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
0 .5
L
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
Deflecting Cavity (initial enginering)
Input RF
coupler
Inter-cell
coupling slot
Nose cone type cells
Side cell
3D model of the X-Band Deflector, from SolidWorks
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
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.
Diffraction pattern from a gold foil
at Pegasus Lab
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Complete System
beam images at the screen
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). .
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Synchronization
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.
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Complete System (initial engineering)
NEG pump
RF deflector
Solenoid
CCD camera
Photo-gun
MCP
Steering magnets
HV feed-through
Laser-sample
Interaction chamber
3D model of the SUED system, from SolidWorks
Tasks to be performed at UCLA
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Conclusions
The SUED system will fabricated and tested at the Pegasus Laboratory at UCLA. The major
components of the system are:
① 100 kV photo-gun for 20 ps, 80 mA electron bunch generation;
② Sample holder
③ RF deflector (20 kV voltage) for electron bunch streaking. Allowing ;
④ MCP detector
⑤ Synchronization System
⑥ Data 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!
Ultrafast Electron Sources for Diffraction and Microscopy Workshop
Thank you !

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