RF GUN BASED MEV HIGH-VOLTAGE ELECTRON MICROSCOPY
Jinfeng Yang #,A), Koichi KanA), Nobuyasu Naruse A), Takafumi Kondoh A), Yoichi Yoshida A),
Katsumi Tanimura A), Toshikazu TakatomiB), Jyunji UrakawaB)
A)
The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki,
Osaka 567-0047, JapanB)
High Energy Accelerator Research Organization , 1-1 Oho, Tsukuba, Ibaraki, Japan
Abstract
The direct visualization of fundamental dynamic
processes in matter occurring on femtosecond time scales
over sub-nanometer (even atomic) spatial dimensions haslong been a goal in science. The electron microscopy is a
powerful tool to observe directly the image fromspecimen with high spatial resolution, however generally
without time resolution. The photocathode radiofrequency (RF) gun is a short-pulse electron source togenerate a high-brightness femtosecond electron beamwith energies of 1~3 MeV. The RF gun based relativistic
ultrafast electron diffraction is a promising new techniquethat has the potential to probe directly structural changes
at the atomic scale with sub-100 fs temporal resolution. In
this paper, the development of a time-resolved relativistic
electron microscopy based on the RF gun is reported. The
requirements and limitations of the beam parameters usedin this technique are discussed.

INTRODUCTION
The direct visualization of fundamental dynamic
processes in matter occurring on femtosecond time scales
over sub-nanometer (even atomic) spatial dimensions haslong been a goal in science. Ultrafast electron diffraction(UED) provides a direct measure (“real-time” probe) of
structural dynamics in matter by recording the change inthe characteristics of electron diffraction peaks (position,intensity, width) in the pump state and the unpump state.
The UED technique has the key advantage with respect to
X-ray diffraction of a much larger interaction crosssection in the matter. A large number of important results,
i.e. phase trans-formations, melting, ultrafast breaking ofchemical bonds and ultrafast reactions, have already beenobtained with picosecond or sub-picosecond temporal
resolution. Most widely used UED instruments are basedon laser-driven photoemission DC guns, generating
typically 50-100 keV electron beams [1,2]. The time
resolution is achieved by operating in the non-spacecharge-limited regime with thousands of electrons perpulse, because of space-charge broadening during
propagation in nonrelativistic beams. For example, in thetheoretical calculation for a 30 keV femtosecond electron
bunch with an electron number of 10,000 per pulse, thelength of the electron bunch increases to ~4 ps by
propagating with a distance of 40 cm [3]. The energy
spread also increases due to the space charge effect during___________________________________#yang@sanken.osaka-u.ac.jp

propagation. To overcome the problem, one is tominimize the propagation distance by placing the sample
in close proximity of the electron source (~ 4.5 cm);another is to decrease the number of electrons in the beam,i.e. 1,000 e-‘s in a 600 fs bunch. However, the techniqueof UED cannot observe the structural dynamics in realspace (imaging over atomic spatial dimensions).
Transmission electron microscopy (TEM) is a
powerful tool to observe directly the image from
specimen with high spatial resolution. When coupled withtime resolution, it, which called ultrafast electronmicroscopy (UEM, also called dynamic transmission
electron microscopy, DTEM), would be the strongest tool
for the study of ultrafast dynamics in material science,physics, chemistry and biology. Currently, the DTEM
with the 15 ns time resolution has been achieved inconventional TEM through the use of photo-activatedelectron source driven by a nanosecond laser [4]. In orderto obtain high time resolution, a stroboscopic imaging ofperiodically driven processes was preformed. In thisconfiguration, a time resolution of 200 ps was achievedfor processes up to 100 MHz, which called the singleelectron-pulse method [5]. These techniques were applied
to ultrasonically driven disruption of crystals andmagneto-elastic effects and magnetic-field-induced
oscillations of the domain magnetization of domain walls
and of their substructures. However, there is no resolution
to achieve the femtosecond-temporal and sub-nanometer(even atomic) spatial resolution in the recent UEM,
because of the long electron bunch length and low bunchcharge due to the space-charge effect.
It is well known that the photocathode RF electron gunhave definitely been a fundamental technologicalbreakthrough in the last 20 years as relativistic electron
sources and are in fact the preferred elect. The RF gun isalso a significant benefit in UED, because it can generate
a femtosecond relativistic electron pulse reducing thespace-charge limitation. Recently, the single-shot MeV
electron diffraction measurement and the time-resolved
measurement have been also succeeded by researchgroups in the world [6-11]. The studies suggest that thephotocathode RF gun is useful for the diffraction
measurement with high time resolution. In this paper, wereport the design and construction of a new time-resolved
near-relativistic electron microscopy using a photocathode RF gun to study the atomic dynamics of phasetransitions in solids. The project was started from 2010. A
new structure femtosecond electron RF gun wasdeveloped in 2010. The beam study and the RF gun based- 282 -

UED experiments have been carried out in 2011. In 2012,a prototype of time-resolved near-relativistic TEM has
been constructed.

PROTOTYPE OF TIME-RESOLVEDRELATIVISTIC ELECTRONMICROSCOPY
The concept of time-resolved relativistic TEM is shownin Fig. 1. It consists of a photocathode rf gun, an injection
system, imaging optical lens system and image
measurement system. The TEM is not only compact, butalso the temporal and spatial resolutions of 100 fs and<10 nm are expectable to be achieved.

Femtosecond RF gun
The electron source required for TEM has to be able togenerate a low emittance and low energy spread beamwith ultrahigh stabilities on charge and energy, and low
dark current. For this reason, we have developed a newstructure RF gun under the KEK/Osaka University
collaboration in 2010 [12]. At the level of 0.1 mm-mrad
or less, the emittance can be affected by a number ofsmall contributions like field asymmetries, the structureand fabrication of the RF cavity and so on. To reducethese effects, the new RF gun has been developed withfollowing optimum design and improvements:(1) A new structure of the RF cavity was used. The shapeof the RF cavity wall is near to the ideal wall contour
to produce an optimum electric field reducing theFourier coefficients of all higher harmonics. The Qvalue of the new RF cavity was 16,300, which is
1.4~1.6 times higher than the old RF gun.
(2) The conventional laser injection ports in the half cellwere removed for good field symmetry. The fieldasymmetries not only lead to an asymmetricalemittance resulting in the emittance growth, but also
cause a distortion on UED image.
(3) A new wall turner system was designed to adjust
precisely the electric field balance in the half and fullcells. The dark current produced from the turnerantennas in old gun is also avoided.
(4) The field emission due to the strong electric field
between the cathode plate and the half-cell cavity isthe biggest problem in the old type RF gun. To
minimize the field emission, a new insertion functionof the photocathode was designed in the new gun as
shown in Fig. 5. The cathode plate was blazed on thehalf cell cavity without the use of the helicon flex
vacuum shield without the use of the helicon flex
vacuum shield. The dark current from the new gun
was greatly suppressed to <0.1 pC/pulse.
(5) The photocathode in the new gun is removable.
Finally, the RF gun is driven by a 90-fs Ti:Sapphirelaser. A copper cathode is used in the electron
microscopy experiment. The expected beam
parameters are listed in Table 1.

Table 1: The expected beam parameters
Beam energy
1~3 MeV
Bunch length
100 fs or lessEmittance
0.1 mm-mrad or less
Energy spread
10-4 (10-5 for challenge)
Bunch charge
107 ~ 108 e-/pulse

Injection System
The electron beam generated from the rf gun ispropagated to the specimen through the injection system.
The injection system constructed with a solenoid magnet
and two condenser lenses. The solenoid magnet is used tocompensate the emittance growth during the beam
propagation due to space-charge effect. The condenser
lenses with a diaphragm precisely control and collimate asmall-size and small-convergence-angle beams on thesample.
The effect of spherical aberration, as a limitation ofspatial resolution, is a significant problem in TEM. It canbe described by ~Csθ3, where Cs is a spherical aberration
coefficient and θ is the convergence angle of beam on thesample. For MeV TEM, Cs ~ a few mm. The previous
experiment suggests a 0.1 mm-mrad low emittanceelectron beam can be achieved in the RF gun by reducing
the laser spot size of 0.2 mm on the copper cathode. By
using the injection system, we can control theconvergence angle θ < 2 mrad on the sample. The

Figure 1: Concept of time-resolved relativistictransmission electron microscopy (TEM) using RF gun- 283 -limitation due to the spherical aberration is less than Csθ3
< 0.1 nm.

Imaging optical Lens System
The imaging system in the prototype of TEM consistsof an objective lens, an intermediate lens and a projectorlens: the objective lens (OL) to provide a back-focal plane(BFP) for expanded images, and the intermediate lens(IL) and the projector lens (PL) to display the images withdesired fashion on the detector. The pole pieces in thethree lenses are made by the Fe-Co alloy materials andfabricated precisely. The lenses can provide the maximum
magnetic field of 2.4 T in the center of the pole pieces
with the maximum Ampere-turns of 38,000 AT.
The magnification (M) of the image in the prototype is
5,000~125,000 for the electron beam energy of 1~3 MeV.
The spatial resolution of < 10 nm is achievable using thelens system with the given electron beam parameters inTable I.

Image Measurement SystemTo achieve high sensitivity and a high damage
threshold of MeV electrons, a scintillater of CsI(Tl)equipped with fiber optic plates was used to convert thediffraction pattern into an optical image with a spatialresolution of 50 μm. The size of the scintillater is 50 x 50mm2. The optical image from the scintillator is thenreflected at 45° into an electron multiplying CCD camera
while passing the electron beam through the mirror toprevent electron and X-ray irradiation of the CCD sensor.
The detecting system has been used successfully in therelativistic UED experiments [8]. The single-shot
diffraction patterns with 5 x 105 ~ 1 x 106 electrons areable to be recorded using the detector.

CONCLUDING REMARKSThe RF gun is a high-brightness femtosecond electron
source and is used successfully in the relativistic UED
experiments. It is also a significant benefit in thedevelopment of next time-resolved TEM and compact
relativistic TEM. However, many developments andimprovements are needed to challenge. One is required toreduce further the transverse emittance and energy spread.
For a low charge beam, the emittance can be minimized
by reducing the laser spot size on the cathode, i.e. 50 μm
or less. However, once again, space charge will limit theminimum beam size to ~ 50μm. The damage problem on
cathode should be considered for the use of such small
UV laser. New cathode materials with small excess
kinetic energy Ek are needed to develop to obtain the lowemittance with the use of a large laser spot size. Thespatial resolution is determined by the energy spread.
Especially for the electron microscopy, ΔE/E ~ 10-5 has
been used to obtain a sub-nanometer resolution. For theRF gun, the energy spread is limited to ΔE/E ~ 10-4 using
a 100 fs laser. It is possible theoretically to reduce to 10-5
using a 10 fs laser. However, a drastically different
approach is required to solve this problem. Finally, thestabilities (charge and energy) and the synchronization of
the laser with accelerating RF are needed to be improved.
The detection of every electron is also essential in future
developments because of small signal levels.

ACKNOWLEDGEMENT
This work was supported by a specially promoted
research (Grant No. 22246127) of Grant-in-Aid for
Scientific Research from the MEXT, Japan. The authors
acknowledge Professor H. Kobayashi and Professor A.
Ogata for valuable suggestions and discussions.

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[11] http://regae.desy.de/[12] J. Yang, et al., Proceedings of 8th Particle Accelerator
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Figure 2: Prototype of RF gun based time-resolved
relativistic TEM. The height is 3 m and the diameter is
0.7 m.
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