DEVELOPMENT OF A 500 KEV STANDING WAVE LINEAR
ACCELERATOR FOR A TRANSMISSION ELECTRON MICROSCOPE
R. Weber, E. Tanabe, AET, Inc., 2-7-6 Kurigi,Asao-ku, Kawasaki, Kanagawa 215-0033 Japan,
Yukinori Nagatani, National Institute for Physiological Sciences, 5-1 Myodaiji-cho AzaHigashiyama, Okazaki City, Aichi Prefecture 444-8787, Japan
Abstract
This paper demonstrates the versatility of using
advanced 3D electromagnetic and particle dynamics
simulation software for designing a 500keV side coupled
standing wave linear accelerator, decelerator and RFchopper as part of a novel high energy transmission
electron microscope (TEM). Side coupled accelerators
require full 3D electromagnetic simulation. We show theoptimization of single accelerator sections, the fine tuningof the field distribution along the beam axis and thecalculation of beam dynamics. A simulation of different
RF chopper designs shows the advantage of using theelectric field for beam deflection instead of the commonlyused magnetic field. A 500kV TEM has been successfully
built and tested.

0.86c forcing the accelerator to be non-uniform along thebeam axis. To achieve the desired beam properties wefirst treat the single sections of the accelerator
independently. A single section as shown in Figure 1 may
be terminated with electric or periodic boundaries. We
optimize the coupling slot lengths and nose gaps in theaccelerator cavities and the nose gap in coupling cavity
for each section. The goal is to obtain the desired E-fieldratio in the accelerator cavities and remove fields from thecoupling cavity while maintaining the π/2 mode
frequency and dispersion. The optimization is performed
with the Eigenmode Solver of CST Microwave Studio [1]on a second order curved tetrahedral mesh.

INTRODUCTION
High energy TEMs are an important tool commonlyused in the life sciences since they allow observing thicksamples with nanometer resolution. These machines
typically use high voltage DC accelerators which haveexcellent beam stability but also drive up weight, size andcost. TEMs with beam energies of 500keV commonly
exceed $10M and can weight over 10 tons. Using astanding wave linear accelerator reduces the cost
substantially but great care must be taken to meet thestringent beam stability and emittance requirements
needed for TEM. Therefore the new system replaces thephoto cathode with an electron gun that emits 100keVelectrons, a linear accelerator then increases the energy to500keV while a 250 femtosecond RF chopper limits theenergy spread to ΔE/E ≤ 10-5. After passing the sample adecelerator decreases the energy to 200keV to protect thesensor chip. The accelerator, decelerator and RF-chopper
are designed using state of the art 3D electromagnetic andparticle dynamics simulation software from CST [1]. CSTsoftware is based on the Finite Integration Technique(FIT) [2] and calculates 3D electromagnetic fields in timeand frequency domain on hexahedral and tetrahedral
meshes. Simulation accuracy is greatly improved by thePerfect Boundary Approximation technique (PBA) [3] forhexahedral meshes and second order curved elements fortetrahedral meshes. A wide range of built in optimizers
enables to meet design specifications with minimum
effort.

SINGLE SECTION OPTIMIZATION
We use an S-band, side coupled standing wave
accelerator to replace the static accelerator. The speed ofthe electrons along the accelerator changes from 0.55c toFigure 1: Vacuum internal of a single side coupled
accelerator section and electric field of the π/2 mode.

ACCELERATOR TUNING
Since the boundary conditions of the single section do
not accurately describe the alternating coupling cavities,
varying geometry along the beam axis and termination of
the accelerator it is necessary to fine tune all couplingslots and nose gaps again in the complete assembly as
shown in Figure 2. The goal of the tuning is to obtain thedesired field distribution along the beam axis, remove
fields from the coupling cavities and maintain the π/2mode frequency. Optimization of such a large model is- 1325 -feasible only with advanced 3D electromagnetic
simulation software such as CST Microwave Studio.

reveals the energy spectrum. Beam dynamics areinvestigated with CST Particle Studio [1] which simulates
charged particle dynamics in 3D electromagnetic fields
including particle tracking, wake field and particle in cell(PIC) calculations.

Figure 2: Contour plot of the electric field distribution ofthe π/2 mode in the tuned accelerator.
The decelerator shown in Figure 3 consists of the last
five section of the accelerator with opposite beam
direction. Because of the changed length the decelerator
requires fine tuning as well. After fine tuning the target
frequencies for cold testing and mechanical tuning can becalculated accurately enabling to build highly efficientcomponents.

Figure 4: Snapshot of electron bunches in the accelerator.
The diameter of the beam is scaled for better viewing.

Figure 5: Electron trajectory in decelerator for the π/2
mode with injection at optimum phase. The diameter ofthe beam is scaled for better viewing.

CHOPPER

Figure 3: Arrow plot of the electric field distribution ofthe π/2 mode in the tuned decelerator.

BEAM DYNAMICS

The RF-Chopper is used to limit the energy spread of
the 500keV electrons leaving the accelerator to ΔE/E ≤10-5 so a clear image can be obtained. RF-Choppers
typically use magnetic fields to deflect the electron beam.
A simulation based study of different deflectors shown inFigure 6 however shows that using the electric field ismore efficient.

A beam dynamics simulation determines the input
power necessary to achieve the desired beam energy and- 1326 -Figure 6: Contour plot of magnetic field in deflector A(left) and deflector B (centre) and electric field in chopper
C (right). Sizes are scaled for better viewing.
Chopper A requires 5550W for a beam deflection of 9
degrees, chopper B requires 322W and chopper C 68W,
making the electric chopper the most efficient. Deflector
C shown in Figure 7 also has the lowest temporal energy
spread while the spatial energy spread is similar in all
deflectors.

Figure 8: Main body (left) and microwave circuits (right)of the constructed 500kV TEM. The height of the main
body is 3.75m.
We have performed first tests of the TEM. We obtained
TEM images using only the linear accelerator. The beamenergy is confirmed to be 200keV by an omega energy
filter inside the TEM. We have also taken the first imageof the 500kV TEM with both the linear accelerator anddecelerator activated. A beam energy of 500kV is
confirmed by the Au[100] incident diffraction pattern.

CONCLUSION
Figure 7: Deflector C with chopper iris showing theelectric field and electron beam. The size of the beam andiris is scaled for better viewing.

EXPERIMENTAL RESULTS
A 500kV TEM as constructed in the National Institutefor Physiological Science [4] is shown in Figure 8. The
100kV field emission electron gun (FEG) is installed on
the top (silver part) of the main body, the 400kV linear
accelerator and the 300kV linear decelerator are installedin the blue-painted parts. The object lens and thespecimen holder are installed in the white-painted part
between the accelerator and the decelerator. The chopper
system is installed in the white-painted part between theFEG and the accelerator. A two-dimensional image sensor
for the electron beam is installed at the bottom. The
whole beam system is installed on a air-suspension
system to isolate it from ground vibrations.

We presented the simulation based design of a side
coupled standing wave accelerator and decelerator using
advanced 3D electromagnetic and particle dynamics
simulation software. We demonstrated the optimization of
single sections and the fine tuning of the electric field
distribution along the beam axis in the complete assembly
as well as the calculation of beam dynamics and selectionof the most efficient chopper design based on simulation
results. The automatic optimization process enables tosignificantly reduce the development time and cost. The
simulation results agree well with measurements. Finally,
the successful first tests of the world first microwave
TEM verifies the optimization techniques.

REFERENCES[1] Computer

Simulation

Technology(CST),

http://www.cst.com/[2] T. Weiland, “A discretization method for the solution of
Maxwell's equations for six-component fields”, Electronics
and Communication (AEÜ), vol. 31, p. 116, 1977.
[3] B. Krietenstein, R. Schuhmann, P. Thoma, and T. Weiland,“The Perfect Boundary Approximation technique facing- 1327 -the challenge of high precision field computation”, inProceedings of the XIX International Linear Accelerator
Conference (LINAC’98), Chicago, USA, 1998, pp. 860862.
[4] National
Institutefor
PhysiologicalSciences,
http://www.nips.ac.jp/eng/- 1328 -