Electron Microscope R&D
When observing bio-samples with an electron microscope at nano-meter and higher resolutions, the most serious problem is the sample damage due to electron bombardment associated with probe beam irradiation. In order to reduce this damage, the ice embedded sample technique has been widely used, but the 100 keV ~ 300 keV electron beam still causes evident damage to the sample, so the irradiation dose has to be maintained at a minimum, and as a result, the signal to the noise ratio becomes worse and lowers the image resolution. Only in special cases such as taking images of viruses - which have highly rotational symmetric structures - resolution can be dramatically improved by averaging over a large number of identical particles. However, this does not work for non-symmetric structures, i.e., most other bio-structures..
In order to overcome this difficulty, we searched for a possible solution using low energy electron beams in TEM (Transmission Electron Microscope) to produce less damage and consequently improve the S/N of the produced image.
We focused on advances in today's SEM (Scanning Electron Microscope), especially how it uses low-energy electron beams. Some recent models can image at below 2 nm resolutions using beam voltages as low as 1 kV. This development is driven by market needs for microscopic imaging on ceramics and plastics with higher spatial resolution. When the beam energy is 500 eV or lower, the electrical charging phenomena disappears, eliminating the need for sputtering gold on the sample surface. Therefore, low energy SEM has been widely used in various fields, including bio-science.
Matthias Germann (Nature 2009) reported test results on electron irradiation of DNA stretched on carbon foil. He found very high DNA resistance for low energy electrons at 300 V and lower voltages. The radiation allowance reached 106 electrons in square Angstroms, which is 105 times higher than in the case of a TEM, indicating that much higher S/N can be expected. Additionally, the image contrast becomes higher for bio-samples (low-Z elements), so image quality will be improved.
There is one technical difficulty in using low energy electron beams for TEM instruments, i.e., a beam passing through samples will diffract into fairly large angles, and electrons take large off-axis trajectories in the object lens, resulting in spherical aberration effects that lower image quality. To address this problem, we decided to try holographic imaging.
Prof. Dennis Gabor invented holography in the 1940’s. His original purpose was to the cure spherical aberration problems in electron microscopes. However, due to technical problems like insufficient beam quality, his original idea was not realized. After the invention of optical lasers in the 1960’s, holography was realized and widely utilized for three-dimensional image recording, high-precision geometrical measurement, and compressed data recording. Prof. Dennis Gabor was awarded the Nobel Prize in Physics in 1971 "for his invention and development of the holographic method”.
In our research, we take up the challenge of realizing his original idea using today’s advanced electron microscopy technology and numerical data analyzing capability.
Our equipment uses a high-performance SEM column as a coherent electron source. We place a cryo-sample on a rotatable stage at target position. First, we inspect the sample in SEM mode, then we cut the sample into thin membranes using FIB: Focused Ion Beam tool. We then put the electron beam on the membrane sample and take a diffraction from downstream using a two-dimensional electron detector. Using numerical processing, the electron phase is recovered, and the real image is reconstructed through back-Fourier transformation. Currently, we are developing a beam transport system, magnetic focusing lenses, and energy filters. Before the end of this year, the system will be in operation.
This microscopy will be applicable to the following the research areas in bio-science, physics and technology.
(1) DNA and virus imaging, (2) Protein structure determination using two-dimensional protein crystals, (3) Membrane protein analysis using micron-sized crystals, (4) Ice embedded single bio-particle imaging, (5) Carbon nanotube, fullerene and various kinds of nano-particles.
Ocean Current Energy Development
We are developing technology to harness boundless energy from ocean current flows. There are many technical problems to be solved, but in the end, this project will make possible large-scale tidal power generation. Japan is an island nation, surrounded by seas, and this is where we can find our own energy resources. We believe that we scientists should contribute to green energy development and leave a clean and safe energy resource to the next generation - our sons and daughters, our grandchildren, and their children after them.
We have a prominent ocean current near Japan called Kuroshio. It begins off the east coast of Taiwan and flows northeastward, passing the southern coasts of the Japanese archipelago. Its average flow speed is 1 ~ 2 m/sec. Since water density is 800 times greater than that of air, the water flow at this speed has the same power as the wind at 8 ~ 16 m/sec, which is enough to provide power using a windmill type generator. Ocean current flows every day throughout all seasons, day and night. This energy flow has great potential to become one of the major energy resources in Japan.
Development of submerged ocean current turbine
In the case of a typhoon, there are high risks of failure of any manmade structures due to strong waves and rainstorms. However, as we go down deep in the water, the wave amplitude decays becomes quiet at 50 m or deeper. We plan to place our turbine generator at 100 m deep. The turbine is tethered to the sea floor and utilizes sustained ocean currents to produce electricity. To keep the turbine at the desired depth, we use the buoyancy of a float, as shown in the sketch. It is known that there are a large amount of turbulent components mixed with the ocean current. Importantly, the float will keep the generator at the desired depth and normal direction, and the tilt angle can be corrected using reaction torque from the rotation blades.
Air and water obey the same fluid dynamic equations, but the density of water is 800 greater than air. Therefore, we can use the same blade design that is used in windmills by scaling for the speed of flow. In our full-scale design, an 80 m diameter windmill blade will be used in the water. While we do need to care for water protection and higher mechanical stress, the design will generate 3 MW electric power at a flow speed of 1.3 m/sec, which corresponds to a 10 m/sec wind speed. Rotation speed is one or two cycles per minute, and the tip speed is 20 km/h, which is much slower than a whale's maximum swimming speed of 50 km/sec, so we can conclude that the risk of being hit by the blades will be fairly low.
The pitch of the blade is variable, and when pitch is turned OFF, blades stop, and without the drag force, the turbine rises and floats flat on the surface, facilitating maintenance from technicians on ships.
The generated electric power will be sent through high-power electric cables laid on the seabed. The anchoring wire and power cable technologies are common to offshore windmills. Currently, three big offshore windmill farms funded by NEDO are under development in Japan, and these key technologies are undergoing field tests there. We anticipate that we can leverage these key technologies to the ocean current power plant in future.
300 ocean current turbines will produce 1 GW
If we place a 300-unit grid of ocean current turbine units in the Kuroshio current, it will generate 1 GW electric power - equivalent to the power to one unit of nuclear reactor. To realize this generator farm, there are a number of technical difficulties to overcome: fabricating high strength blades; protecting generators from salt water leakage; preventing metals from rust; protecting generator surfaces from barnacles, seaweed, and environmental risks; establishing maintenance procedures; and extending the lifetime of components in such a severe environment.
Scale model tests at OIST
In order to demonstrate the concept of our ocean current generator, we are testing a scaled-down model. The key concepts are
- use of the same windmill turbine blade design,
- use of a float to provide buoyance and torque compensation,
- use of a tether to the ocean bed.
In 2012, we developed a 1/40 scale model which used 2 m diameter blades and a 1 kW generator which was taken from a commercial windmill product. The float was designed at OIST and made by FRP at the Oshiro ship factory in Okinawa. In the summer of 2012, we tested the model by touring with a motorboat in a coral reef. At a speed of 1 m/sec, we got 400 Watt from the turbine as designed. In 2013 we used a technical testing pool in Kyushu University to do a precise power generation test.
In 2014, we will start a careful study of all components to optimize them for ocean current, followed by power generation tests in test pools. Field tests using tidal current will be performed in 2015 at a narrow channel in front of Okinawa Churaumi Aquarium.
Scale down model, blades are 2 m diameter.
Testing in coral reef in front of Maeda port near OIST. (2012/09/05)