Review Series to Celebrate Our 100th Volume
Invention of the split-anode magnetron
2024 年 100 巻 5 号 p. 281-292
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2024 年 100 巻 5 号 p. 281-292
Magnetron production and use far exceed that of other microwave tubes due to their high operational efficiency, power efficiency, and cost-effectiveness in production. The magnetron was named by A. W. Hull; however, the device invented by Hull differs from the magnetron utilized as a microwave tube. The magnetron widely used today is based on the split-anode magnetron invented by K. Okabe. This overview introduces two papers published by Okabe in the Proceedings of the Imperial Academy and discusses the events that led to the discovery of the split-anode magnetron. In addition, the operation mechanisms of magnetrons are explained.
The notion of controlling the flow of electrons inside a vacuum tube by using an external magnetic field to create amplifiers or oscillators originated from a proposal by A. W. Hull in 1921.1) As shown in Fig. 1, when a solenoid is wound around the outside of a cylindrical diode vacuum tube and a magnetic field is applied along the axis of the cylinder, electrons emitted from the filament are deflected in their trajectory because of the influence of the magnetic field. As depicted in Fig. 2, when the strength of the magnetic field \(B\) is altered while a constant anode voltage is maintained, the trajectory that should remain straight in the absence of a magnetic field, identified as “a,” bends with rising \(B\) , resulting in trajectories “b” and “c.” At a certain magnetic field strength \(B_c\) , the trajectory returns to the filament after contacting the anode. This magnetic field strength is referred to as the critical magnetic field. At magnetic field strengths higher than the critical magnetic field, electrons fail to reach the anode and instead return to the filament (path “d”). The relationship between this magnetic field and the anode current is shown by the solid line in Fig. 3. Until the magnetic field strength hits \(B_c\) , the anode current doesn’t change; however, when the magnetic field strength reaches the critical value \(B_c\) , the anode current sharply decreases, reaching zero. Hull suggested using this property to build current amplifiers or oscillators. For instance, when a direct-current magnetic field with a strength approaching \(B_c\) is applied and a small input AC current is superimposed on the solenoid coil providing the magnetic field, substantial AC current flows through the anode circuit. Hull dubbed such a vacuum tube a magnetron. Currently, however, the device that operates in this manner is not referred to as a magnetron. Furthermore, Hull’s device could generate electromagnetic waves with frequency of only a few hundred kilohertz because the solenoid was wrapped around the diode tube to produce a magnetic field and the same solenoid was utilized in the resonance circuit of the electromagnetic waves generated by the diode tube.
Cylindrical diode-type magnetron proposed by A.W. Hull.
Electron trajectories in the cylindrical diode-type magnetron. The electrons remain straight in the absence of a magnetic field (a). The trajectory bends with the increase in magnetic field (b and c). The electrons fail to reach the anode and return to the filament (d).
Anode current versus intensity of applied magnetic field in the single-anode magnetron. The solid line and dashed line show the expected result and actual experimental one, respectively.
Additionally, A. Žáček discovered oscillation at a wavelength of 29 cm for a single-anode magnetron in 1924. Nevertheless, because this discovery was not released in English, it received little notice.2) Also in 1924, E. Habann achieved oscillation using negative resistance in a magnetron with a split cylinder-type anode and a grid; however, this oscillation was limited to a few tens of megahertz.3)-5) Neither the device fabricated by Žáček nor that fabricated by Habann is currently referred to as magnetrons.
In 1927, Kinjiro Okabe was performing a physics experiment to determine \(e/m\) (the relationship between the charge of an electron, \(e\) , and its mass, \(m\) ) at Tohoku Imperial University. Using a single-anode magnetron, he measured the strength of the magnetic field ( \(B\) ) in relation to the anode current, which is similar to the solid line in Fig. 3 to determine \(B_c\) . The objective was to ascertain the value of \(e/m\) from the determined \(B_c\) , along with the anode voltage and anode diameter. Okabe speculated that, when the strength of the magnetic field reached \(B_c\) , the anode current would sharply decrease and become zero. Therefore, he believed \(B_c\) could be reliably computed in certain circumstances. However, in the experiment report submitted by a student, a line representing the values of the anode current, which decreased at field strength greater than \(B_c\) (dashed line in Fig. 3), increased slightly from the middle, creating a small peak resembling a camel’s hump, before subsequently decreasing again. Okabe thought that this outcome might be attributable to experimental error and chose to replicate the experiment himself. However, the hump appeared again. When Okabe held his palm close to the circuit, he saw variations in the anode current, indicating oscillation.6) Okabe conducted experiments with a single-anode magnetron and a Lecher wire (Fig. 4) and discovered oscillations of very short wavelengths, in the range of several tens of centimeters, at magnetic field strength near \(B_c\) . The shortest oscillation wavelength recorded at that time was 24 cm, which was created using the Barkhausen-Kurz oscillation tube developed by Barkhausen and Kurz in Germany.7) The oscillation wavelength observed by Okabe was close to this record.6)
Schematic circuit for the oscillation of short wavelengths in the range of several tens of centimeters with a single-anode magnetron.
Experimental findings obtained using a single-anode magnetron were published in the Proceedings of the Imperial Academy in 1927.8) The author reported that, when the magnetic field strength is maintained at approximately \(B_c\) and a high voltage is applied to the anode, the magnetron can generate exceptionally short and intense electromagnetic waves that do not decay. The shortest wavelength obtained at this moment was less than 30 cm. Table 1 shows the oscillation wavelengths measured using the Lecher wire method, the qualitative intensity of the oscillations, and the semi-theoretical oscillation wavelengths with respect to the anode voltages. The diameter of the anode measured 1.32 cm, and the wavelength was practically independent of the degree of filament-heating.
Oscillation wavelengths, qualitative intensity of the oscillations, and semi-theoretical oscillation wavelengths with respect to the anode voltages in the single-anode magnetron8)
| Anode voltage (volts) |
\(\lambda\)
(cm.) |
Intensity of the oscillations |
\(\lambda_0\)
(cm.) |
|---|---|---|---|
| 190 | 150 | weak | 87 |
| 230 | 122 | weak | 79 |
| 280 | 88 | middle | 72 |
| 450 | 63 | strong | 58 |
| 500 | – | – | 55 |
| 1000 | 32 | very strong | 38 |
| 1300 | 26.5 | very strong | 35 |
| 5000 | – | – | 17 |
| 20000 | – | – | 8.5 |
Okabe proposed that the oscillation mechanism involves electrons oscillating with a period equal to the time required for electrons to depart from the cathode and return to it. He computed the semi-theoretical oscillation wavelength based on his expectation. The semi-theoretical oscillation wavelength can be calculated as follows: The electron motion is considered when a homogeneous magnetic field of intensity \(B\) is applied along the axis of a diode vacuum tube consisting of a thermal cathode with a radius \(r_0\) and an anode with a radius \(R\) (Fig. 5). The equations for the electron motion are as follows9),10):
Cross section of the single-anode magnetron consisting of a thermal cathode with a radius \(r_0\) and an anode with a radius \(R\) . \(V_a\) is an anode voltage. \(\theta\) is an angle formed by the basic axis and the line connecting the electron and center.
| \begin{aligned} \left. \begin{array}{l} \displaystyle m\frac{d^2r}{{dt}^2}-mr\left(\frac{d\theta}{dt}\right)^2 = Ee-Ber\frac{d\theta}{dt},\\[10pt] \displaystyle \frac{m}{r}\frac{d}{dt}\left(r^2\frac{d\theta}{dt}\right) =Be\frac{dr}{dt},\\[10pt] \displaystyle m\frac{d^2z}{{dt}^2}=0. \end{array} \right\}\end{aligned} | [1] |
Here, \(m\) and \(e\) represent the mass and charge of the electron, respectively, \(t\) is time, \(E\) is the strength of the electric field, \(z\) denotes the distance along the tube axis, \(r\) is the distance of the electron from the center, and \(\theta\) represents the angle formed by the basic axis and the line connecting the electron and center. Because the magnetic field does not influence the electron motion in the \(z\) axis direction, only the electron motion on one plane perpendicular to the \(z\) axis needs to be considered. For simplicity, setting the initial velocities of the electrons in the radial and tangential directions to zero and integrating Eq. [1] produce
| \begin{aligned} \left. \begin{array}{l} \displaystyle\left(\frac{dr}{dt}\right)^2=\beta F-\alpha^2r^2\left(1-\frac{r_0^2}{r^2}\right)^2,\\[10pt] \displaystyle\frac{d\theta}{dt}=\alpha\left(1-\frac{r_0^2}{r^2}\right). \end{array} \right\}\end{aligned} | [2] |
| \begin{aligned} \left. \begin{array}{l} \displaystyle F=\frac{\log{\frac{r}{r_0}}}{\log{\frac{R}{r_0}}},\\[10pt] \displaystyle\alpha=\frac{Be}{2m},\\[8pt] \displaystyle\beta=\frac{2eV_a}{m}. \end{array} \right\}\end{aligned} | [3] |
Here, \(V_a\) represents the anode voltage. Setting \(r_0\risingdotseq0\) in Eq. [3] results in \(F\risingdotseq1\) , leading to Eq. [4]:
| \begin{aligned} \displaystyle\frac{dt}{dr}\fallingdotseq\frac{1}{\sqrt{\beta-\alpha^2r^2}}.\end{aligned} | [4] |
Integrating Eq. [4] with regard to \(r\) yields Eq. [5]:
| \begin{aligned} \displaystyle t=\frac{1}{\alpha}\sin^{-1}{\frac{\alpha}{\sqrt\beta}}r.\end{aligned} | [5] |
When \(dr/dt=0\) , \(r\) is written as \(r_m\) , which is given by Eq. [6]:
| \begin{aligned} r_m=\displaystyle\frac{\sqrt\beta}{\alpha}.\end{aligned} | [6] |
To calculate the time \(t_e\) needed for an electron to travel to the point where \(dr/dt = 0\) from the cathode, we obtain Eq. [7]:
| \begin{aligned} \displaystyle t_e\fallingdotseq\frac{1}{\alpha}\sin^{-1}{\frac{\alpha}{\sqrt\beta}}r_m=\frac{1}{\alpha}\sin^{-1}{1}=\frac{1}{\alpha}\frac{\pi}{2}=\frac{\pi m}{eB_c}.\end{aligned} | [7] |
The magnetic field that gives Eqs. [6] and [7] is the critical magnetic field. Because \(r_m=\sqrt\beta/\alpha=R\) , \(R\) is expressed as Eq. [8]:
| \begin{aligned} \displaystyle R=\frac{\sqrt{\frac{2eV_a}{m}}}{\frac{{eB}_c}{2m}}=\frac{2}{B_c}\sqrt{\frac{2m}{e}}\sqrt{V_a}.\end{aligned} | [8] |
From Eq. [8], the strength of the critical field is provided by Eq. [9]:
| \begin{aligned} \displaystyle B_c=\frac{6.72\sqrt{V_a}}{R}.\end{aligned} | [9] |
In Eq. [9], \(B_c\) is the magnetic flux density in Gauss, \(R\) is in centimeter, and \(V_a\) is in volt. The strength of \(B_c\) is controlled by the anode radius and the anode voltage. Hence, the wavelength corresponding to the time required for an electron to depart from the cathode, follow a circular trajectory, and return to the cathode is given by Eq. [10]:
| \begin{aligned} \lambda_e=2\times c \times t_e = \frac{10650}{B_c} = \frac{10650R}{6.72\sqrt{V_a}}.\end{aligned} | [10] |
In Ref. 8, the diameter of the anode is 1.32 cm; therefore, \(R\) is 0.66 cm. Equation [10] does not account for the influence of space charge. When the effect of space charge is considered, Eq. [7] grows significantly.10) In Ref. 8, \(\sim12{,}000\) is used instead of 10,650 for the calculation of the semi-theoretical oscillation wavelengths. As illustrated in Table 1, the empirically acquired wavelengths and the semi-theoretical wavelengths agree nicely. This similarity indicates that the oscillations obtained in the single-anode magnetron arise from electron oscillations with a period equal to the time required for electrons to depart from the cathode, follow a circular trajectory, and return to the cathode. Okabe projected that oscillations with wavelengths on the order of several centimeters would be obtained in a single-anode magnetron.
The experimental results obtained with the split-anode magnetron were published in the Proceedings of the Imperial Academy in 1927.11) Producing extra-short electromagnetic waves for a magnetron necessitates a high anode voltage, a magnetic field strength exceeding \(B_c\) , and a high vacuum. The shortest wavelength of oscillations obtained in the single-anode magnetron was 17 cm12); it finally reached 12 cm,13) despite the incredibly modest oscillation. However, in the single-anode magnetron, it was not possible to obtain shorter wavelengths or stronger oscillations. Consequently, Okabe created a split-anode magnetron with a divided cylindrical anode, where the cylindrical anode of the magnetron was divided into two or more pieces, each of which was led outside the tube through independent lead wires. He observed oscillations in the split-anode magnetron several times larger than those in the single-anode magnetron.14)
Circuit connection for the split-anode magnetron. The lead wires from each anode are brought together and arranged to approach the cathode lead at a specific position such as B and then linked to the positive terminal of the high-tension anode battery.11)
Figure 6 depicts the circuit connection for the split-anode magnetron.11) The lead wires from each anode are brought together and made to approach the cathode lead at a certain point such as B in the figure; they are then linked to the positive terminal of the high-tension anode battery. A fundamental oscillation with a wavelength of 12 cm was obtained in a small magnetron, and a harmonic oscillation with a wavelength of 8 cm was produced in one case with an abnormally strong magnetic field. A typical case of an extremely powerful oscillation at about 42 cm was produced in a split-anode magnetron in which the anode diameter, cathode diameter, anode length, and cathode length were 1.4 cm, 0.014 cm, 2.6 cm, and 3 cm, respectively. Table 2 shows the oscillation wavelengths and intensities for various anode voltages. The oscillations were tuned by adjusting the magnetic coil current and the filament current to attain the greatest oscillation amplitude. In the split-anode magnetron, the wavelength remained constant within a certain range even as the anode voltage was varied. This behavior implies that a resonant circuit is formed in the split-anode magnetron. The anode lead and the anode segment appear to form a resonant circuit, the natural frequency of which can be altered by varying the length of the lead between point B and the anode segment. When the distance between the anode lead and the cathode lead at point B is appropriately adjusted, tuning appears to happen; thus, the maximum oscillation obtained is generally several times stronger than that obtained in the single-anode magnetron. The variations in the wavelength due to the changes of the anode voltage and the filament current are minimal. The shortest wavelength of the oscillations obtained in the single-anode magnetron is 12 cm; however, the power of the oscillation is quite weak. By contrast, the split-anode magnetron generates very strong oscillations even at a wavelength of 12 cm. Furthermore, when the number of divisions is increased, for example, by dividing the configuration into four parts while maintaining parameters such as the anode diameter, anode voltage, and magnetic field strength the same as for the two-part configuration, the wavelength is confirmed to be roughly halved. Therefore, to generate oscillations with a shorter wavelength, the number of divisions should be increased, and each anode segment should be connected by a resonant circuit. The split-anode magnetron paved the way to microwave applications.
Oscillation wavelengths and intensities for various anode voltages in the split-anode magnetron, where the anode diameter, the cathode diameter, the anode length, and the cathode length were 1.4 cm, 0.014 cm, 2.6 cm, and 3 cm, respectively11)
| Anode voltage | Wavelength (cm.) |
Intensity (arbitrary) |
|---|---|---|
| 951 | 34.5 | 5.3 |
| 724 | 41.5 | 15.5 |
| 670 | 42 | 16 |
| 500 | 42.5 | 6.7 |
| 400 | 42.5 | 4 |
| 320 | 42.5 | 1.8 |
The split-anode magnetron invented by Okabe enabled effective and steady production of microwaves with a wavelength of 3 cm (i.e., a frequency of 10 GHz).15) In addition, a split-anode magnetron capable of stable oscillation with a wavelength of 2 mm was completed in 1939.16) H. Yagi gave a speech at the Institute of Radio Engineers in the United States in 1928, where he discussed not only experimental findings for the Yagi-Uda antenna but also results for the split-anode magnetron. The content of the presentation was published in the Proceedings of the Institute of Radio Engineers in 1928.17) The split-anode magnetron subsequently garnered significant interest not only in the United States but also in Europe, prompting substantial research on it. As a result, magnetrons are widely used today as a microwave tube in applications such as microwave ovens, radar systems for aircrafts and ships, weather radars, accelerators, and industrial applications.
Nowadays, a multicavity resonator magnetron is commonly used. The multicavity resonator magnetron contains numerous cavity resonators within a single tube instead of the anode being divided and each segment connected to a tuning circuit individually. The operation mechanism of the multicavity magnetron is comparable to that of the multi-split magnetron.18) Figure 7 shows a cross-sectional view of the internal structure of the multicavity magnetron, along with the applied DC electric and magnetic fields. The anode in this magnetron is known as a hole-and-slot-type anode. In the case of such multicavity configurations, the \(\pi\) -mode oscillation pattern is commonly used. In this instance, every other cavity oscillates in the same phase, whereas nearby cavities have a phase difference of \(\pi\) radian. The operating mechanism of such a magnetron is complex; nevertheless, it functions somewhat similarly to an M-type traveling wave tube.19)
Cross section of the internal structure of the multicavity magnetron.
Here, the qualitative operational mechanism of the magnetron is explained according to Ref. 19. Figure 8 depicts the operating principle of the M-type traveling wave tube. The cathode is in a planar configuration, where electrons are not emitted, and the anode is stretched parallel to it. The DC electric field is applied parallel to the paper from the anode to the cathode \((-E_y)\) , and the magnetic field is applied perpendicular to the paper \((B_x)\) . The electrons are assumed to originate from an electron cannon located at the left end. The equation for the electron motion in the presence of both the electric and magnetic fields is given by
| \begin{aligned} m\frac{d\boldsymbol{v}}{dt}=-e(\boldsymbol{E}+\boldsymbol{v}\times\boldsymbol{B}).\end{aligned} | [11] |
In the coordinates of Fig. 8, Eq. [11] is written as Eq. [12]:
| \begin{aligned} \left. \begin{array}{l} \displaystyle\frac{d^2y}{{dt}^2}= -\frac{e}{m}\left(-E_y + B_x\frac{dz}{dt}\right),\\[10pt] \displaystyle\frac{d^2z}{{dt}^2} = \frac{e}{m}B_x\frac{dy}{dt}. \end{array} \right\}\end{aligned} | [12] |
Integrating Eq. [12] under the condition \(dz/dt = v_0\) and \(y = 0\) when \(t = 0\) yields Eq. [13]:
| \begin{aligned} \left. \begin{array}{l} \displaystyle y = \frac{1}{\omega_c}\left(\frac{E_y}{B_x}-v_0\right)(1-\cos{\omega_ct)},\\[10pt] \displaystyle z = -\frac{1}{\omega_c}\left(\frac{E_y}{B_x}-v_0\right)\sin{\omega_ct + }\frac{E_y}{B_x}t, \end{array} \right\}\end{aligned} | [13] |
where \(\omega_c = \displaystyle\frac{e}{m}B_x\) .
Operation principle of the M-type traveling wave tube. The cathode is in a planar configuration, and the anode is stretched parallel to it. The DC electric field is applied parallel to the paper from the anode to the cathode, and the magnetic field is applied perpendicular to the paper. The electrons are assumed to originate from an electron cannon located at the left end.
When the initial velocity of the electrons is \(v_0 = E_y/B_x\) , the average velocity \(v_z\) of these electrons in the \(z\) direction is determined by the ratio between the electric field and the magnetic field:
| \begin{aligned} v_z = \frac{dz}{dt} = \frac{E_y}{B_x}.\end{aligned} | [14] |
In addition, when the initial velocity of the electron is equal to \(E_y/B_x\) , \(y = 0\) and the electron moves to the right because the forces exerted on the electrons by the electric field and the magnetic field balance each other. Consequently, because the structure of the anode was created such that the phase velocity of the electromagnetic wave is equal to \(E_y/B_x\) , the electromagnetic wave and electron interact. Assuming that both the initial velocity and phase velocity of the electron are equal to \(E_y/B_x\) (Fig. 8), the electron travels through the electric field of the electromagnetic wave and thus experiences force from the electromagnetic wave. At point A in Fig. 8, the electron experiences upward force due to the electric field \(\varepsilon_y\) of the electromagnetic wave and also experiences force from the magnetic field \(B_x\) , resulting in an increase in velocity in the \(z\) direction. Nevertheless, at point D, the velocity of the electron in the \(z\) direction drops. With such an increase or decrease in velocity, electrons converge in the P-region. In this region, where the electric field \(\varepsilon_z\) of the electromagnetic wave functions as a decelerating electric field, the electromagnetic wave is amplified by gaining energy from electrons. The electrons approach sufficiently close to the anode to compensate for the decrease in energy and lose their potential energy in the process. That is, in the M-type traveling wave operation, the \(\varepsilon_y\) component clusters electrons, and, even though the electrons lose their significant energy, the electron velocity in the \(z\) directional remains unchanged. Thus, synchronization is consistently maintained, and efficient oscillation or amplification is possible. This behavior clarifies the qualitative operational mechanism of the magnetron.19)
The structure of the coupling circuit in the multicavity magnetron involves the placement of several resonators inside a metal cylinder. This arrangement resembles the structure joining the input and output terminals of the M-type traveling wave tube. The electron flow becomes a cluster inside the magnetron, and it rotates as a collective motion. Thus, the beginning and the terminus are no longer identifiable in the operation of the magnetron. That is, the operation of the magnetron inherently includes positive feedback. Therefore, the magnetron functions similarly to an oscillator.
Compared with other microwave tubes, the magnetron is produced and used as a microwave power source in applications such as microwave ovens and radars on a substantially larger scale. This preference for the magnetron is attributable to its excellent operational efficiency, high-power operation capacity, and cost-effectiveness.
The idea of controlling electron flow inside a vacuum tube by applying an external magnetic field to obtain amplifiers and oscillators was proposal by Hull, who named such a tube a magnetron. However, the technology proposed by Hull is currently not called a magnetron. Okabe found that undamped extra-short and very intense electromagnetic waves were obtained for a single-anode magnetron. The oscillation process of the single-anode magnetron is attributed to the electron oscillations with a period equal to the time required for electrons to leave the cathode and return to it. The shortest wavelength of oscillations obtained in the single-anode magnetron was 12 cm. Nevertheless, in the single-anode magnetron, shorter wavelengths and stronger oscillations were not achievable. Okabe later fabricated a split-anode magnetron with a divided cylindrical anode. In this magnetron, the anode leads and the anode segments create resonant circuits, which are interconnected. Okabe discovered strong oscillations in the split-anode magnetron several times larger than those in the single-anode magnetron. The split-anode magnetron paved the way to microwave applications. Thus, the magnetron often utilized today as a microwave tube is based on the split-anode magnetron developed by Okabe.
Currently, a multicavity resonator magnetron is widely used. This magnetron has many cavity resonators within a single tube instead of the anode being split and each segment being connected to a tuning circuit individually. The split-anode magnetron and multicavity magnetron have nearly the same operation mechanism, which is also almost approximately the same as that of the M-type traveling wave tube. The clustered electrons lose their potential energy in the decelerating electric field, approach the anode, and transfer energy to the electromagnetic waves. The operation mechanism of the magnetron absolutely differs from that of other microwave tubes, including traveling wave tubes and velocity-modulated tubes, which accounts for its high efficiency.
Edited by Makoto KOBAYASHI, M.J.A
Correspondence should be addressed to: H. Mimura, Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku Chuo-ku Hamamatsu, Shizuoka 432-8011, Japan (e-mail: mimura.hidenori@shizuoka.ac.jp).
This paper commemorates the 100th anniversary of this journal and introduces the following papers previously published in this journal. Okabe, K. (1927) A new method for producing undamped extra-short electromagnetic waves. Proc. Imp. Acad. 3 (4), 204 ( https://doi.org/10.2183/pjab1912.3.204); Okabe, K. (1927) Production of extra short electromagnetic waves by split-anode magnetron. Proc. Imp. Acad. 3 (8), 514-515 ( https://doi.org/10.2183/pjab1912.3.514).
[From Proc. Imp. Acad., Vol. 3 No. 4, p. 204 (1927)]
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[From Proc. Imp. Acad., Vol. 3 No. 8, pp. 514–515 (1927)]
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Hidenori Mimura, born in Kyoto in 1956, graduated from the Department of Electronics, Faculty of Engineering, Shizuoka University, in 1979. He received M.S. and Ph.D. degrees from Shizuoka University in 1981 in 1987, respectively. He worked as Research Associate in Shizuoka University (1981–1984). He worked as Senior Researcher in Nippon Steel Corporation (1987–1994) and the ATR Optical Radio Communication Research Laboratories (1994–1996). He joined and worked as Associate Professor in Research Institute of Electrical Communication (RIEC), Tohoku University (1996–2003). From 1998 to 1999, he was a Visiting Researcher of the Technical University of Darmstadt, Germany. He relocated to the Research Institute of Electronics (RIE), Shizuoka University, in 2003 and became Professor in RIEC (2003–2004). He worked as Professor in RIE (2003–2022) and was in charge of a director of RIE (2007–2022). He was a Research Fellow of Shizuoka University (2011–2022). He was an Editor of IEEE ED letters (2016–2022). He was a Visiting Professor of Ocean University of China, Kaunas University of Technology, and Tokyo Medical and Dental University and an Adjunct Professor at the University of Indonesia. He retired from Shizuoka University in 2022. He is currently Specially Appointed Professor at RIE, Professor Emeritus at Shizuoka University, and Honorary Research Fellow at Shizuoka University. He is an Honorary Professor of St. Petersburg State Institute of Technology, an Adjunct Professor of Ontario Technical University, an Honorary Doctor of Liga Technical University, and an Honorary Member of the Academy of Science of Moldova. His research activity is electron devices including vacuum electronics and optical devices including imaging devices. While he was a student, he attended Professor Okabe’s final lecture on the invention of magnetron (1979).
