1. Development of Frequency-Tunable Terahertz Radiation Sources
Tsun-Hsu Chang 張存續
Department of Physics,
National Tsing Hua University, Taiwan
2008 FISFES Workshop November 6 – 8, NCKU, Tainan, Taiwan

2. Introduction:

3. Objective: Filling the Terahertz Gap
Science, 23 November 2007
R. Kleiner, “Filling the terahertz gap”, Science 318, 1254 (2007). 3

4. Terahertz Research THz photonics THz spectroscopy THz plasmonics
Plasma diagnostics
Fusion
ESR
DNP NMR
Material processing
Number of publications in Physical Review Letters.
Search title/abstract using the key word--- “terahertz”.

5. Applications: high power
ESR
DNP
NMR

6. How to Generate Terahertz Radiations
Generate THz radiations:
How to Generate Terahertz Radiations
Electronics: (high power, coherent)
Free electron laser
Electron cyclotron maser - gyrotron
Backward-Wave Oscillator
Photonics: (low power, non-coherent)
Josephson effect
Quantum cascade laser
Far infrared laser
Femtosecond laser

7. Terahertz vacuum electron devices
Generate THz radiations: high power
Terahertz vacuum electron devices
Terahertz FEL
FEL
gyrotron
BWO

8. 高頻電磁實驗室 High Frequency Electrodynamics Laboratory
指導教授:朱國瑞老師,張存續老師 博士後:邱陳琦,姜惟元 博士班:高士翔,戴玲潔,吳家勳,吳光磊,陳乃慶,袁景濱 碩士班:林冠男,林柏年,康迺豪,劉煜,林柏宏, 姚仁傑,吳智遠,徐複樺,吳俊潭,姚欣佑 學士班：姜博瀚,郭彥廷,任德育,李育浚 校外合作:工研院材化所,中科院,同步輻射,高速電腦 國際合作:UC-Davis, USA, Fukui Univ., Japan
Terahertz gyrotron term

9. one photon multiple-photon multiple photon
Toward Bridging the Terahertz Gap
one photon multiple-photon multiple photon
per excitation, per electron, per electron,
  large interaction large interaction interaction space
space space ~ wavelength
  
Terahertz gap
Frontier in science and technology.

10. ECM based Devices --- gyrotrons
The gyrotron is a coherent radiation source based on the
electron cyclotron maser (ECM) interaction
gyro-monotron
high average power
gyro-BWO
continuous frequency tunability
(relatively unexploited)
利用ECM機制來產生微波的元件, 我們統稱為gyrotrons.
從震盪機制上來說, 又可分為下面兩類, gyro-monotron (磁旋單腔管)與gyro-BWO(磁旋返波震盪器).
Gyro-monotron是利用兩端反射機制, 形成共振模式, 因此它的功率輸出可以很大.
而gyro-BWO則是利用內部回饋機制, 它的最大特性是頻率可調的特性.
Gyro-BWO 的困難度與門檻相對較高, 使得它的神秘面紗, 一直到最近幾年才逐漸被揭露.
由於 gyrotron 特別是 gyro-BWO 本身容易受單一或多模競爭而產生非穩態現象.
為了進一步說明這些非穩態特性, 我們來看電磁場的建立過程.
60 (330)

11. Difficulties for Terahertz gyro-BWO
利用ECM機制來產生微波的元件, 我們統稱為gyrotrons.
從震盪機制上來說, 又可分為下面兩類, gyro-monotron (磁旋單腔管)與gyro-BWO(磁旋返波震盪器).
Gyro-monotron是利用兩端反射機制, 形成共振模式, 因此它的功率輸出可以很大.
而gyro-BWO則是利用內部回饋機制, 它的最大特性是頻率可調的特性.
Gyro-BWO 的困難度與門檻相對較高, 使得它的神秘面紗, 一直到最近幾年才逐漸被揭露.
由於 gyrotron 特別是 gyro-BWO 本身容易受單一或多模競爭而產生非穩態現象.
為了進一步說明這些非穩態特性, 我們來看電磁場的建立過程.
60 (330)

12. Difficulties: Underlying Physics
2000 Nonlinear field contraction
2001 Nonstationary and chaotic behavior
利用ECM機制來產生微波的元件, 我們統稱為gyrotrons.
從震盪機制上來說, 又可分為下面兩類, gyro-monotron (磁旋單腔管)與gyro-BWO(磁旋返波震盪器).
Gyro-monotron是利用兩端反射機制, 形成共振模式, 因此它的功率輸出可以很大.
而gyro-BWO則是利用內部回饋機制, 它的最大特性是頻率可調的特性.
Gyro-BWO 的困難度與門檻相對較高, 使得它的神秘面紗, 一直到最近幾年才逐漸被揭露.
由於 gyrotron 特別是 gyro-BWO 本身容易受單一或多模競爭而產生非穩態現象.
為了進一步說明這些非穩態特性, 我們來看電磁場的建立過程.
60 (330)
2002 Linear and time-dependent behavior of gyro-BWO
2005 Dynamics of mode competition
All published in PRL.

13. The high efficiency gyro-BWO
T. H. Chang, C. T. Fan, K. F. Pao, K. R. Chu, and S. H. Chen, “Stability and tunability of Gyrotron Backward-Wave Oscillator”, Appl. Phys. Lett. 90, (2007).

14. W-band TE01 gyro-BWO @ 3-5 A, 100 kV Difficulty #1: 110 GHz PNA
Difficulty #2: THz mode converter
Difficult #3: Electron gun
Outer electrode
Electron emitter
Anode
Vacuum container
Center electrode
(Cathode nose)
@ 3-5 A, 40.4 kG
Good morning, ladies and gentlemen!
I am very happy to know that gyro-BWO becomes a hot subject,
because there are four talks related to gyro-BWO in this session.
Today, I want to talk about W-band gyro-BWO.
It is a joint effort of Tsing Hua’s group and Prof. Yeh’s group.
25s
Difficulty #4: Magnet
T. H. Chang, et al., “W-band TE01 gyrotron backward-wave oscillator with distributed loss”, Phys. Plasmas 15, (2008). 14 14

15. First difficulty: Basic diagnostic system
Solved
2006: vector network analyzer
E8363B 2,800,000 NTD.
2007: test set controller
N5260A 2,400,000 NTD.
Good morning, ladies and gentlemen!
I am very happy to know that gyro-BWO becomes a hot subject,
because there are four talks related to gyro-BWO in this session.
Today, I want to talk about W-band gyro-BWO.
It is a joint effort of Tsing Hua’s group and Prof. Yeh’s group.
25s
2008: Millimeter wave head module
N5260AW10 2,600,000 NTD. 15 15

16. Second difficulty: TE01 mode converter
Solved
22 GHz

17. Second difficulty  a main vantage
T. H. Chang, C. H. Li, C. N. Wu, and C. F. Yu, “Exciting circular TEmn modes at low terahertz region”, Appl. Phys. Lett. 93, (2008).

18. Terahertz Devices Using LIGA
Require high precision machining (<2 um)  LIGA technique
1. irradiation (mask)
3. electroforming
2. development (SU-8)
4. cold test
200 GHz TE02 mode converter (for Fukui University, Japan)
400 GHz TE41 mode converter (Fourth harmonic gyrotron)

19. Scaled experiment: Ka-band TE01 gyro-BWO
Tuning range: 15.8%
Peak efficiency: 23.7%
Good morning, ladies and gentlemen!
I am very happy to know that gyro-BWO becomes a hot subject,
because there are four talks related to gyro-BWO in this session.
Today, I want to talk about W-band gyro-BWO.
It is a joint effort of Tsing Hua’s group and Prof. Yeh’s group.
25s
Distributed loss suppresses the axial-mode competition.
Mode-selective circuit suppresses the transverse-mode competition. 19 19

20. Third difficulty: Electron gun
Magnetron Injection Gun (MIG) for W-band gyro-BWO.
CUSP gun for high harmonic gyro-BWO.
Problem for gyro-BWO
New design
(for gyro-BWO)
Old design
(for gyro-TWT)
W-band MIG gun is ready for fabrication.
CUSP gun simulation

21. Fourth difficulty: Magnet
Superconducting magnet
(8 Tesla, 6,000,000 NTD)
Pulsed magnet
(40 Tesla, 3,000,000 NTD)
Our old magnet is aging
and has hysteresis effect.
2 Tesla is barely okay.
Magnet is the biggest problem to us.
Sufficient research funding can solve the problem.

22. Reducing the Magnetic Field Requirement
Second harmonic slotted gyro-BWO:
Good morning, ladies and gentlemen!
I am very happy to know that gyro-BWO becomes a hot subject,
because there are four talks related to gyro-BWO in this session.
Today, I want to talk about W-band gyro-BWO.
It is a joint effort of Tsing Hua’s group and Prof. Yeh’s group.
25s
N. C. Chen, C. F. Yu, and T. H. Chang, “A TE21 second-harmonic gyrotron backward-wave oscillator with slotted structure”, Phys. Plasmas, 14, (2007). 22 22

23. Gyrotrons at Fukui FIR Center
0.4 THz, 1.5 kW
1 THz 0.25 kW
A series of LHe free superconducting magnets:
8T, 12 T, 17 T, and 21 T.

24. International Cooperation: 394 GHz Frequency tunable gyrotron24

25. International Cooperation: 203 GHz TE02 gyro-BWO
Novel TE02 mode converter
using LIGA technique.
Novel mode-selective circuit 25

26. Conclusion and Foresight
Terahertz gap
NTHU Terahertz Research Center