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

Introduction:

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

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”.

Applications: high power ESR DNP NMR

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

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

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

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.

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)

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)

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.

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, 191501 (2007).

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, 073105 (2008). 14 14

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

Second difficulty: TE01 mode converter Solved 22 GHz

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, 111503 (2008).

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)

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

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

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.

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, 123105 (2007). 22 22

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.

International Cooperation: 394 GHz Frequency tunable gyrotron 24

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

Conclusion and Foresight Terahertz gap NTHU Terahertz Research Center