1

Development of Frequency-Tunable Terahertz Radiation Sources

Tsun-Hsu Chang 張存續Department of Physics,

National Tsing Hua University, Taiwan

2008 FISFES WorkshopNovember 6 – 8, NCKU, Tainan, Taiwan

2

Introduction:

3

Objective: Filling the Terahertz Gap

R. Kleiner, “Filling the terahertz gap”, Science 318, 1254 (2007).

Science, 23 November 2007

4

Terahertz Research

THz photonics THz spectroscopy THz plasmonics Plasma diagnostics

Fusion ESR DNP NMR Material processing

Number of publications in Physical Review Letters.

2 0 0 0 2 0 0 1 2 0 0 2 2 0 0 3 20 0 4 2 0 0 5 2 0 0 6 2 0 0 7 2 0 0 8Y e ar

0

2 0

4 0

6 0

8 0

Num

ber o

f pub

licat

ion fo rec a st

Search title/abstract using the key word--- “terahertz”.

5

Applications: high power

ESR

DNPNMR

6

How to Generate Terahertz Radiations

Photonics: (low power, non-coherent) Josephson effect Quantum cascade laser Far infrared laser Femtosecond laser

Electronics: (high power, coherent) Free electron laser Electron cyclotron maser - gyrotron Backward-Wave Oscillator

Generate THz radiations:

7

Terahertz vacuum electron devices

Terahertz FEL

Generate THz radiations: high power

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

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

Frontier in science and technology.

Terahertz gap

10

ECM based Devices --- gyrotrons

gyro-monotronhigh average power

gyro-BWO continuous frequency tunability

(relatively unexploited)

The gyrotron is a coherent radiation source based on the electron cyclotron maser (ECM) interaction

11

Difficulties for Terahertz gyro-BWO

12

Difficulties: Underlying Physics

• 2000 Nonlinear field contraction

• 2001 Nonstationary and chaotic behavior

• 2002 Linear and time-dependent behavior of gyro-BWO

• 2005 Dynamics of mode competition

All published in PRL.

13

The high efficiency gyro-BWO

1 3 .5 1 4 1 4 .5 1 5 1 5 .5 1 6B o (k G )

3 2 .5

3 3

3 3 .5

3 4

3 4 .5

3 5

Freq

uenc

y (G

Hz)

0

5

1 0

1 5

2 0

2 5

3 0

Eff

icie

ncy

(%)

0

2 5

5 0

7 5

1 0 0

1 2 5

1 5 0

Pow

er (k

W)

ex p e rim en t

s im u la tio n

ex p erim en t

sim u la tio n

5 0 60 7 0 8 0 90 10 0 11 0V b (k V )

32 .5

3 3

33 .5

3 4

34 .5

3 5

Freq

uenc

y (G

Hz)

0

5

1 0

1 5

2 0

2 5

3 0

Effic

ienc

y (%

)

ex p erim en t

sim u la tio n

ex p erim en t

sim u la tio n

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).

14

Outer electrodeElectron emitter

AnodeVacuum container

Center electrode(Cathode nose)

14

W-band TE01 gyro-BWO

40 5 0 60 7 0 80 9 0 1 00 11 0 1 20V 0 (k V )

0

30

60

90

12 0

P out

(kW

)

9 5

9 6

9 7

9 8

9 9

Freq

. (G

Hz)

39 4 0 4 1 4 2 43 44B 0 (k G )

0

3 0

6 0

9 0

1 20

P out

(kW

)

9 5

9 6

9 7

9 8

Freq

. (G

Hz)

Vb=100 kV , B0=40.4 kG , =1.0, rc /rw =0.466-0.4763, rw =0.189-0.193 cm TE01 S1

Ib= 5 A

4 A3 A

Ib= 5 A4 A3 A

(a )

(b )

@ 3-5 A, 100 kV

@ 3-5 A, 40.4 kG

Difficulty #1: 110 GHz PNA

Difficult #3: Electron gun

Difficulty #4: Magnet

Difficulty #2: THz mode converter

40 50 60 70 80 90 100 11 0 120V 0 (k V )

0

30

60

90

120

P out

(kW

)

95

96

97

98

99

Freq

. (G

Hz)

39 40 41 42 43 44B 0 (k G )

0

30

60

90

120

P out

(kW

)

95

96

97

98

Freq

. (G

Hz)

Vb=100 kV , B0=40.4 kG , =1.0, rc /rw =0.466-0.4763, rw =0.189-0.193 cm TE 01 S1

Ib= 5 A

4 A3 A

Ib= 5 A4 A3 A

(a )

(b )

T. H. Chang, et al., “W-band TE01 gyrotron backward-wave oscillator with distributed loss”, Phys. Plasmas 15, 073105 (2008).

1515

First difficulty: Basic diagnostic system

2006: vector network analyzer E8363B 2,800,000 NTD.

2007: test set controller N5260A 2,400,000 NTD.

2008: Millimeter wave head module N5260AW10 2,600,000 NTD.

Solved

16

Second difficulty: TE01 mode converter

85 90 95 100 105 110Frequency (G H z)

-6

-5

-4

-3

-2

-1

0

Tran

smis

sion

(dB

)Exp.Th.

22 GHz

Solved

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, 111503 (2008).

18

Terahertz Devices Using LIGA

Require high precision machining (<2 um) LIGA technique

1. irradiation (mask)

2. development (SU-8)

3. electroforming

200 GHz TE02 mode converter (for Fukui University, Japan) 400 GHz TE41 mode converter (Fourth harmonic gyrotron)

4. cold test

1919

Scaled experiment: Ka-band TE01 gyro-BWO

1 3 1 4 1 5 1 6 1 7 1 8 1 9B z (k G )

0

10

20

30

5

15

25

effic

ienc

y (%

)

3 0

3 2

3 4

3 6

3 8

freq

(GH

z)

dotted line - experim ent broken line - sim ulation

1 3 1 4 1 5 1 6 1 7 1 8 1 9B 0 (k G )

0.6

0.8

1

1.2

1.4

00.040.080.120.160.2

vz/v

z0

0 2 4 6 8 10 12 14 16Z (c m )

0.50.520.540.560.58

0.6

r w (c

m)

A quadaqC oating

(a ) 4-P in Interface

6-P inInterface

(c)

(b )

v z/v z 0

Distributed loss suppresses the axial-mode competition. Mode-selective circuit suppresses the transverse-mode competition.

Tuning range: 15.8%Peak efficiency: 23.7%

20

Problem for gyro-BWO

Old design(for gyro-TWT)

Third difficulty: Electron gun

1 3 1 3 . 5 1 4 1 4 . 5 1 5 1 5 . 5 1 6 (k G )

6 0

7 0

8 0

9 0

1 0 0

1 1 0

(

kV)

bV

B0

Magnetron Injection Gun (MIG) for W-band gyro-BWO. CUSP gun for high harmonic gyro-BWO.

0 10 20 30 40 50Z (cm )

0

2

4

6

8

10

12

r (c

m)

0

10000

20000

30000

40000

Bz (

G)

Struc tureM agnetic fie ldEqual-potentia l lineE lec tron tra jec to ry

5 A m p s 7 0 k V

v z/v z = 2 .3 %v /v= 5 .4 %r g = 0 .9 5 9 m mr L = 0 .1 9 9 m m

3 8 k G

New design(for gyro-BWO)

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 agingand has hysteresis effect.2 Tesla is barely okay.

Magnet is the biggest problem to us. Sufficient research funding can solve the problem.

2222

Reducing the Magnetic Field Requirement Second harmonic slotted gyro-BWO:

6 6.4 6.8 7.2 7.6 8B z (k G )

0

4

8

12

16

20

24

effic

ienc

y (%

)

3 1

3 2

3 3

3 4

3 5

3 6

freq

(GH

z)

I b= 5 (A )I b= 4 (A )I b= 3 (A )

p z/p z 0= 0.0 5

- 8 - 4 0 4 8k z ( c m -1)

0

20

40

60

80

f (G

Hz)

b o rken l in e : sm o o th-b o re w a veg u id eso l id l in e : s lo t ted -bo re w avegu ide (b /a = 1 .5 )

B z= 6 .8 k G

m o d e (T E 2 1)/2 m o d e (T E 1 1)

2 m o d e (T E 0 1)

/2 m o d e ()

s= 1

s= 2

s= 3

ab

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).

23

Gyrotrons at Fukui FIR Center

0.4 THz, 1.5 kW1 THz 0.25 kW

A series of LHe free superconducting magnets:

8T, 12 T, 17 T, and 21 T.

2424

International Cooperation: 394 GHz Frequency tunable gyrotron

7 .1 5 7 .20 7 .2 5 7 .3 0 7 .3 5 7 .40 7 .4 5B o (T )

1 0 -2

1 0 -1

1 0 0

1 0 1

I st (A

)Vb=13.7 kV , =1.6, rc /rw =0.16 rw =0.2372 cm

TE06 S2 starting currentw ith s lightly W G taper 02 lossless

2 .5 o

3 .0 om ild ly tap e red

0 .3 5 A

l =1 2 3

4 5

u n ifo rm

0 .4 cm 2 .0 cm 0 .5 cm

0 .4 c mrw = 0 .2 3 7 2 cm

(a )

(b )

7 .2 0 7 .25 7 .30 7 .35 7 .4 0 7 .4 5B o (T )

0

5 0

1 0 0

1 5 0

2 0 0

P out

(W)

394

395

396

397

398

399

Freq

(GH

z)

l =1

l = 2

l = 3

l = 4

l = 5

pumping port

pumping port

collector

water jacket

super conductingmagnet

interaction section

output window

MIG gun

gun coils

2525

International Cooperation: 203 GHz TE02 gyro-BWO

Novel TE02 mode converter using LIGA technique.

Novel mode-selective circuit

7 2 7 6 8 0 8 4 8 8 9 2B 0 (k G )

0

10

20

30

effic

ienc

y (%

)

190

200

210

220

230

freq

(GH

z)

v z/v z 0= 5 %Ib= 0 .7 A

Ib= 0 .5 A

Ib= 0 .4 A

203 G H z

26

Conclusion and Foresight

Terahertz gap

NTHU Terahertz Research Center