Revival of Vacuum Electronics and UNIST Research Activities€¦ · Challenges in high power THz source High EM power density Dense electron beams breakdown Precise circuit fabrication

THz Vacuum Electronics and Electrodynamics (TEE) Lab.

Revival of Vacuum Electronics and UNIST Research Activities

EunMi ChoiOn behalf of our TEEM current members and alumni

Dept. Physics

Ulsan National Institute of Science and Technology (UNIST),

Ulsan, South Korea

Nov. 15, 2017@ Gyeongju, ICABU

CONTENTS

• Introduction

• Current mm & sub-mm Vacuum Device development

• UNIST recent activities

• Conclusions

Doubling of Pf2 every two years! Ref: Levush, IVEC 2007

IntroductionSame principle, but different medium!

Solid-state electronics Vacuum electronics

semiconductor vacuummedium

Ref: Slides by J. Booske

Introduction

Solid state source

• For solid state, peak power ~ average power• For vacuum, peak power >> average power

Ref: Levush, IVEC 2007

Introduction

Ref: Slides by J. BooskeTwo frontiers:Constant Pf2 limit of HPM (1-100 GHz)THz gap (100-1000 GHz): < 𝑃 >∝ 1/𝑓2

Require1) High EM power density2) Dense electron beams

Nuclear fusion application24 MW

20 MW

160 m

Total 24 MW power at 170 GHz should be produced. 20 MW power delivered to plasma. Total loss budget less than 17 % Gaussian output mode purity greater than 95 %

• For ITER, 63.5 mm diameter corrugated Al waveguide is being developed.

• Losses due to 1) ohmic loss, 2) mode conversion of HE11 should be mitigated by proper design of transmission line

내부의 Corrugation 구조(단위 : mm)

Highly overmodedcircular HE11 waveguides DEMO reactor

• Frequency is above 200 GHz!

DNP-NMR

NMR frequency

Magnetic field

DNPfrequency

400 MHz 9.4 T 260 GHz

600 MHz 14 T 390 GHz

800 MHz 19 T 520 GHz

900 MHz 21 T 585 GHz

1000 MHz 23.5 T 650 GHz

Bruker 527 GHz DNP-NMR

DNP NMR brings an excellent application for gyrotrons Hundreds of high field NMR spectrometers all over the world

NMR is an important tool for determination of structures in material science/structural biology. Low in sensitivity of nuclear spin

Transfer of electron spin polarization to nuclear spin polarization

It is also applicable for remote sensing of clouds

- Typical cloud sensing radars are a few kW, X/Ka-band, and recently, W-band

Advantages?Radar cross section of a cloud droplet of radius r ~ r6/λ4

• high frequency is much better• Laser radars (Lidars) cannot penetrate visibly opaque clouds millimeter-wave radar well suited to cloud imaging

Disadvantages?Water absorption, therefore, precise frequency selection (35 GHz, 94 GHz)

High power mm-wave radar

Ref: IEEE Spectrum, Sep. 2012,

The obstacle

Realistic considerations

Power needed to send data at THz frequencies transmitting less than 100 m only realistic

Identifying unknown substances at a distance the sample’s distinctive feature is washed out at 10 m and 100 m!

Ref: IEEE Spectrum, Sep. 2012,

Challenges in high power THz source

High EM power density

Dense electron beams

breakdown

Precise circuit fabrication

Beam generation

Beam confinement

Key challenges• Micromachined interaction structures• Advanced cathode technology• Magnetics

Levush et al., IRMMW2009

Minimum beam thickness~𝑇1/2𝐽𝑏/𝐵𝐽𝑐

Power TWT ~ 𝑁 × 24(1

𝑓)8/3𝑉𝑏

13/6𝐽4/3

Recent research trends in VETowards THz, Towards Compactness

MEMS

3D printing

Cold cathode

“nano” vacuum tube

He leak check

Hwu et al., IVEC2016: W-band 3D printing, Innosys

• Surface roughness ~ 30 nm• No leak• W-band structure

Recent innovative microfab. technologies

UV-LIGA

Han et al., APL100, 213505 (2012)

Northrop Grumman 0.85 THz TWT (2013년 11월공식발표)

Field Emission DC

UNIST recent activities

• Microfabricated vacuum electronics• High power gyrotron development and its application• Intrinsic Orbital angular momentum of gyrotron beam

Microfabricated vacuum device (Precision machining)

• Circuit size is reduced f > 100 GHz

• Relatively simple mechanical machining can be used up to 400 GHz

• Up to 1 THz circuit fabrication is possible

• Need to consider fabrication time, price, and tolerance

NRL (USA): C. Joye et al., IEEE Trans Elec. Dev.,

vol. 61, June 2014

• Circuit power (hot test)

~ >60W

• Electronic efficiency ~

5.5 %

• 11.5 kV, ~100mA beam

energy

• BW 15 GHz

• 500 usec, 2 Hz rep

rate

THz Vacuum Electronics

C. Joye et al., J. Micromech. Microeng. (2012)

Folded waveguide fabricated by nanoCNCSolid-state / vacuum integration system

• Beam tunnel elimination• High current

Modified sine-waveguide slow wave structure

Elliptical beam

Folded waveguide fabricated by nanoCNCMeasured with mechanical assembly

Measured with diffusion bonding

W.J. Choi et al., IEEE-TED, vol. 64 (2017)

Analysis with simulation

RF breakdown

Air breakdown

Required power vs. Frequency

For breakdown, @ 100 GHz: Pout > hundreds of kW@ 300 GHz: Pout > tens of kW

1000

10000

100000

1000000

10000000

100000000

1.E+10 1.E+11 1.E+12

Frequency (Hz)

Pow

er (W

)

1

10

100

1000

10000

100000

Pow

er (

kW)

10 100 1000

Frequency (GHz)

he

igh

t: 1

65

cm

width: 85 cm

W-band gyrotron at UNIST

Gyrotron development

Avalanche gas breakdown

It is the first high power (>10kW),

high frequency (>90 GHz) gyrotron

development in Korea!

• 95 GHz• 60 kW

UNIST gyrotron performance

Ref: S.G.Kim at al., IEEETST (2015)S.G.Kim et al., Jour. Infra.Milli.THz (2016)

Stand-off radioactive material detection

DARPA’s SIGMA program: to prevent attacks involving “dirty bombs” and other nuclear threats

Gas collection: (ex) xenon, inert gas, a fission product in nuclear reactors

• Takes long time to detect• Strongly affected by wind direction, etc• Sensitivity issue

• Goal: prevent attacks involving dirty bombs

• City-scale, dynamic, real-time map• Real-time map of background

radiation by networked detectors• Logging more than 100,000 hours

covering 150,000 miles• Distinguish benign sources and

threatening ones

• Sensitivity issue

Gamma-ray enhanced emission of fluorescence (γ-REEF)

THz-REEF experiment from air-plasma using a single color laser pulse

Electron acceleration in the THz field and collision with molecules

THz-enhanced fluorescence spectra of N2 gas-plasma

Laser-induced 플라즈마에서는레이저광자의흡수를통해많은 high lying states 들이존재하여이러한상태의분자들은 energetic electron과의충돌에의한 ionization 이 ground state에있는분자들과의충돌보다더쉽게일어난다

THz-REEF

Replace THz wave with gamma ray has not yet demonstrated

IR

Courtesy: X.C.Zhang at Univ. Rochester

A new proposal for remote detection

G. S. Nusinovich et al, Journal of Infrared, Millimeter, and Terahertz Waves 32 (3), 380-402 (2010).

fs

The total delay time:

Delay time

Forward power

Breakdown

Breakdown delay time

sf+ττ=τ

The total delay time:

n

N

nN exp)(P

1

t

di dtttn0

'' )(exp)(

𝜏𝑓 : The exponentiation time of electron

avalanche ionization that proceeds from an initial

seed electron.

𝜏𝑠 : The waiting time for a seed electron to

appear to initiate a breakdown.

Delay time

Forward power

Breakdown

Formative delay time Statistical delay time with radioactivity

)exp(),0(P2 tStn

P = P1 + P2 : The total probability for breakdown

discharge.

( ) ( )tStSn

n n ΔexpΔ!

1=)(P2

S = 6 μs-1

(average rate of electron generation by the seeding

source)

( ) ( )nndNNPtnN cr

n

cr

cr

/exp1==),<(P0

1 --∫

Experimental setupExperimental setup using UNIST 95 GHz gyrotron

Time gap between RF detector & Photodiode

⇒ Comparison of with & without source

• Frequency : 95 GHz

• Output power : ~ 32 kW

• Pulse length : 20 μs

• Beam radius at focal point : ~ 5 mm.

Radioactive source : Electric field ↓, pressure range ↑

Observation of plasma breakdown even in 760 Torr

(under-threshold condition)

Pressure (gas) Plasma density

760 Torr (air) 6.44×1013 cm-3

760 Torr (Ar) 6.23×1013 cm-3

60 Torr (air) 5.87×1013 cm-3

Breakdown experiment with radioactive material

D. S. Kim et al., Nat. Commu. 8, 15394 (2017)

With source (red cross), Without source (blue circles), and Calculated distribution (black

line)

Elimination of statistical delay time with radioactive source

• Plasma delay time measurement (Ar)



Real-time detectability


• Ar gas

• Output power : 19 kW

• Inner pressure : 250 Torr

• Distance : 20 cm ~ 120 cm

20 cm

Clear difference in delay time w & w/o radioactive material

Breakdown occurred with only 30 kW in air as well as Ar! Only with the presence of radioactive material


Ar gas

Air


Analysis on breakdown conditionObservation on the reduction of the required electric field:16 kV/cm (w/o) 3.4 kV/cm (w/)

Postulate: the increased conductivity in the breakdown-prone volume leads to the reduction in the electric field amplitude

Introduce β: field-reduction factorE0: applied RF fieldEcr: required RF field amplitude for breakdown

n0: seed electron density w/o radioactive material (~ 1-10 /cm3)n0*: seed electron density w/ radioactive material


Analysis on breakdown condition

MCNPX simulation

• The average number & energy of high energy electrons are 50 and 0.44 MeV

12600 of secondary knock-on electrons produced

• The total time for generation of 12600 secondary knock-on electrons is ~ 5x10-9 sec

• For a duration of 1 μs before the plasma breakdown is induced, the number density of the total secondary knock-on electrons is 1.3x108/cm3

Primary high energy electrons due to Compton scattering

Secondary knock-on electrons


Result

Field reduction

Analysis 2.5

Experiment 4

Orbital angular momentum (OAM) beam

Analogy: Photon vs. Electron

Circularly polarized beam Helically rotating beam

Orbital angular momentum

Vaziri et al., J. Opt. B: Quantum

Semiclass. Opt. 4 (2002)

Spin angular momentum l

OAM provides an additional dimension to multiplexing techniques that can be employed to achieve higher data rates

Using l=±1, ±4, ±15, 532nm 20mW laser, spatially send OAM beams. Using pattern recognition algorithm (no phase measurement), identify mode patterns

3 km

M. Krenn et al., New Jour Phys 16, 2014

Free space, wireless communications

Orbital angular momentum (OAM) beam

R. M. Henderson, IEEE microwave magazine, 2017

UNIST OAM gyrotron

Gyrotron: a high power millimeter/THz vacuum tubes using a rotating electron beam

Electron

emission

Interaction

cavity

Mode

converter

Gyrotron schematic

e

cm

eB

22222 ckckz

czz nvk

Gaussian beam (TEM00)

Rotating TE modes (TE6,2)

e

cm

eB

Higher order OAM gyrotron mode generation

• Quasi-optical mode generator can mimickthe gyrotron rotating modes.

• TE6,2 & TE10,1

The perforated mode generator was manufactured by 3D printing technique

Measurement result: time-averaged measured amplitude and phase of TE6,2 and TE10,1 modes

TE6,2 TE10,1

A.Sawant et al., Sci. Rep. 3372 (2017)

Experimental results

Gyrotron OAM beam experiment

Phase information? Indirect measurement

Reference Sztul et al., OL 31, 2006

1l 2l

Conclusions

• Vacuum electronics pushes the technological limit in power and frequency by means

of recent microfabrication technology.

• UNIST’s first gyrotron demonstrated the long distance detectability of radioactive

material.

• Nano-CNC machining allowed the precise manufacturing of TWT circuits and showed

outstanding results so far.

• A new concept of OAM in ECM introduced

Documents

Revival of Vacuum Electronics and UNIST Research Activities€¦ · Challenges in high power THz source High EM power density Dense electron beams breakdown Precise circuit fabrication