Revival of Vacuum Electronics and UNIST Research Activities

EunMi Choi On behalf of our TEEM current members and alumni THz Vacuum Electronics and Electrodynamics (TEE) Lab. 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 Introduction Same principle, but different medium! Solid-state electronics Vacuum electronics medium semiconductor vacuum

Ref: Slides by J. Booske

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

Solid state source

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

Ref: Levush, IVEC 2007 Introduction

Two frontiers: Require Ref: Slides by J. Booske Constant Pf2 limit of HPM (1-100 GHz) 1) High EM power density THz gap (100-1000 GHz): < 푃 >∝ 1/푓2 2) Dense electron beams 24 MW Nuclear fusion application

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 160 m waveguide is being developed. • Losses due to 1) ohmic loss, 2) mode conversion of HE11 should be mitigated by proper design of transmission line Highly overmoded 20 MW circular HE11 waveguides DEMO reactor • Frequency is above 200 GHz!

내부의 Corrugation 구조 (단위 : mm) DNP-NMR NMR is an important tool for determination of structures in Bruker 527 GHz DNP-NMR material science/structural biology.  Low in sensitivity of nuclear spin Transfer of electron spin polarization to nuclear spin polarization

NMR Magnetic DNP frequency field frequency 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 DNP NMR brings an excellent application for gyrotrons  Hundreds of high field NMR spectrometers all over the world 1000 MHz 23.5 T 650 GHz High power mm-wave radar

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) The obstacle

Ref: IEEE Spectrum, Sep. 2012, Realistic considerations

Power needed to send data at THz frequencies Identifying unknown substances at a distance  transmitting less than 100 m only realistic  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

breakdown High EM power density Precise circuit fabrication

Beam generation Dense electron beams Beam confinement

1 Power TWT ~ 푁 × 24( )8/3푉 13/6퐽4/3 푓 푏

1/2 Minimum beam thickness~푇 퐽푏/퐵퐽푐

Key challenges • Micromachined interaction structures • Advanced cathode technology • Magnetics Levush et al., IRMMW2009 Recent research trends in VE Towards THz, Towards Compactness

Cold cathode

Field Emission DC

Northrop Grumman 0.85 THz TWT (2013년 11월 공식발표)

Recent innovative microfab. technologies

MEMS “nano” vacuum tube 3D printing He leak check UV-LIGA

Hwu et al., IVEC2016: W-band 3D printing, Innosys • Surface roughness ~ 30 nm Han et al., APL100, 213505 (2012) • No leak • W-band structure 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 THz Vacuum Electronics 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 C. Joye et al., J. Micromech. Microeng. (2012) Folded waveguide fabricated by nanoCNC

Solid-state / vacuum integration system

Modified sine-waveguide slow wave structure • Beam tunnel elimination • High current

Elliptical beam Folded waveguide fabricated by nanoCNC Measured with mechanical assembly

Analysis with simulation

Measured with diffusion bonding

W.J. Choi et al., IEEE-TED, vol. 64 (2017) Gyrotron development RF breakdown W-band gyrotron at UNIST It is the first high power (>10kW), Air breakdown high frequency (>90 GHz) gyrotron development in Korea!  Avalanche gas breakdown

Required power vs. Frequency

100000000100000

1000000010000

10000001000

100000100 width: 85 cm

Power (W) Power (kW) Power

10000

10 cm 165 height:

10001 1.E+1010 1.E+11100 10001.E+12 FrequencyFrequency (GHz) (Hz) For breakdown,

@ 100 GHz: Pout > hundreds of kW @ 300 GHz: P > tens of kW • 95 GHz out • 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 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

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

Laser-induced 플라즈마에서는 레이저광자의 흡수를 통해 많은 high lying states 들이 존재하여 이러한 IR 상태의 분자들은 energetic electron과의 충돌에 의한 ionization 이 ground state에 있는 분자들과의 충돌보다 더 쉽게 일어난다

THz-REEF experiment from air- Electron acceleration in the THz THz-enhanced fluorescence plasma using a single color laser field and collision with spectra of N2 gas-plasma pulse molecules

Replace THz wave with gamma ray  has not yet demonstrated

Courtesy: X.C.Zhang at Univ. Rochester A new proposal for remote detection

Forward power The total delay time:

Delay time   s  f Breakdown

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

Forward power 휏푓 : The exponentiation time of electron The total delay time: avalanche ionization that proceeds from an initial Delay time τ=τ f +τ s seed electron.

Breakdown 휏푠 : The waiting time for a seed electron to appear to initiate a breakdown. . Formative delay time . Statistical delay time with radioactivity 1  N  P(N)  exp  1 n   P (n) = (SΔt) exp(SΔt) n  n  2 n! t  ' '  n(t)  exp  (t )  dt P2 (n  0,t)  exp(S t) 0 i d 

-1 ncr S = 6 μs P1(N < ncr ,t) = ∫ P(N)dN = 1- exp(- ncr / n) (average rate of electron generation by the seeding 0 source)

P = P1 + P2 : The total probability for breakdown discharge. Experimental setup Experimental setup using UNIST 95 GHz gyrotron

• Frequency : 95 GHz  Time gap between RF detector & Photodiode • Output power : ~ 32 kW • Pulse length : 20 μs ⇒ Comparison of with & without source • Beam radius at focal point : ~ 5 mm. Breakdown experiment with radioactive material

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

 Radioactive source : Electric field ↓, pressure range ↑  Observation of plasma breakdown even in 760 Torr (under-threshold condition)

D. S. Kim et al., Nat. Commu. 8, 15394 (2017) Breakdown experiment with radioactive material • Plasma delay time measurement (Ar)

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

 Elimination of statistical delay time with radioactive source

D. S. Kim et al., Nat. Commu. 8, 15394 (2017) 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 D. S. Kim et al., Nat. Commu. 8, 15394 (2017) Breakdown experiment with radioactive material

Ar gas

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

D. S. Kim et al., Nat. Commu. 8, 15394 (2017) Analysis on breakdown condition

Observation 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 factor E0: applied RF field Ecr: required RF field amplitude for breakdown

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

D. S. Kim et al., Nat. Commu. 8, 15394 (2017) Analysis on breakdown condition

• The average number & energy of high energy electrons are 50 and 0.44 MeV  12600 of secondary knock-on electrons produced Primary high energy • The total time for generation of 12600 secondary -9 electrons due Secondary knock-on electrons is ~ 5x10 sec to Compton knock-on scattering electrons • 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 MCNPX simulation

Result

Field reduction Analysis 2.5 Experiment 4

D. S. Kim et al., Nat. Commu. 8, 15394 (2017) Orbital angular momentum (OAM) beam Analogy: Photon vs. Electron Circularly polarized beam Helically rotating beam

Spin angular Orbital angular momentum   momentum  l

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

Vaziri et al., J. Opt. B: Quantum Semiclass. Opt. 4 (2002) Orbital angular momentum (OAM) beam Free space, wireless communications 3 km

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

R. M. Henderson, IEEE microwave magazine, 2017

M. Krenn et al., New Jour Phys 16, 2014 UNIST OAM gyrotron Gyrotron: a high power millimeter/THz Gyrotron schematic vacuum tubes using a rotating electron beam

  kzvz  nc eB   c m e Mode 2 2 2 2 2 converter   kz c  kc

Gaussian beam (TEM00)

Interaction Rotating TE modes (TE6,2) cavity

eB c  me Electron emission Higher order OAM gyrotron mode generation

• Quasi-optical mode generator can mimick the gyrotron rotating modes. • TE6,2 & TE10,1

The perforated mode generator was manufactured by 3D printing technique

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

A.Sawant et al., Sci. Rep. 3372 (2017) Experimental results Gyrotron OAM beam experiment Phase information? Indirect measurement

l  1 l  2

Reference Sztul et al., OL 31, 2006 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