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)
D. S. Kim et al., Nat. Commu. 8, 15394 (2017)
Breakdown experiment with radioactive material
Real-time detectability
D. S. Kim et al., Nat. Commu. 8, 15394 (2017)
• 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
D. S. Kim et al., Nat. Commu. 8, 15394 (2017)
Ar gas
Air
Breakdown experiment with radioactive material
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
D. S. Kim et al., Nat. Commu. 8, 15394 (2017)
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
D. S. Kim et al., Nat. Commu. 8, 15394 (2017)
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