Transcript Phase 2: Hollow cathode discharge

Millimetre Wave and THz
Research at QMUL
Professor Xiaodong Chen
School of Electronic Engineering and
Computing Science
Queen Mary University of London
Email: [email protected]
Department of
Electronic Engineering
Outline

Where am I from?

History of QMUL Group

Some New Topics

Summary
Department of
Electronic Engineering
Queen Mary, University of London
Queen Mary and Westfield College was founded in 1889, one of four
major Colleges of University of London, ranked in12/13 place last year.
The newly merged Medical Hospital of the College was founded in
1373, the first teaching hospital in London!
Sir Peter Mansfield (Co-winner of 2003 Nobel prize in medicine (MRI))
was a graduate in physics at Queen Mary, University of London.
Department of
Electronic Engineering
Department of Electronic Engineering
Queen Mary, University of London

Antenna & Electromagnetics Group (since
1968)
Networks Group
Centre for Digital Music
Multimedia & Vision Group

22 full staff + 10 teaching staff



Department of
Electronic Engineering
The Antenna and Electromagnetic Group





Prof. Clive Parini (Director of Graduate School)
Prof. Xiaodong Chen (Director of Graduate Studies)
Prof. Yang Hao
Dr Robert Donnan (Lecturer)
Dr Akram Alomainy (Lecturer)

Prof Peter Clarricoats, FRS (part time)
Prof. Derek Martin (part time)
Prof. Brian Collins (visiting professor)
George Hockings (visiting professor)

10 Postdcotoral Research Assistants,

20 PhD Research Students



Department of
Electronic Engineering
Brief History: Major Milestones








1969-70 Analysis and Design of Corrugated Horns.
1973
First UK Compact Antenna Range.
1974
First text on Geometric Theory of
Diffraction.
1976
First use of Optimisation in Reflector
Antenna Design
1977
First Design of Array Feeds with Mutual
Coupling for Satellite Antennas
1982
First Design Tools for Shaped-beam
Antennas for Spacecraft Applications
1983
Reflector Surface Metrology using
Ultrasound or Millimetrewaves.
1984
First text on Corrugated Horns.
Department of
Electronic Engineering
1985:Reflector Design of James Clerk
Maxwell Radio Telescope.
With a diameter of 15m the James Clerk Maxwell Telescope (JCMT) is the
largest astronomical telescope in the world designed specifically to
operate in the submillimeter wavelength region of the spectrum. The JCMT
is used to study our Solar System, interstellar dust and gas, and distant
galaxies. It is situated close to the summit of Mauna Kea, Hawaii, at an
altitude of 4092m.
Department of
Electronic Engineering
1991:200GHz clean room operation
of single offset CATR
Department of
Electronic Engineering
5GHz to 200GHz single offset CATR
Department of
Electronic Engineering
1992: Successful Measurement of
Advanced Microwave Sounding Unit -B
Department of
Electronic Engineering
Mounted on NOAA weather satellite AMSU-B uses
passive radiometry to determine upper atmospheric
water vapour content
15km
50Km
Swath width approx 2000Km
Department of
Electronic Engineering
Orbit covers the globe (except near poles)
530 miles
2 satellites cover
the complete globe
in 12 hours
28.8°
Earth rotation
Per orbit
Orbit plane rotates
Eastward 1° per
day
Department of
Electronic Engineering
Passive radiometry around the water
vapour absorption line (183.3GHz)
AMSU-B channels:90GHz
150GHz
183GHz
Department of
Electronic Engineering
AMSU-B measured upper
atmosphere water vapour content
Department of
Electronic Engineering
AMSU-B QUASI-OPTICS
Mirrors and diochroic plates are
used to select the various
channels
Diochroic
plate
Input
signal
Frequency 1
Frequency 2
Department of
Electronic Engineering
New Tri-reflector CATR System (2005)
Makes efficient use of main reflector
Field
Magnetude
Field
Magnetude
QUIET
ZONE
QUIET
ZONE
TRI-REFLECTOR
RANGE
Absorber
Department of
Electronic Engineering
SINGLE
REFLECTOR
RANGE
300GHz Tri-reflector CATR Demonstrator
Currently under test at QM
*Spherical Main reflector
diameter = 1M
*Shaped subreflectors of order
350mm in diameter
* rms error on all reflectors
about 8 microns
* Quiet zone size is 75% of main
reflector diameter.
* Spherical main reflector permits
manufacture of large sizes with 1
micron rms for 1 THz operation
using optical mirror technology
Department of
Electronic Engineering
New Research Topics:




Antenna Technology
Wireless Communications/GPS
EM Healthcare
Quasi-optics and Millimetre Wave



New analysis algorithm –DGBA
Quasi-optical components/system
High Power THz Generation
Department of
Electronic Engineering
Analysing Qausi-optical system
Summary of the Problem
High-frequency methods of analysing reflectors
Physical Optics (PO)
+
GTD
•Simple
•Ray-based method
•Flexible
•Numerically efficient
•Non-singular fields
•Inefficient for λ << D
λ: signal wavelength
•Non-modular
•caustics
D: reflector diameter
Analysis Objective: modular, efficient analysis tool
06/23
Introducing: DGBA
(Diffracted Gaussian Beam Analysis)
Component Structure
Modular Analysis
diffracted
beams
reflected beams
}
diffracted
beams
input
plane
GB expansion
}
output
plane
Gaussian Beam Expansion
f ( x, y) 
    A
mn
m
n
w( x  mL) exp( jnx )
w( y   L) exp( j y )
09/23
Gaussian Beam Reflection
reflected
beam
x
sr
incident
beam
1
1

i 
i  j
q
R
 wi2
reflected
beam:
1
1

r 
r  j
q
R
 wr2
hr
r
n
xi
incident
beam:
si
hi
q
q
h
x
Gaussian beam optics:
•
wr  wi
R r by Geometrical Optics:
1
1 1
“lens formula”: R r  Ri  f
10/23
Gaussian Beam Diffraction
- canonical problem -
q

d
11/23
Gaussian Beam Diffraction
- solution of the canonical problem -


1/2
 1
1
) x  xs
1  2 erfc( j jk0 (d ( ys )  d ( y p ))
U ( P )  Ui ( P )  
;
1
/
2
x  xs
 1 erfc(  j jk (d ( y )  d ( y1 ))
)
s
p
0
2


U ( P) : Total diffracted field at the observation point
U i ( P ) : Unperturbed (incident) field at the observation point
xs
: Shadow boundary
k0d  ys  : Complex phase at the stationary point
 
of the boundary-diffraction integrand
k 0d y 1p : Complex phase at the (first) pole
of the boundary-diffraction integrand
12/23
Gaussian Beam Diffraction
- normal incidence -
x
w0
a
half
screen
^s
P(x,y,z)
s
Q(x0,y0 ,z0 )
y
Gaussian beam z=-z0
amplitude (+1)
z
•Boundary diffraction theory gives asymptotic solution
•GO incident beam is complemented by a diffracted field
in terms of complementary error functions
•Solution is valid for normal incidence within the paraxial region13/23
DGBA test application:
A Cassegrain-Gregorian Compact Antenna Range
(CATR) – the spherical tri-reflector @ 90 GHz spherical
300
shaped
shaped
100
100
feed
16/23
DGBA - Numerical Results
- spherical tri-reflector CATR test case field in the quiet zone (1200 from main reflector)
field in dB
-10
co-polar
-20
DGBA; E-plane
DGBA; H-plane
PO; E-plane
PO; H-plane
-30
-40
x-polar
-50
-1
-0.5
0
radius in m
0.5
1
20/23
Dichroic


Dichroics are well known for their frequency selective characteristics at
millimeter and sub-millimeter wave frequencies
There are two basic types of dichroic mirrors: Patch and Slot.
28
Two channel Quasi-Optical Network
(QON)


Two channels: 54GHz (oxygen lines) and 89GHz
(atmospheric windows)
High pass dichroic (transmits at 89GHz and reflects at
54GHz) is needed to achieve high pass QO system
M189
H-89
M1-54
D
H-54
M2
29
High-pass dichroic
D

Porosity value
S
High cutoff
frequency
The final design: D = 2.16mm, S = 2.46mm,
Thickness = 2.5mm
30
Measurement

Transmission measurement above 75GHz was conducted by placing it in
a quasi-optical measurement bench
31
H
Results analysis - 1
32
Integration of DGBA and PMM
Dual Channel Quasi-optical system
Integration of DGBA and PMM
Results – 54GHz
-8.68dB Beamwidth Deg.
Simulation.：H-21.51 E-20.92
Measured： H-20.09 E-19.34
M1-54
Horn-54
M2
Integration of DGBA and PMM
Results – 89GHz
-8.68dB Beamwidth Deg.
Simulation：H-21.44 E-21.22
Measured：H-19.15 E-19.69
M1-89
Dichroic
Horn-89
M2
THz sources
 One of the most difficult components to realise in
sub-millimeter bands is the THz sources.
 THz sources can be broadly divided into three
categories:
 Solid state sources;
 Vacuum tube sources;
 Optical style sources.
 Each of them has its strength and weakness.
36
Overview: State of the art
Gyrotron
BWO
THz-emission power as a function of frequency
Solid line: Conventional THz sources; Ovals: recent THz sources
*1: M. Tonouchi, ‘Cutting-edge terahertz technology’, Nature photonics, Feb, 2007
37
Overview: Physical limitations
 Solid state sources: are limited by reactive parasitics, or
transit times (RC) rolloff, or heavy resistive losses;
 Vacuum tube sources: suffer from physical scaling
problem, metallic losses and need for extremely high
fields;
 Optical style sources: the photon energy level (~meV)
too close to that of lattice phonons, needing cryogenic
cooling.
38
Micro-klystron
39
Micro-klystron beam source
PSD Experimental setup to test the scale down effect
Experimental measured A-K voltage and current
40
Introduction – What is Pseudo-Spark
Discharge?
VB [V]
vacuum
breakdown
1400
hollow
cathode
1200
1000
800
deff
p
insulator
600
seu
d
o
sp
ark
reg
io
n
pseudospark
region
400
anode
electron
beam
Single gap PSD geometry
200
0
(pd)min
2
4
6
8
10
12
14
16
pd [torr x cm]
Paschen curve and pseudospark region
• Occurs in special confining geometry
• In various gases such as helium, nitrogen, argon, et al
• Low pressure, 50-500mtorr, self-sustained, transient hollow cathode discharge, for a gap separation of
several mm
• High quality electron beam and ion beam extraction before and during the conductive phase
41
PSD Process



Phase 1: Townsend discharge
- low current pre-discharge
- plasma formation
Phase 2: Hollow cathode discharge
- hollow cathode effect
- plasma expansion
Phase 3: Superdense glow discharge (conductive phase)
- high-current phase (10 kA cm-2 )
42
Phase 1: Townsend discharge
Seed electrons
Pre-discharge
Plasma formation
43
Phase 2: Hollow cathode discharge
Hollow cathode effect
Plasma expansion
Secondary emission
44
Phase 3: Superdense glow discharge
Sheath contraction
Primary emission
Conductive phase
45
PSD Numerical Simulation
MAGIC: Particle-In-Cell and Monte-Carlo Collision
(PIC-MCC)
Ref: C.K. Birdsall et al, Computer Phy. Comm 87, 1995.
46
PSD - Gas Ionisation
1. Electron-induced ionisation
2. Ion-induced ionisation
The cross section depends on:
1. The energy of the impact electron;
2. The gas type.
For different gases, the cross sections are different functions of impact
electron energy. The functions can be achieved from experimental results.
47
PSD-2D Computational Model
MAGIC 2D Model:
Constant A-K voltage 10kV
AK gap d=6mm
Radius = 25mm
Room temperature
Insulator: 6mm thick Perspex
Anode aperture: 0.5mm radius
Anode thickness: 12mm
Cathode aperture: 1.5mm radius
Trigger radius: 1mm, cable outer
radius: 6mm
Nitrogen 100mTorr
48
PSD-2D Phase 1&2
Plasma formation at 30ns
Plasma expansion at 50ns
49
PSD-2D Phase 3
Plasma expansion and
emission at 80ns
50
PSD Process
Detailed motion of all the particles in the system.
51
Simulation results
Observed voltage between the anode and the cathode.
Observed current at the anode aperture.
52
Summary



MM/THz technology becomes increasingly
beneficial to our society. MM/THz technology
has been advancing over one century – an
old and young topic.
New applications have posed many technical
challenges in MM/THz technology – needing
fresh blood of microwave engineers.
Solutions lies in understanding and innovation
in methodology and technology.
Department of
Electronic Engineering
Thank you!
Department of
Electronic Engineering