Technological Aspects: High Voltage - Indico · 2018. 11. 14. · breakdown (as a rule of thumb) Or...
Transcript of Technological Aspects: High Voltage - Indico · 2018. 11. 14. · breakdown (as a rule of thumb) Or...
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Technological Aspects: High Voltage
Dan Faircloth
Ion Source Section Leader
Rutherford Appleton Laboratory
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Talk Outline
• High voltage and ion sources • Electric field calculation • Electrical breakdown • Insulators • Partial breakdown • Statistical variability • Factors affecting breakdown voltage • Cables and terminations • Ancillary equipment • Earthing and safety
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Uses of High Voltages in Ion Sources
• Extracting beams (up to 50 kV)
• Accelerating beams (up to 28000 kV)
• Initiating discharges / pre-ionising gases (up to 20 kV)
• Focusing and deflecting beams (up to 50 kV)
• Suppressing unwanted particles (up to 5 kV)
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Ion sources are particularly challenging for HV design
Explosive gasses (e.g. Hydrogen)
High temperatures
Other contaminants (e.g. Cs)
Magnetic fields
Large amounts of charge carriers
Stray beams: electrons and ions
Compact design
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Main Aim of High Voltage Design for Ion Sources
Produce reliable breakdown
... where we want it
prevent breakdown
...where we don’t want it
The two regions are only mm apart!
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High Voltage Breakdown
• Electric field strength is the primary factor
• In general high voltage breakdown is most likely to occur where the electric field is highest, but this depends on: Materials and gasses
Pressures
Temperatures
Surfaces
Magnetic fields
Stray beams
Charges
Photons
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Electric Field
• Potential gradient, electric field strength, electric field intensity, stress, E
• Units of Vm-1, kVm-1, kVmm-1, kVcm-1
• Equations, Analytical , Empirical, Numerical
dVE =
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Maxwell’s Equations
t∂∂
+=×∇DJH
t∂∂
−=×∇BE
0=⋅∇ B
ρ=⋅∇ D
For electrostatic fields:
0=×∇ E
ρ=⋅∇ D
φgrad−=E
ED ε=
ερφ =∇2
Laplace’s Equation 02
2
2
2
2
2
=∂∂
+∂∂
+∂∂
zyxφφφ
or
ερφφφ
=∂∂
+∂∂
+∂∂
2
2
2
2
2
2
zyxPoisson’s Equation
if ρ = 0:
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Using Laplace's Equation
Infinite parallel plates:
d
V 0
02
2
2
2
2
2
=∂∂
+∂∂
+∂∂
zyxφφφ
02
2
=∂∂
∴xφ
0=∂∂
+∂∂
zyφφ
φ does not vary with y or z:
1cx=
∂∂
⇒φ
21)( cxcx +=⇒φ
E
At x = 0, and at x = d, 0=φ V=φ
xdVx =∴ )(φ
dVc =∴ 1 02 =cand
φgrad−=E
dV
−=∴E
dV
=⇒ E
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Similarly.... 0112
2
2
2
2 =∂∂
+∂∂
+
∂∂
∂∂
⋅zrr
rrr
φϕφφ
r1
r2
φ1
φ2
( )1
2
1
211 lnln
ln)( rr
rr
r −
−
+=φφφφ
−
=
1
2
21
ln)(
rrr
rE φφ
−
=
1
21
21max
lnrrr
E φφ
Fine for simple geometries...
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Finding Electric Field Distributions
Teledeltos paper
Silver Paint
Manually find equipotentials
Power supply, voltmeter and probe
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Automated
Electrolytic tank
Finding Electric Field Distributions
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Thankfully we have Computers
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ερφφφ
=∂∂
+∂∂
+∂∂
2
2
2
2
2
2
zyx
Numerically Solving Poisson’s Equation
Direct methods • Gaussian elimination • LU decomposition method
Iterative methods • Mesh relaxation methods
- Jacobi - Gauss-Seidel - Successive over-relaxation method (SOR) - Alternating directions implicit (ADI) method
• Matrix methods - Thomas tridiagonal form - Sparse matrix methods: Stone’s Strongly Implicit Procedure (SIP) Incomplete Lower-Upper (ILU) decomposition - Conjugate Gradient (CG) methods: Incomplete Choleski Conjugate Gradient (ICCG) Bi-CGSTAB - Multi–Grid (MG) method Take your pick!
Apply discrete form to finite elements
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... but you won’t be the first
There are many solvers available to download Standard Masters degree project Written in every language, run on every platform
Mostly 2D Poor geometry input Poor meshing Poor post processing No/poor support If you want a free 2D solver:
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Commercial 3D Modelling Software
Plus others...
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Electrical Discharges
Arc Discharge
Glow Discharge
Log I
Breakdown Voltage
µA
nA
A
100 A A
ppro
xim
ate
Curr
ent R
ange
Thermal Arc Discharge
70 V
Dark Discharge Townsend Breakdown
Voltage Between Anode and Cathode
Background Ionisation
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Arc Discharge
Glow Discharge
Breakdown Voltage
µA
nA
A
100 A A
ppro
xim
ate
Curr
ent R
ange
Thermal Arc Discharge
70 V
Dark Discharge Townsend Breakdown
Voltage Between Anode and Cathode
Background Ionisation
Quark Gluon Plasma
Fusion
Z-Pinch 1M A
1G A
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Breakdown Voltage
µA
nA
App
roxi
mat
e Cu
rren
t Ran
ge
70 V
Dark Discharge Townsend Breakdown
Voltage Between Anode and Cathode
Background Ionisation
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Electrical Breakdown
• Global Breakdown – Complete rupture or failure of the insulation
between two electrodes
• Local Breakdown – Partial breakdown of part of the insulation
between two electrodes
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• Global break down can only occur when a highly conductive channel is formed between the two electrodes
• The journey towards high voltage breakdown depends on the degree of non uniformarity of the electric field
• Geometry of electrodes and materials and environment all play a critical role
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Avalanche
dxndn xx α=
John Townsend "Townsend discharge"
1897
E
xx enn α
0=
- +
By integration and nx = n0 at x = 0 x = 0
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Avalanche - + Electron
Swam
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Avalanche - +
x
ρ
Electron Swam
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Avalanche - +
x
E
E0
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Avalanche - +
x
E
Electron Swam
E0
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- +
Streamer
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- +
Streamer
Cathode directed streamer
Anode directed streamer
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- +
Streamer
CRACK
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Townsend Secondary Ionisation Coefficient, γ
dxndn xx α=
John Townsend "Townsend discharge"
1897
xx enn α
0=
Self sustaining discharge resulting in breakdown when:
1=deαγ
mpion γγγγ ++=
)1(10
−−= d
d
eeII α
α
λ
Townsend Criterion for Breakdown
γ is the number of secondary electrons produced per electron in the primary avalanche
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Paschen Curve
Friedrich Paschen 1889
Shorter mean
free path
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Paschen Curve
Friedrich Paschen 1889
Operating just below the Paschen Minimum:
Longer gaps have
lower breakdown voltages!
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Vacuum Breakdown Processes
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Vacuum Breakdown
Extraction conditioning Kilpatrick
Insulating micro- inclusions can also cause field enhancement
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Insulators
Something has to hold up the electrodes
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Surface Breakdown
Electrons Photons Neutral Gas Molecules Neutral Surface Molecules Positive Ions (Gas Molecules) Negative Ions (Gas Molecules) Positive Ions (Surface Molecules) Negative Ions (Surface Molecules)
Ambient Electric
Field
Localised Electric
Field
Insulator surfaces are the weakest part of the insulation system
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Impulse voltages applied to a rod-plane gap with PTFE insulator between
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Surface Charging
Dust Figure
Charge Distribution
+36 kV Impulse Charge Distribution
-70 kV Impulse
Polarity is very important if the gap is
asymmetrical
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Triple junctions always exist at some scale
Plane Electrodes
Insulator
Detail
Triple Junction Region
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Triple Junction Effect 1.00
0.93
0.86
0.79
0.72
0.65
0.58
0.51
0.44
0.37
0.30
Field Strength (kVm-1)
Equi
pote
ntia
ls
4mm Triple Junction
INSULATOR AIR
Fiel
d St
reng
th
INSULATOR AIR
2mm Triple Junction
INSULATOR AIR
INSULATOR AIR
1mm Triple Junction
INSULATOR AIR
INSULATOR AIR
0.5mm Triple Junction
INSULATOR AIR
INSULATOR AIR
PTFE (εr = 2.2) ambient field of 0.5 kVmm-1
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Triple Junction Effect
1mm triple junction ambient field of 0.5 kVmm-1
1.50
1.37
1.24
1.11
0.98
0.85
0.72
0.59
0.46
0.33
0.2
Field Strength (kVmm-1)
Equi
pote
ntia
ls
Fiel
d St
reng
th
εr = 1.0
INSULATOR AIR
INSULATOR AIR
εr = 2.2
INSULATOR AIR
INSULATOR AIR
εr = 3.6
INSULATOR AIR
INSULATOR AIR
εr = 7.0
INSULATOR AIR
INSULATOR AIR
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Common Insulators Material Relative
Permittivity, εr
Dielectric Strength (kV/mm)
Air 1 3
PTFE 2.2 19.7
Al2O3 8-10 13.4
Mica 6 118
Epoxy Resin 3.6 20
SF6 1.002 7.5
Oil 2.5 - 4 10 - 15
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Triple Junction Screening
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Field Strength (kVmm-1)
No Screening Inside
Insulator Outside
Ring
1 mm PTFE triple junction ambient field of 0.5 kVm-1
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Triple Junction Screening
No Screening Recessed Insulator
Inside and Recessed
1 mm PTFE triple junction ambient field of 0.5 kVm-1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Field Strength (kVmm-1)
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Antenna power feed sparking
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A simple 2D model of the antenna power feed
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Demonstrated fields in excess of 3 kV mm-1
5.4 kV mm-1
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Partial Discharges
Cb Cv
Cx
Cy
Void
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Insulator Materials Depends on application! For example: Al2O3 is commonly used in sources in vacuum AlN is used when a high thermal conductivity is required Macor is used when a complex shape needs to be machined Porcelain is used in compression Epoxy resin is used to impregnate and pot Mica is used for thin high voltage withstand Glass is used when visible transparency is required
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Corona •Corona is another type of partial discharge occurring in very divergent fields
•Divergent fields are caused by sharp points
Equipontentials
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Corona •Corona is another type of partial discharge occurring in very divergent fields
•Divergent fields are caused by sharp points
Electric Field
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Corona •Corona is another type of partial discharge occurring in very divergent fields
•Divergent fields are caused by sharp points
•Discharge behaviour is dependant on polarity
+ Negative point
-
- Positive point
+
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Electrode Design •Minimise Electric Field by making smooth rounded electrodes •Shield any sharp points with corona shields
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Breakdown strength of Air
In air at normal room conditions two electrodes require about 30 kV for each cm of spacing to breakdown (as a rule of thumb)
Or 3 kVmm-1
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Statistical Variability
Even with identical conditions the same electrode gap will breakdown at different voltages each time the voltage is applied. This is because of the statistical nature of high voltage breakdown: no two sparks are ever the same.
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Environmental conditions
Higher temperatures and lower pressures lead to lower flashover voltages. A correction factor for V50% can be found from this equation:
where P is in mmHg and t is in degrees centigrade.
tP
+×
273386.0
-6 2
10
18
26
34
42
50
5860
0 660 72
0 780
0%
20%
40%
60%
80%
100%
120%
Percentage change in V50%
Temperature (Deg C)
Presure (mmHg)
Humidity can also affect breakdown voltage
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Polarity is important in Non Uniform Gaps
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Geometry Scaling factors
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Type of Applied
Voltage is Important
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Additional Complications
• Magnetic Fields
• Xrays
• Space charge
• Insulator surface charge
• Stray beam
• Contamination
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High Voltage Design High voltage platform or
internal isolation?
Pros and cons
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Cables and Terminations
Correct termination of high voltage cables is essential
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Connectors or Bushings?
Depends on...
• Application
• Maintenance
• Permanence
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Big Bushings
800 kV Outdoor
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Commercial Insulators
• Dirt and Dust • Sheds • Tracking A well designed
insulation system is one you don’t ever have to worry about
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High voltage platforms don’t have to be too complicated, but...
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Clean lab conditions!
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Water and home-made insulators don’t mix
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Commercial insulators are relatively cheap (≈€200) and will work in all conditions
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Power to the Platform
How to get power to the equipment on the HV platform?
• Motor alternator set
• Isolating transformer
• Waveguide DC break
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Solid Insulation Isolation
Transformer
1:1
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Oil Filled Isolation
Transformer
Pro: Compact design Con: Bund required
Oil dielectric strength 10 – 15 kVmm-1
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RF Waveguide DC Breaks
Air Vacuum
Water Cooled
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Voltage Dividers SF6 Filled
1
2
Compensated
If R1C1 = R2C2
For all frequencies
SF6 dielectric strength 2.5 times air = 7.5 kVmm-1
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Insulation Test Equipment
Current limited test of insulation withstand strength
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Power Supply Technologies
• Semiconductors: Thyristor, IGBT, GTO • Tube- tetrode • PFN • Cascade rectifier (Greinacher/ Cockcroft–Walton
multiplier ) • Vandergraph, peloton • Linear (Usually front end only) • Switched mode-transformer -HV Diode and
Capacitor
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High Voltage Power Supply Manufacturers
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Custom Built Power Supplies
• Tight specification is essential
• Or of course you could make your own if it is specialised e.g. pulsed extraction.
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Tetrode used for ISIS 17 kV pulsed extraction power supply
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Earthing Solid single point earth
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High voltage platform “Local earth”
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Safety
• Electric Shocks can kill
• Stored energy in capacitors
½CV2 = 0.5 x 1 µF x 30 kV = 450 J
• X-rays
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Electric Shocks
Hand to hand resistance: 100 kΩ dry/thick skin 1 kΩ wet/broken skin • The stratum corneum
breaks down 450–600 V leaving 500 Ω
• You can feel 5 mA • 60 mA can fibrillate the
heart
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HV Safety Philosophy
1. Impossible to accidently lock someone in the HV area.
2. Ability to shut down the power inside and outside the HV area.
3. Impossible to power on the HV without locking the area.
4. Impossible to enter the HV area without making it safe.
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HV Safety Philosophy
1. Impossible to accidently lock someone in the HV area.
2. Ability to shut down the power inside and outside the HV area.
3. Impossible to power on the HV without locking the area.
4. Impossible to enter the HV area without making it safe.
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HV Safety Philosophy
1. Impossible to accidently lock someone in the HV area.
2. Ability to shut down the power inside and outside the HV area.
3. Impossible to power on the HV without locking the area.
4. Impossible to enter the HV area without making it safe.
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HV Safety Philosophy
1. Impossible to accidently lock someone in the HV area.
2. Ability to shut down the power inside and outside the HV area.
3. Impossible to power on the HV without locking the area.
4. Impossible to enter the HV area without making it safe.
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Automatic Earthing System
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Automatic Earthing System
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Earth Stick
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Earth stick should be hung just inside the entrance of the high voltage area
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You can never prevent humans from circumventing safety systems...
But you must make sure that they require some effort to wilfully bypass
Complacency and familiarity can kill
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Cautionary tale of Dr. Jon Osterman...
Example of very bad safety systems:
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Let that be a lesson!
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Thank you for listening