Electrostatic Accelerators and Pulsed High Voltage
Transcript of Electrostatic Accelerators and Pulsed High Voltage
Particle Accelerator Engineering, Spring 2021
Electrostatic Accelerators and Pulsed
High Voltage
Spring, 2021
Kyoung-Jae Chung
Department of Nuclear Engineering
Seoul National University
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Resistors, capacitors and inductors
πΆ =2πππ
lnπ ππ π
πΏ =ππ
2πln
π ππ π
πΏ =ππ2π΄
π
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RC circuit
β« This is the simplest model for a pulsed voltage circuit; electrical energy is stored
in a capacitor and then dumped into a load resistor via a switch.
πΆππ
ππ‘+π
π = 0
π(π‘) = π0 exp βπ‘
π πΆ
β« Passive integrator (keep πππ’π‘ βͺ πππ by keeping the product π πΆ large)
πΌ = πΆππππ’π‘ππ‘
=πππ β πππ’π‘
π βππππ
πππ’π‘(π‘) β1
π πΆΰΆ±πππ π‘ ππ‘
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RL circuit
β« Usually, we want a rapid rise time for power into the load. The time for initiation
of current flow to the load is limited by the undesirable (or parasitic) inductance.
πΏππ
ππ‘+ π π = 0
π(π‘) = π0 1 β exp βπ‘
Ξ€πΏ π
β« The πΏ/π time determines how fast current and voltage can be induced in the
load.
ππππ π~2.2πΏ/π
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Transformer
β« The transformer is a prime component in all high-voltage supplies. It utilizes
magnetic coupling to convert a low-voltage ac input to a high-voltage ac output
at reduced current.
β« The transformer does not produce energy. The product of voltage times current
at the output is equal to or less than that at the input.
πΏ1 =π0π1
2π΄π‘2πππ‘
πΏ2 =π0π2
2π΄π‘2πππ‘
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Transformer circuit
β« For ideal coupling:
π1πΞ¦
ππ‘= π1 π2
πΞ¦
ππ‘= π2
π1π1 β π2π2
π2 = π1π2π1
= π2π 2
π1 = π1π1π2
2
π 2
Open load Infinite πΏ1
Transformed load resistance
when viewed from the primary
β« To reduce the leakage current, the
primary inductance should be high
β use of ferromagnetic material as a
core
π 1 =π1π2
2
π 2
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Limitation of pulse transformer
β« If the transformer is used to amplify the voltage of a square pulse from a pulsed
voltage generator (common accelerator application), then leakage currents
contribute to droop of the output voltage waveform.
β« Volt-sec limitation
π1πΞ¦
ππ‘= π1
π1π΄1 π΅ π‘ β π΅ 0 = ΰΆ±π π‘ ππ‘
π0Ξπ‘ volt β sec β€ 2π1π΄1π΅π
β« The output pulse is a square pulse
when Ξ€πΏ π 1 β« Ξπ‘.
β« Pulse transformers with low primary
inductance have poor energy transfer
efficiency and a variable voltage
output waveform.
Energy remains in transformer magnetic fields
at the end of the main pulse; this energy
appears as a useless negative post-pulse.
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High-voltage dc power supply: half-wave rectifier
β« A capacitor is included to reduce ripple in the voltage.
β« The fractional drop in voltage during the negative half-cycle is on, the order
(1/2π)/π πΆ, where π is the load resistance.
β« Output voltage is controlled by a variable autotransformer in the primary.
β« Because of the core volume and insulation required, transformers are
inconvenient to use at voltages above 100 kV. β Use of ladder network for
higher voltage.
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Cockcroft-Waltonβs voltage multiplier
β« The first nuclear disintegration by nuclear projectiles artificially produced in a
man-made accelerator (1932).
7Li + 1H β 4He + 4He (+17.3 MeV)
Cockcroft and Walton, Proc. Roy. Soc. A136, 619 (1932)
Cockcroft and Walton, Proc. Roy. Soc. A137, 229 (1932)Nobel prize in physics (1951)
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Analysis of Cockcroft-Waltonβs voltage multiplier circuit
β« The voltage multiplier circuit is a combination of a clamping circuit and a peak
detector circuit (rectifier circuit).
Clamping circuit
(positive clamper)
Peak detector circuit
(rectifier circuit)
πππ’π‘ = 2πππ = ππππ
1 stage
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Van de Graaff accelerator
β« A Van de Graaff [R. J. Van de Graaff, Phys. Rev.
38, 1919 (1931)] accelerator is a low-current
electrostatic MeV-range high-voltage generator
using corona discharge from an array of needles
in gas.
β« The electrons drift toward the positive electrode
and are deposited on a moving belt. The belt
composed of an insulating material with high
dielectric strength, is immersed in insulating gas
at high pressure. The attached charge is carried
mechanically against the potential gradient into a
high-voltage metal terminal.
β« The terminal acts as a Faraday cage; there is no
electric field inside the terminal other than that
from the charge on the belt. The charge flows off
the belt when it comes in contact with a metal
brush and is deposited on the terminal.LANL 7 MeV VDG, as injector
for 24.5 MeV tandem VDG
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1.5 MeV SNU Van de Graaff accelerator
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Tandem Van de Graaff accelerator
β« The output beam energy of a Van de Graaff accelerator can be extended a factor
of 2 through the tandem configuration.
β« Negative ions produced by a source at ground potential are accelerated to a
positive high-voltage terminal and pass through a stripping cell. Collisions in the
cell remove electrons, and some of the initial negative ions are converted to
positive ions. They are further accelerated traveling from the high-voltage
terminal back to ground.
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Optimum size of high-voltage terminal
β« The solution to the Laplace equation is spherical coordinates gives the radial
variation of potential and electric field between the spheres:
π π =π0π 0
π 2 β π 0
π 2πβ 1
πΈπ π = βππ
ππ=
π0π 0π 2 β π 0
π 2π2
β« The electric field is maximum on the inner sphere
πΈπ,πππ₯ =π0
π 2 β π 0
π 2π 0
β« The condition to minimize the peak electric field isπ 0π 2
=1
2
β« For concentric cylinders, the optimum ratio of inner to outer cylinder radii is
π 0π 2
= πβ1 β 0.368
πΈπππ₯ =4π0π 2
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β« A better electric field distribution can be obtained through the use of
equipotential shields which are biased electrodes located between the high-
voltage terminal and ground. Voltage on the shields is maintained at specific
intermediate values by a high-voltage resistive divider circuit.
β« The fields on the outer surfaces of the electrodes are
πΈ0 =(π0 β π1)/π 01 β π 0/π 1
β« The optimum parameter are found to be
π 0π 2
=1
2π 0 =1
2π 1
πΈπππ₯ =4π0π 2
πΈ1 =π1/π 1
1 β π 1/π 2
π 1 =5
8π 2 π1 =
3
5π0
β« The peak electric field is reduced to
πΈπππ₯ =192
75
π0π 2
β 2.56π0π 2
Use of equipotential shields between the electrodes
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RLC circuit
πΏπ2π
ππ‘2+ π
ππ
ππ‘+π
πΆ= 0
π2π
ππ‘2+ 2πΌ
ππ
ππ‘+ π0
2π = 0 π€βπππ, πΌ =π
2πΏ, π0 =
1
πΏπΆ
β’ Initial conditions: π π‘ = 0 = πΆπ0, Ξ€ππ ππ‘ π‘ = 0 = 0
(1) Overdamping
(2) Critical damping
(3) Underdamping
π > 2 Ξ€πΏ πΆ
π = 2 Ξ€πΏ πΆ
π < 2 Ξ€πΏ πΆ
π π‘ =π0π½πΏ
πβπΌπ‘ sinhπ½π‘ (π½2 = πΌ2 β π02)
π π‘ =π0πΏπ‘πβπΌπ‘
π π‘ =π0πππΏ
πβπΌπ‘ sinπππ‘ (ππ2 = π0
2 β πΌ2)
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RLC circuit
Time
0s 0.2ms 0.4ms 0.6ms 0.8ms 1.0ms 1.2ms 1.4ms 1.6ms 1.8ms 2.0ms
-I(R1)
-40KA
0A
40KA
80KA
C=8.3mF
C=6.64mFC=4.98mFC=3.32mFC=1.66mF
Time
0s 0.2ms 0.4ms 0.6ms 0.8ms 1.0ms 1.2ms 1.4ms 1.6ms 1.8ms 2.0ms
-I(R1)
0A
25KA
50KA
-20KA
45uH
35uH
25uH
15uH
5uH
β« Variable C
β« Variable L
C = variable
L = 15 mH
R = 100 mW
V0 = 7 kV
C = 1.66 mF
L = variable
R = 100 mW
V0 = 7 kV
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RLC circuit
β« Variable R
β« Variable V0
Time
0s 0.2ms 0.4ms 0.6ms 0.8ms 1.0ms 1.2ms 1.4ms 1.6ms 1.8ms 2.0ms
-I(R1)
0A
40KA
-30KA
60KA
250m
200m
150m
100m
50m
Time
0s 0.2ms 0.4ms 0.6ms 0.8ms 1.0ms 1.2ms 1.4ms 1.6ms 1.8ms 2.0ms
-I(R1)
0A
20KA
40KA
60KA
9k
8k
7k
6k
5k
C = 1.66 mF
L = 15 mH
R = variable
V0 = 7 kV
C = 1.66 mF
L = 15 mH
R = 100 mW
V0 = variable
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Pulse modulator with inductive energy storage and opening
switch
β« Compare energy density stored in inductor
and in capacitor.
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Marx generator (Erwin Marx, 1923)
ππ = ππ0
πΆπ = Ξ€πΆ0 π
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Marx generator: equivalent circuit
Self-discharge time:
When L is used instead of R:
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Transmission line
β« Most accelerator applications for pulse
modulators require a constant-voltage
pulse.
β« The critically damped waveform is the
closest a modulator with a single
capacitor and inductor can approach
constant voltage.
β« Better waveforms can be generated by
modulators with multiple elements. Such
circuits are called pulse-forming networks
(PFNs). The transmission line is the
continuous limit of a PFN.
β« There are various kinds of transmission
line, but coaxial type is the most widely
used in the pulsed power system.
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Coaxial transmission line
β« Lumped parameter model for a lossless transmission line
πΆβ² =2ππ
lnπ ππ π
F
m
πΏβ² =π
2πln
π ππ π
H
m
β« Kirchhoffβs law:
βπΆβ²βπ§πππππ‘
= πΌπ β πΌπβ1
πΏβ²βπ§ππΌπππ‘
= ππ β ππ+1
β« The voltage and current differences are approximated as:
ππ+1 β ππ +ππ(π§π, π‘)
ππ§βπ§ πΌπ+1 β πΌπ β
ππΌ(π§π, π‘)
ππ§βπ§
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Coaxial transmission line
β« Combining equations, we obtain the continuous partial differential equations:
ππ
ππ§= βπΏβ²
ππΌ
ππ‘
ππΌ
ππ§= βπΆβ²
ππ
ππ‘Telegrapherβs equation
β« Wave equation:
π2π
ππ§2= πΏβ²πΆβ²
π2π
ππ‘2π(π§, π‘) = πΉ π‘ Β±
π§
π£
π£ =1
πΏβ²πΆβ²=
1
ππβ
π
ππ
β« Phase velocity:
β No geometric effects
β« Characteristic impedance:
π0 =πΏβ²
πΆβ²=
1
2π
π
πln
π ππ π
β60
ππln
π ππ π
π0πππ‘
β60
ππ
(Voltage hold-off is maximized when Ξ€π π π π = π)
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Coaxial transmission line
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Pulse forming line (PFL)
β« There are numerous applications in both physics and electrical engineering for
short (~10 ππ < π‘π < 100 ππ ) electrical pulses. These applications often require
that the pulses have a βgoodβ square shape.
β« Although there are many ways for generating such pulses, the pulse-forming line
(PFL) is one of the simplest techniques and can be used even at extremely high
pulsed power levels.
β« A transmission line of any geometry of length π and characteristic impedance π0makes a pulse forming line (PFL), which when combined with a closing switch πmakes the simple transmission line pulser.
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Simple PFL
β« When the switch closes, the incident wave ππΌ, with a peak voltage of ( Ξ€1 2)π0,
travels toward the load, while the reverse-going wave ππ , also with a peak
voltage of ( Ξ€1 2)π0, travels in the opposite direction.
β« The incident wave ππΌ, then, supplies a voltage of ( Ξ€1 2)π0 for a time determined
by the electrical length of the transmission line ππ to the load. The reverse-going
wave ππ travels along the transmission line for a duration ππ and then reflects
from the high impedance of the voltage source, and becomes a forward-going
wave traveling toward the load with peak voltage ( Ξ€1 2)π0 and duration ππ.
β« The two waves add at the load to produce a pulse of amplitude ( Ξ€1 2)π0 and
pulse duration ππ = 2ππ.
β« Pulse characteristics
π =π02
ππ = 2ππ =2π
π£πβ2π
πππ
β« Matching condition: π πΏ = π0
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Coaxial PFL
β« Basic parameters
πΏβ² =π
2πln
π 2π 1
π£π =1
ππ=
1
πΏβ²πΆβ²=
1
ππ=
π
ππππβ
30
ππ
cm
ns
πΆβ² =2ππ
ln Ξ€π 2 π 1
π0 =πΏβ²
πΆβ²=
1
2π
π
πln
π 2π 1
β 60ππππln
π 2π 1
β« Pulse characteristics
π =π02
ππ = 2ππ =2π
π£πβ2π
πππ =
π cm
15ππ [ns]
β« Matching condition: ππΏ = π0
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Coaxial PFL
β« Electric field
πΈ(π) =π0
π ln Ξ€π 2 π 1
β« Optimum impedance for maximum voltage
πΈπππ₯(π = π 1) =π0
π 1 ln Ξ€π 2 π 1
β« Voltage at the maximum electric field
π0 = πΈπππ₯π 1 lnπ 2π 1
β« The value of π 2/π 1 that optimizes the inner conductor voltage occurs when
ππ0/ππ 1 = 0, yielding
lnπ 2π 1
= 1
ππππ‘ =1
2π
π
π= 60
ππππ
β60
ππ
ππππ‘π€ππ‘ππ =
60
81= 6.7 Ξ© ππππ‘
πππ =60
2.4= 38.7 Ξ©
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Analysis of simple PFL
β« On closure of the switch, the voltage on the load rises from zero to a value
determined by
ππΏ = πππΏ
ππΏ + π0ππΏ =
π
2(matched)
β« Simultaneously, a voltage step ππ is propagated away from the load towards the
charging end of the line. It takes πΏ = π/π£π for the wave to reach the charging end.
ππ = ππΏ β π = πππΏ
ππΏ + π0β 1 = π
βπ0ππΏ + π0
ππ = βπ
2(matched)
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Analysis of simple PFL
β« Potential distribution (matched load)
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Lattice diagram representation of pulse-forming action
β« On closure of the switch, the voltage on the load rises from zero to a value
determined by
ππΏ = πππΏ
ππΏ + π0= πΌπ
β« The potential on the load is given by
ππΏ = πΌπ (0 < π‘ < 2πΏ)
ππΏ = πΌπ + (πΌ β 1)πΎπ (2πΏ < π‘ < 4πΏ)
πΎ = π½ + 1
ππΏ = πΌπ + πΌ β 1 πΎπ + π½(πΌ β 1)πΎπ
(4πΏ < π‘ < 6πΏ)
β« Finally
ππΏ = π πΌ + πΌ β 1 πΎ 1 + π½ + π½2 +β―
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Typical waveforms from PFL under matched and
unmatched conditions
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Coaxial simple PFL
β« The discharge of a simple transmission line through a shorting switch into a
matched resistive load produces a constant-voltage pulse.
β« The magnitude and duration of the pulse:
π =π02
π‘π =2πΏ
π£
Open Matched
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Blumlein PFL
β« An important disadvantage of the simple PFL is that the pulse generated into a
matched load is only equal to V/2.
: This can be serious if one generates pulses at the high voltage, as the power
supply used to charge the line must have an output potential which is twice that
of the required pulse amplitude.
β« This problem can be avoided using the Blumlein PFL invented by A. D. Blumlein.
β« The magnitude and duration of the pulse:
π = π0 π‘π =2πΏ
π£
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Pulse forming networks (PFNs)
β« Transmission lines are well suited for output pulse lengths in the range 5 ππ <π‘π < 200 ππ , but they are impractical for pulse lengths above 1 ΞΌs.
β« Discrete element circuits that
provide a shaped waveform are
called pulse-forming networks.
β« The magnitude and duration of
the pulse:
π =π02
π‘π =2
π£= 2π πΏπΆ
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Pulsed power compression
β« Power compression is the technique that has allowed the generation of such
high output. Energy is transferred from one stage of energy storage to the next
in an increasingly rapid sequence. Each storage stage has higher energy density
and lower inductance. Even though energy is lost in each transfer, the decrease
in transfer time is sufficient to raise the peak power.
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Peaking capacitor circuit: CLC circuit
β« Transfer of energy between capacitors forms the basis of most pulsed power
generators.
π(π‘) =π0ππΏ
sinππ‘ π =1
πΏπΆ=
πΆ1 + πΆ2πΏπΆ1πΆ2
π1(π‘) = π0 1 βπΆ2
πΆ1 + πΆ21 β cosππ‘ π2(π‘) =
πΆ1π0πΆ1 + πΆ2
1 β cosππ‘
π‘ = π/π
α€π2π0 πππ₯
=2πΆ1
πΆ1 + πΆ2
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Magnetic switching: saturable core
β« A two-stage power compression circuit
β« At early times, the right-hand portion of the circuit is approximately an open
circuit because of the high inductance of the winding around the high π core.
Energy flows from πΆ0 to πΆ1.
β« The core reaches saturation when
β« After saturation, the inductance πΏ2 decreases by a large factor, approaching the
vacuum inductance of the winding. The transition from high to low inductance is
a bootstrapping process that occurs rapidly.
ΰΆ±π1 π‘ ππ‘ = ππ΄π(π΅π + π΅π) π1 π‘ = ππ΄πππ΅
ππ‘
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Magnetic switching: saturable core
β« The optimum core parameter is obtained by integrating for 0 β€ π‘ β€ π/π.
ππ΄π π΅π + π΅π =π0π
2π
β« For a multiple compression
ππππβ1
β πΏππΏπβ1
β 1
π 1 < π <
π
π0
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Pulse shaping with saturable core magnetic switches
β« Saturable core inductors can also be used for pulse length shortening and rise
time sharpening if efficiency is not a prime concern.
β« The following circuit can produce a short, fast-rising voltage pulse from a slow
pulse generator.