Cathode and Hibachi Development on Electra

Cathode and Hibachi Development on Electra
Naval Research Laboratory
Plasma Physics Division
Electra title page
Matt Myers
Naval Research Laboratory
Code 6733
[email protected]
Electra is a repetitively pulsed KrF laser that will be used to develop the
technology required for an Inertial Fusion Energy (IFE) power plant
ELECTRA
The cathode must meet IFE requirements for efficiency and durability
while pumping the laser gas uniformly
ELECTRA
• Turn-on time: τrise < 40 ns
– Over heating of anode foil heating by low energy electrons
– ASE (amplified spontaneous emission) may result in laser pre-pulse
• Gap Closure: must sustain a flat-top voltage pulse for ~600 ns
–
–
–
–
Need to be in this regime to satisfy IFE requirements of energy on target
Pulse must be flat-topped to avoid uneven pumping of laser gas
If voltage droops anode foil may be overheated by low energy electrons
Poor efficiency
• Uniformity: variation in Jbeam < 10%
– Local overheating of anode foil
– Non-uniform pumping may result in spatial non-uniformity in laser pulse
• Longevity: 3 x 108 shots continuous at 5 Hz (2 years)
– IFE requirements for reliability and maintenance
– Rep-rated experiments presently limited by insufficient shielding of “sky
shine” radiation
We have looked at 17 types of cathodes/materials
ELECTRA
•Dielectric Fiber
-Double Velvet
-Other Velvets
-Glass
•Carbon
-Fiber
-Cloth
-Foam 100 ppi
-Foam 500 ppi
-Flock (2%)
•Metal/Dielectric
-Ceramic/Honeycomb
-RHEPP
-Fine scale RHEPP
•Patched Cathodes
-Silicon Carbide
-Carbon/Carbon Fiber
-Diamond-like Carbon
•CsI Coated Patches
-Double Velvet
-Carbon/Carbon ESLI
-Silicon Carbide
Experimental Standards
ELECTRA
• Beam nominally Vdiode=500 kV, Idiode=90-100 kA, Tpulse=
100 ns flat-top
• External magnetic field, Bext~1.4 kG
• Initial diode pressure (8 inch Cryo-pump), Pdiode ≤ 8E-6 T
• AK gap is ~5.2 cm
• Anode is aluminum (1 mil foil, 1 mil aluminized Kapton,
or cooled Al plate)
• For these experiments all cathodes are 27 cm x 97 cm
Diagnostics
Vdiode
capacitive probe
Idiode
Rogowski
ELECTRA
4-frame GOI
(time-resolved
uniformity)
scintillator
Jbeam
Faraday cup
Ebeam
B-dot array
Pdiode(t) cold cathode gauge
radiachromic film
(time-int. uniformity)
Comparison of Turn-On Times
ELECTRA
normalized diode current
0
-2
Double Velvet
Glass Fiber
Carbon Flock 2%
Carbon Fiber
Carbon Foam 100 ppi
Carbon Cloth
RHEPP
Honeycomb/Ceramic
-4
-6
-8
-10
0
20
40
60
80
time (ns)
•Dielectric fiber cathodes exhibit fast turn-on.
•Carbon Flock, Carbon Fiber, and RHEPP metal/dielectric turn-on is marginal.
Comparison of Diode Impedance History
ELECTRA
normalized diode impedance
1.5
1.4
1.3
1.2
1.1
100 ns into current pulse
Double Velvet
Carbon Flock 2%
Carbon Fiber
RHEPP
1.0
0.9
0.8
0.7
0.6
diode current
0.5
time (20ns/div)
•Velvet cathode impedance history is the standard for comparison.
•Carbon Flock and Carbon Fiber look promising.
Comparison of Uniformity and Longevity
ELECTRA
UNIFORMITY
CATHODE
Double Velvet
Glass Fiber
Carbon Flock (ESLI)
Carbon Fiber (Eusebio)
Carbon Foam (100 ppi)
Carbon Cloth
RHEPP
Fine Scale RHEPP
Honeycomb/Ceramic
Current Density
Variation (rad. film)
6%
25%
15%
41%
19%
57%
50%
25%
32%
LONGEVITY
Change in V or I Pressure @ end of
No. Shots
first-last ?
run
500
No
2.00E-04
….
….
….
3365
Slight
2.30E-05
3100
No
2.20E-05
….
….
….
….
….
….
300
Slight
2.00E-05
….
….
….
….
….
….
• Velvet is standard for uniformity but not expected to meet longevity requirements
• Carbon flock uniformity and longevity promising out to ~3000 shots at 5 Hz. Turn-on
slightly improved with number of shots.
• Carbon fiber longevity okay out to ~3000 shots at 5 Hz.
• RHEPP longevity for short pulses is established. Promising for “patterned” beam schemes
NRL Velvet Cathode
ELECTRA
• Standard velvet (Eagle brand, double velvet, superfine quality)
– 18 µm fiber diameter, 4.4% volume coverage
Velvet : Turn-on and Gap Closure
10
ELECTRA
40 ns 10-90% rise
8
6
Vd
(1.0e05)
Id
(1.0e04)
Zdiode
(5.0e00)
4
2
diode impedance ~5.5 Ω
0
time (20 ns/div)
Velvet : Single Shot, Time Integrated Uniformity
ELECTRA
1mm bandout
1 cm/div
current density (A/cm2)
(A/cm2)
70
60
50
40
30
20
10
0
current density (A/cm2)
Distance (1 cm/div)
70
60
50
40
30
20
10
0
horizontal distance (1 cm/div)
1 cm/div
JAVE=33.1 A/cm2
JSTDEV=1.3 A/cm2 (4%)
Velvet : Single Shot, Time Resolved Uniformity
ELECTRA
GOI Frame 1
2.0
raw intensity
250
0.0
200
150
100
50
0
200-250
150-200
100-150
50-100
0-50
izstack2t
(5.0e03)
goi1t
(1.0e00)
goi3t
(1.0e00)
-2.0
-4.0
-6.0
-8.0
3800
3900
time (ns)
• Direct illumination of scintillator: EJ-212 w/ honeycomb
• 2 ns gate, gain=3, tpulse= 65 ns
• 100 x 50 pixel bandout: INTmean=131, INTstdev=8.2 (6%)
• Faraday Cup Data: Jmean=31.0 A/cm2, Jstdev=1.98 A/cm2 (6.3%)
4000
Velvet: Longevity
ELECTRA
1.0
• Diode voltage and impedance
unchanged after 500 shots at 5 Hz
Vdiode (1E5 Volts)
0.0
-1.0
-2.0
2nd Shot
500th Shot
-3.0
-4.0
• Not expected to meet longevity
requirements
-5.0
-6.0
3700
• Cathode turn-on unchanged at 500
shots
3800
3900
4000
time (ns)
pressure (Torr)
1.E-03
500 shots
•Diode vacuum reaches ~ 2E-4 Torr
and stabilizes
1.E-04
•Uniformity at 500 shots: unknown
1.E-05
1.E-06
time (50 sec/div)
Carbon Flock (2%) Cathode
• Energy Science Laboratories Inc. (San Diego, CA):
ELECTRA
Electrostatically deposited carbon fiber onto aluminum back
plate: 7.0 µm dia., 1.5 mm length, 2% volume. coverage
Carbon Flock (2%): Turn-on and Gap Closure
ELECTRA
10
53 ns 10-90% rise
8
6
Vd
(1.0e05)
Id
(1.0e04)
Zdiode
(5.0e00)
4
diode impedance ~6.5 ↔ 5.9 Ω
2
0
-2
3800
3900
time (ns)
4000
Carbon Flock (2%): Single Shot Uniformity
ELECTRA
• Faraday Cup Data: Jmean=32.7 A/cm2, Jstdev=2.52 A/cm2 (7.7%)
(A/cm2)
1 cm/div
current density (A/cm2)
1mm bandout
70
60
50
40
30
20
10
0
horizontal distance (1 cm/div)
JAVE=40.1 A/cm2
JSTDEV=4.8 A/cm2 (12%)
1 cm/div
Carbon Flock (2%): Uniformity at 3365 Shots
ELECTRA
• Overall uniformity slightly worse after 3000 Shots; more hot spots
(A/cm2)
1 cm/div
current density (A/cm2)
1mm bandout
90
80
70
60
50
40
30
20
10
0
horizontal distance (1 cm/div)
JAVE=52.2 A/cm2
JSTDEV=8.26 A/cm2 (15%)
1 cm/div
Carbon Flock (2%): Longevity
ELECTRA
Vdiode (1E5 Volts)
0.0
• Diode voltage unchanged
throughout 3365 shot run at 5 Hz
-1.0
-2.0
1 Shot
500 Shots
2000 Shots
-3.0
• Diode current unchanged
• Cathode turn-on slightly improved
with number of shots
-4.0
-5.0
3800
4.0E-05
3900
4000
time (ns)
3365 shots
pressure (Torr)
3.5E-05
3.0E-05
2.5E-05
2.0E-05
• Diode vacuum stabilizes at about
2.3E-5 Torr after 1000 shots.
1.5E-05
1.0E-05
5.0E-06
0
200
400
600
800
time (s)
1000
1200
1400
Additional Requirements for KrF Laser Cathodes
ELECTRA
• Magnetic guide field required to prevent beam from pinching
• Cathode Compatibility with external guide field (1-3 x Bself)
• Beam rotation - must get through hibachi structure efficiently
• Possible solution: rotate cathode to compensate
• Transit time instability
• Large area, parallel-plane, low impedance diodes are vulnerable
• Causes spatial and temporal non-uniformities in e-beam resulting in
poor diode efficiency and anode foil overheating
• Possible solution requires columnar or patched cathode emitters
• Edge effect: Jedge~2Jbeam
• Anode foil damage/overheating
Develop The Emitter & Hibachi As A Single System
ELECTRA
Baseline Design:
Pattern emitter to miss hibachi ribs
Flow water through ribs for cooling
1. Transmission
2. Foil cooling
3. Pattern the beam (to miss the ribs)
Laser Gas
Kr + Ar
1.33 atm
4. Ribs provide a electrically flat anode
5. Beam uniformly pumps laser gas
Vacuum
.001”Ti
FoilGas
Laser
e-beam
e-beam
Water
cooled rib
S. B. Swanekamp, Jaycor, McLean, VA, contributed to this work
Step 1: 1-D Energy deposition profile
shows 78.6%deposited in gas @ 500 keV
35
Edeposited (keV/electron)
Emitter
30
25
20
15
84% at 700 keV
10
5
0
0
5
10
15
X (CM)
20
25
30
The Electron Beam Can Be Patterned To “Match” the Hibachi
ELECTRA
PATCH VELVET CATHODE (3 cm x 3 cm)
PATCH METAL/
DIELECTRIC
CATHODE
(3 cm x 1 cm)
Closely Space Ribs Approximate Electrically Flat Anode
ELECTRA
Vac
Uniform
Emitter
.001”Ti
Foil
1 cm/div
w/ Anode Foil
e-beam
1 atm
air
Rib
0
Radiachromic Film
on Downstream Side
of Hibachi Foil
10 20 30
Amps/cm2
1 mm Wide Band-Out
No Anode Foil
Uniform
Emitter
.01”Ti
Foil
1 cm/div
Vac
e-beam
1 atm
air
Rib
0
Ribs can be a ferritic material !
10 20 30
Amps/cm2
Beam rotation compensated
Cathode strip without rotation
RC films at the anode
4° rotation at
the anode
Overlaid position of a cathode strip
Part of the
electron beam
hits the ribs
Electron
beam at
cathode
anode
Cathode strip with 4° rotation
Beam enters
the hibachi
vertically
Rib positions
Electron
beam at
cathode
anode
Beam losses at
ribs are
minimized
Normalized current
density
Electron beam spreads inside hibachi
Current transmission efficiency
With anode foil: 83%
Without anode foil: 82%
Normalized amplitude
I diode (with anode foil) = 101.1 kA
I diode (without anode foil) = 88.5 kA
(a)
(b)
1
2
3
4
Distance (cm)
1.2
1
0.8
0.6
0.4
0.2
0
0
2
4
6
8 10 12 14
Distance (cm)
FWHM (cm)
(c)
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
With anode foil
Without anode foil
5
(a)
J diode with anode foil
2.37
(b)
J laser cell with anode foil
2.61
(c)
J laser cell without anode foil
3.15
Transit Time Instability
ELECTRA
2-D Modeling:
Diode acts as parallel plate transmission line,
guides growing TEM mode
Experiment:
1600
DC current
AC current
1400
Current (A)
1200
1000
800
Energy flow:
RF to electrons
at cathode
600
400
200
0
-100
-80
-60
-40
X (CM)
Edge of
Cathode
0.8
0
20
cathode
Center of
Cathode
anode
1.5
2.5 GHz
0.6
0.4
1.0
0.5
0.0
f=2.5 GHz
-0.5
-1.0
-1.5
0
5
10
15
20
Time (ns)
0.2
V (MV)
B-dot signal (Arb. units)
FFT (B-dot signal)
-20
Energy flow:
electrons to RF
at anode
0.8
0.4
0.0
0.0
0
2
4
6
8
10
12
14
16
Frequency (GHz)
S. B. Swanekamp, Jaycor, McLean, VA, contributed to this work
0
50
100
150
Time (ns)
200
2D PIC simulations predict that resistive, tuned slots in the cathode
stabilize the instability
ELECTRA
V (MV)
Anode
f=2.5 GHz
0.8
0.4
0.0
0
e-
100
150
Time (ns)
200
-1.0
-5 0 5
Z (cm)
0.4
0.2
0.0
0
50
100
150
Time (ns)
200
dN/dE (Arb. units)
V (MV)
0.0
-0.5
X (m)
Cathode
Slot with
Resistive
Wire
1.0
stopped
by foils
Not stopped
by gas
0.5
0.0
0.0
0.5
E (MeV)
1.0
After
λ/4
0.5
1.0
Cathode
50
dN/dE (Arb. units)
Before
1.0
stopped
by foils
Not stopped
by gas
0.5
0.0
0.0
S. B. Swanekamp, Jaycor, McLean, VA, contributed to this work
0.5
E (MeV)
1.0
Elimination of edge effects will extend anode foil lifetime and allow
more uniform pumping of the laser gas
ELECTRA
Simulation
0 20
40 60 80
current density (A/cm2)
Base
Emitter
Radiachromic
film
Experiment
0 20
40 60 80
current density (A/cm2)
Experiment
A metallic field shaping element
reduces the electric field at the
boundary, suppressing the high
current density beam edge.
0 10 20 30
current density (A/cm2)
S. B. Swanekamp, Jaycor, McLean, VA, contributed to this work
Status Summary
ELECTRA
• Velvet sets standard for turn-on, gap closure, and uniformity but
probably won’t meet longevity requirements.
• RHEPP metal dielectric has promise for longevity and for
patterned beam schemes. Longevity for longer pulses needs to be
examined.
• Carbon flock uniformity and longevity is promising to 3000 shots.
• Carbon fiber longevity is promising to 3000 shots but uniformity is
questionable.
• Need to improve optical diagnostics to get more time resolved
uniformity data and to look at turn-on physics.