Transcript ppt

Studies of Enhanced Edge Emission of a
Large Area Cathode
Naval Research Laboratory
Laser Plasma Branch, Plasma Physics Division
Frank Hegeler
Naval Research Laboratory
Code 6730
[email protected]
Abstract
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Studies of Enhanced Edge Emission of a Large Area Cathode *
F. Hegeler†, M. Friedman, M.C. Myers, S.B. Swanekamp‡, and J.D. Sethian
Laser Plasma Branch, Plasma Physics Division
Naval Research Laboratory
Washington, DC 20375
Electra is a repetitively pulsed, electron beam pumped krypton fluoride (KrF) laser that is
used to develop the technology required for inertial fusion energy (IFE). A full scale fusion
KrF laser will be pumped with electron beams with cross-sections of 2,500 to 10,000 cm2.
Understanding the mechanisms that govern uniform electron beam emission over large area
cathodes is important for overall system efficiency and durability. This paper presents
measurements of the enhanced current density along the edges of a large area electron beam
as well as successful techniques that eliminate this edge effect/beam halo. The spatial and
temporal current density data is obtained with a Faraday cup array at the anode, and the
spatial, time-integrated current density is obtained with radiachromic film. MAGIC particlein-cell (PIC) simulations support the experimental results. Experiments and simulations
showed that recessing the cathode minimizes the electric field at the edge and eliminates the
edge effect. However the field shaper itself emits under long term repetitive operation. In
contrast using a metallic electric field shaper, which is electrically insulated from the
cathode, eliminates the beam halo and prevents or minimizes electron emission from its
surface during repetitive operation.
*Work supported by the U.S. Department of Energy
† Commonwealth Technology, Inc., Alexandria, VA 22315
‡ JAYCOR, McLean, VA 22102
Electra is a repetitively pulsed KrF laser that will be used to develop the
technology required for an Inertial Fusion Energy (IFE) power plant
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The laser cell is pumped from two sides by 500 kV, 110 kA, 100 ns flattop electron beams at 5 Hz
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Transformer
1:12 Step up
Pulse forming lines
2 x 8.5  (50 ns one-way)
Output Switch
1.07 MV @ 3.94 s
SF6-insulated, laser triggered
Based on Nike
Magnet
0.5-1.5 kG
Prime switch
E-beam
Diode
Air-insulated
CL
Prime Caps
11.2 kJ
@  43 kV
Cathode
Hibachi
Laser
Cell
Experimental Standards
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• Beam nominally Vdiode = 450-550 kV, Idiode = 70-110 kA,
Tpulse= 100 ns flat-top
• External magnetic field, Bext~1.4 kG
• Initial diode pressure (8 inch Cryo-pump), Pdiode  8E-6 Torr
• AK gap is 4.2-5.2 cm
• Anode is aluminum (1 mil aluminized Kapton)
• Cathode sizes: 27 cm x 97 cm and 35 cm x 106 cm
Diagnostics
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Vdiode
capacitive probe
Idiode
Rogowski
4-frame GOI
(time-resolved
uniformity)
scintillator
Jbeam
Faraday cup
Ebeam
B-dot array
Pdiode(t) cold cathode gauge
radiachromic film
(time-int. uniformity)
Voltage and Current
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-40
0
40
80
120
160
200
240
time (ns)
Typical voltage and current density waveforms for velvet cathode experiments, with an A-K
gap of 5.2 cm, and t = 0 corresponds to the start of the current pulse.
The Enhanced Edge Emission Effect
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It is well known that the current density is enhanced near the
edges of planar explosive emission cathodes. This beam
halo is caused by a discontinuity between the vacuum electric
field outside the diode and the electric field inside the diode
where space-charge dominates the electric field. Recessing
the cathode into the metallic shroud to produce a field shaper
at the discontinuity reduces the electric field to zero at the
triple point [1]. For relativistic e-beam applications with
average electric fields on the order of 100 kV/cm, the
elimination of the halo becomes more problematic since
electron emission from the electric field shaper must be
suppressed.
[1] J.R. Pierce, “Theory and design of electron beams,” 2nd ed.,
New York: D. Van Nostrand Company, 1954.
PIC Simulation: The beam halo is eliminated by
reducing the electric field at the cathode edge
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Simulation of the e-beam halo with PIC code grid resolution of 1 mm. (a)
anode, (b) metallic non emitting shroud, (c) cathode, and (d) A-K gap of 5
cm, with dx ranging from 0 to 10 mm, and dy = 1 mm for all cases.
Using MAGIC 2-D PIC code, developed by Mission Research Corporation, Newington, VA
PIC Simulation: The beam halo amplitude is dependent
on the simulation grid size
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Simulation of the e-beam halo with PIC code grid resolution of 0.5 mm. (a) anode,
(b) metallic non emitting shroud, (c) cathode, and (d) A-K gap of 5 cm, and
dy = 0.5 mm for both cases. The normalized peak current density is 3.4 and 1.18
for dx = 0 and 2 mm, respectively.
Using MAGIC 2-D PIC code, developed by Mission Research Corporation, Newington, VA
Velvet Cathode
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Velvet : Single Shot, Time Integrated Uniformity
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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%)
Faraday Cup Array Sensor to Measure the Beam Halo
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Faraday cup
sensor elements
(5 mm x 20 mm)
insulated by
3 mil Kapton foil
Current of the individual
sensors is measured
by Pearson monitors.
Electron Beam Rotation
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The beam pinches due to its self magnetic field.
The inward pinched beam is subject to a JxB
force with the applied magnetic field, causing
the beam to rotate while it travels from the
cathode towards the anode. For small rotations,
the rotation angle q r is given by
x By
tan(q r ) 
z Bx
where x is the beam propagation distance, z is
the cathode point horizontal coordinate, By is the
magnitude of the vertical component of the selfmagnetic field, and Bx is the applied magnetic
field magnitude. Due to the current rise and fall
the rotation angle varies in time, with its
maximum value occurring at peak beam current.
Lowering the external magnetic field amplitude
results in a larger rotation angle as shown right,
lines (b) and (c).
Time-Resolved Measurements of the Beam Halo
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Faraday cup array results with an external magnetic field of B = 1.4 kG. The
numbers (1) to (8) correspond to the traces of the individual faraday cups.
Time-Resolved Measurements of the Beam Halo
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e-beam
normalized current density
2
(4)
(3)
(8)
(7)
(6)
(5)
(4)
(3)
(2)
(1)
(3)
(2)
1.5
(2)
1
towards
e-beam
center
(c)
(b)
(a)
(1)
(3)
0.5
(2)
0
-0.5
-40
(1)
0
40
80
120
160
200
240
time (ns)
Faraday cup array results with an external magnetic field of B = 0.6 kG. The
numbers (1) to (5) correspond to the traces of the individual faraday cups, and the
waveforms of faraday cups (6) to (8) are not shown for clarity.
3-D Simulations Show Beam Rotation and Pinching
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B = 1.4 kG
y (10 cm/div)
(b)
B = 0.6 kG
(c)
-50
-25
0
z (cm)
25
50
3-D PIC simulations showing electron beam rotation and pinching. The dashed line
represents the dimensions of the cathode, and the solid lines illustrate the beam edge at the
anode. The vertical e-beam displacements at positions (b) and (c) are 1.1 cm and 1.8 cm
for an external magnetic field of 0.6 and 1.4 kG, respectively. Vdiode = 500 kV, 30 x 100cm2
cathode, and A-K gap = 5.0 cm.
Recessing the Cathode Eliminates the Beam Halo
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cathode
shroud
Electron emission at the cathode edge without recess. (a)
experimental results from a radiachromic film scan and
(b) 2-D PIC code simulation with a 1 mm cell grid size.
Experimental parameters are Vdiode = 500 kV, 27 x 97 cm2
velvet cathode, and A-K gap = 5.2 cm.
cathode
shroud
Electron emission at the cathode edge with a 11 mm
recess into the cathode shroud. (a) experimental results
from a radiachromic film scan and (b) 2-D PIC code
simulation with a 1 mm cell grid size. Experimental
parameters are Vdiode = 420 kV, 35 x 106 cm2 velvet
cathode, and A-K gap = 5.2 cm.
Experiments and simulations showed that recessing the cathode minimizes the
electric field at the edge and eliminates the edge effect. However the field shaper
itself emits under long term repetitive operation.
To allow high e-beam transmission efficiency, the cathode
should be segmented into strips.
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E-beam: Pattern emitter to miss hibachi ribs
Cooling: Flow water through ribs
Deflect recirculating laser gas
TOP VIEW
SIDE VIEW
Laser Gas
Louvers
Kr + Ar
1.33 atm
Vacuum
Vacuum
Emitter
Emitters
Laser Gas
Flow
Pressure
Foil
.001”Ti
e-beam
4.4 cm
e-beam
Rib
1.3 x 1.0 cm
Water cooled
Rib
Pressure
Foil
Foil loading
 1.4 W/cm2
Rotated strip cathode, 2.5 cm wide strips
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Segmented strip cathode has multiple beam halos
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Strip Cathode Requirements:
• The beam halo of every strip must be eliminated.
• The concept of edge elimination must be reliable for repetitive
operation at average electric fields of 100 kV/cm.
Solution: The Floating Field Shaper
• Instead of preventing electron emission from a field shaper that
is in direct contact with the cathode, the field shaper is
electrically isolated from the cathode.
• The floating field shaper is incapable of emitting large currents
until the field shaper is electrically shorted to the cathode by
the expanding cathode plasma.
Successful Repetitive Operation with
Electrically Insulated Field Shaper
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Tested for
3000 shots
at 1 Hz
(a)
1.3"
1.18"
1.181"
0.275"
3/16"
(a)
3/16" electrically insulated field shapers
velvet
0.394"
strip cathode configuration
Beam Halos are Reduced by Field Shapers
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electrically insulated field shapers
(a)
(a)
(b)
(b)
0.537"
0.662"
strip cathode configuration
1.055"
0.930"
0.371"
0.511"
Summary
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• Recessing the cathode into the shroud reduces the electric
field at the cathode edge and eliminates the beam halo.
• This concept works for single shot operation, but with average
A-K gap electric fields of 100 kV/cm, the field shaper itself
emits under repetitive or long-pulse operation.
• By electrically insulating the field shaper from the cathode,
emission from the field shaper can be eliminated.
• The floating field shaper has been successfully tested for
repetitive operation (for 3000 shots).
• The floating field shaper eliminates the electron beam halo, and
it is compact enough to be attached to each strip.