Download Laser Driven Electron Beam production at ELI-NP

The conceptual design of the
Electron Spectrometer for the High
Field Physics experiments at ELI-NP
S. Balascuta, Edmond Turcu
1
Outline
• The acceleration of the electrons by Laser Wake
Fields, using two 10 PW Laser beams at ELI-NP.
• The energy analysis of the electron beam by
dispersion in a static magnetic field
• The parameters of the Electron Spectrometer (ES)
• ES with permanent magnets.
• ES with electromagnets
• The hybrid ES
• Conclusion
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The acceleration of electrons by Wake
Field Laser Acceleration
• Accelerating gradient for conventional RF linear accelerators is <100
MV/m. Laser produces plasma with electric field gradient E0= c· me·ωp/e
with ωp=(4·π·n0·e2/me)0.5 and n0=1018 cm-3, E0=96·109 (V/m).
• High quality electron pulses with energy up to 1 GeV, pulse time τp < 100
fs, accelerated with 40 TW laser beam, with 37 fs pulses, focused to an 8.5
μm spot (FWHM) [1].
• The extension of the Laser propagation beyond Rayleigh length is
necessary to increase the electron beam energy [2]
• Two effects can limit the acceleration distance: a) the diffraction of the
drive laser pulse was overcame by focusing laser beam on preformed
plasma channels and b) particle wake de-phasing was overcame by
operating at lower plasma density.
L dep≈ λp3/ λ2 ≈ 1/np3/2
The energy gain of the electron beam:
n0
1
2
W

E

L


gain
0
dep
E0  4 n0 mec  n0
n03/2 n0
[1] K. Nakamura, B. Nagler, Cs. Toth, C. G. R. Leddes, C. B. Schroeder, E. Esarcy, W. P.
Leemans: Physics of Plasmas 14, 056708 (2007)
[2] W. P. Leemans et al. IEEE Trans Plasma Science 24, 331, 1996
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The E6 area at ELI-NP.
Fig.1 The diagram of the interaction
chamber, the beam stop and the two
10 PW beams (probe and pulse) at
E6 area of ELI-NP.
X Fig.2: Electron Spectrometer magnet
L=400 cm. Electron beam angular
dispersion is ± a.
Y
2a
tg(a) =d/(2L)
d
For a uniform B field
400 cm
For L=400 cm, a=0.5 deg dmin=7 cm
For L=400 cm, a=0.2 deg dmin=3 4cm
Laser driven electron beams.
• High quality electron beams were produced using ultra-short laser pulses
(<50 fs, at low power intensity).
• At ELI-NP two 10 PW beams (probe & pulse beams) will be focused on 1 m
long capillary gas cell.
• Characteristics of the magnet: the transmission and the efficiency
Beam
Dump
ICT
Electron
Spectrometer
Off axis
parabola
H2 plasma
channels
1.5 m
CCD1
CCD2
Fig:3 A view in the vertical plane passing through the center of the interaction
chamber.
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The electron trajectories in Electron Spectrometer up to the Beam Dump.
0.15
Beam Dump
0.75
Fig. 4: The picture of some electron
trajectories for seven energies
Fig. 5: The end displacements of electron
trajectories versus the electron energy
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Electron Spectrometer with two electromagnets
Advantages:
• Electromagnets have adjustable electric current to
assure that the electron beam hits the Beam Stop at
about the same level from the floor.
• Uniform magnetic field inside the Gap. No need for
water cooling.
• It can have a modular design: Four 1 meter long
electromagnets placed in contact with each other.
Disadvantages:
• The Copper coils can pick up the Electromagnetic Pulse created
upon the interaction of the Laser with the Plasma cell.
• If the coils are placed in the vacuum, the water cooling of the coils
increases the volume and complexity of the vacuum system for the
coils. In vacuum the air cooling by convection is absent.
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The geometry of the ES with
electromagnets
•
•
•
Width of the Iron Poles 2W=80 mm, high of the pole H=80 mm, the gap is 60 mm,
high of the C core is 100 cm.
Two solenoids on the straight poles with length 2W=80+10 mm and height 80 mm.
Because of the heat generated in the wire, there is an upper limit of the current
in the wire. It depends on the wire diameter. The dependence is a power function
that was calculated for transmission lines or for wires connected to chassis.
Fig. 6: The measured current
Fig. 7: The magnet seen in the vertical plane
normal to laser beam axis
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The Electron Spectrometer with two solenoids (27 cm long).
Fig. 8: The magnetic field along the X
and Y axes.
Y (mm)
Fig. 9: The upper limit of electric
current as a function of the
diameters of the copper coils.
Fig. 10: The magnetic field along the Y
axes.
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Electron Spectrometer with two squared solenoids 20 cm long.
Fig. 11: N=1000 turns (cupper wire 1 mm diameter) connected at 1.8 kV.
The dimensions in the left figure are in mm.
Fig. 12: A map of the magnetic field lin two planes at G/3 and –G/3 relative
to the upper and lower poles.
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Fig. 13: Magnetic field along two directions (X axis on the left and Y axis on the right)
for 20 cm long electromagnet.
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The 1 meter long electromagnet with a “C” iron yoke.
Fig. 14:An electromagnet with Iron yoke (1 m long), with 1000 turns/solenoid (2.3 m long),
connected to 1.8 KV. The electric current is 1.8E3 /(0.02094*2.3*2*1000) =18.7 Amps.12
Electron Spectrometer with
permanent magnets
Advantages:
• Stable magnetic fields if the temperature variation
inside the E6 chamber is less than 1 degree.
• Magnets plated with Nickel or Chromium can be placed
in vacuum.
• The design can be modular. Four dipoles 1 m long, could
be placed in contact with each other (along the
direction of the incident laser beam) to make the 4
meter long magnet.
Disadvantages:
The cost could be bigger than for the an electro- magnets, because there are many
magnets that have to be attached well to each other and to the Aluminum
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Electron Spectrometer with permanent magnets and straight
Iron Yoke
Fig. 15
Fig. 16
Fig. 17: The total magnetic field along the X
axis.
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Electron Spectrometer with permanent magnets and Iron
Yoke with rounded corners
Fig. 18
Fig. 19
Fig. 20
Fig. 21
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A hybrid magnet with “C” Iron Core and both permanent magnets and
electromagnets.
Z
Fig:22. Magnetic field in the center of the Gap . In
the center of the gap, It is expected a magnetic flux
0.4 + 0.6 =1 Tesla
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Conclusion
• Magnetic field of 1 Tesla calculated for an electromagnet 1
meter long and Iron core 0.4 m wide. For 2000 turns of #18
wire, the field in the center of the gap is 1 Tesla.
• For 1 meter long Electron Spectrometer with permanent
magnets, the field in the center is 0.5 Tesla.
• A hybrid magnet (with both permanent magnets and
electro-magnets) could provide a magnetic flux bigger than
1 Tesla using a low electric current in the copper coils. The
heat generated in the Copper wires is decreased to zero.
• The company ICPE (Bucharest) will be involved with the
evaluation of the cost and the technical design will of the
magnet.
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