Page 1 - Blended Kinetic Simulations of Transport in C-2W

Generation of Relativistic THz Radiation by Laser Irradiation of Microplasma Waveguide with
Introduction
• MFE* devices need internal field measurements, B(r).
• Pulsed Polarimetry provides B, ne (μneBds).
• It is a LIDAR diagnostic. Remote sensing through one port,
arbitrarily aimed sightline using a light bullet sensor.
• Based on a FIR (150 𝜇𝑚 or 2 THz) radiation
• Simulation shows we can generate required radiation * Magnetic fusion energy
Visual Guide to Pulsed Polarimetry [2,3]
Application to Pulsed Polarimetry Contact: anecas@tae.com & RSmith@tae.com
Ales Necas1, Roger Smith1, Longqing Yi 2, Francesco Ceccherini1, Laura Galeotti1, and the TAE team
THz Radiation Simulation Results
1TAE Technologies, Inc., 19631 Pauling, Foothill Ranch, CA 92610 Pulsed polarimetry layout for Field
2Dept. of Physics, Chalmers University of Technology, 41296 Gothenburg, Sweden
Electron energy spectrum (MeV)
• Ultra-relativistic electron energy spectrum is obtained • 60 nC of electron charge with energy > 10 MeV
Generation of THz Radiation [4,5]
n Intense laser (𝑎; ≫ 1) irradiates a microplasma waveguide (MPW) to generate ultra-relativistic electrons
Accelerating Electric Field (V/m)
𝑦 (𝜇𝑚)
Reversed Configuration (FRC) plasma
Synthetic Diagnostics
Ppol
Spol
n Electrons are extracted from the MPW walls by the laser transverse electric field and injected into the MPW
n Electrons are accelerated by two mechanisms: n Ponderomotive force
n Longitudinal component of the waveguide mode. This mode is setup by the laser and the dominant mode is the 𝑇𝑀!!
n As the electrons pass through the small hole at the end of the MPW, they generate diffraction radiation.
n Thus generated electron beam can be also used for transition radiation by placing a metallic foil at the exit.
Laser
DistributedBackscatter
Mirror Dish Thomsonscattering
Polarized 150 μm FIR pulse, 5mJ (THz Radiation)
Relativistic e beam
l
Double pass polarimeter
Ip
s=ct/2
ne μ Is(t) +Ip(t)=I(t) Ip/I= cos2(2a(t))
B
toandback
2l
Grid analyzer
Is
Magnetized plasma, n , B e
sub-ps pulse
•
𝑥 (𝜇𝑚)
𝑇𝑀"" mode can be observed inside the MPW, potentially higher modes as well
Electron longitudinal momentum and density
• •
• •
Pulse is linearly polarized
A sightline trajectory through the center of the FRC is shown
Pulse’s polarization undergoes Faraday rotation and the backscatter inherits the polarization at the scattering volume as the pulse propagates through the plasma. Thomson backscatter is a weak reflection and Faraday rotation is doubled upon reflection
𝑛 /𝑛 %
' fs
𝑝&%/(𝑚%𝑐)
𝑛%/𝑛' fs
𝑝&%/(𝑚%𝑐)
800nm, 100TW high contrast, 200fs TiSap laser
Is/I= sin2(2a(t))
Faraday rotation: 𝑑𝛼(𝑑𝑠) = 2 ×2.63𝑥10!"#×l$𝑛%𝐵×𝑑𝑠×180/𝜋 °
3° @ ne=4x1019, B=0.1T, ds=10 cm, l= 500 μm
Verbal Guide to Pulsed Polarimetry
• Laser generates sub-ps pulse THz (~150 𝜇𝑚) radiation by laser acceleration of electrons to relativistic energies
• Electron diverted into a dump
• Light is linearly polarized and directed by mirror into the plasma
• Faraday rotation is a non-reciprocal effect, retro-reflected light gets
double rotation
• Thomson back-scattered light from plasma is collected by a
Laser
THz radiation
FIR pulse
μ-plasma waveguide
Simulation Setup
Critical density:
𝑛 = 1.1×10"#/𝜆" (𝑐𝑚&') ! $%
E! Electric Field [V/m] fs
Time trace of 𝐸! @ single spatial location Low pass filter
Frequency spectrum of 𝐸!
𝑥 (𝜇𝑚)
n Simulation is performed with PIC code EPOCH [1] n Simulation is limited to 2D
n MPW is composed of pre-ionized carbon atoms n Axial density profile is uniform
n Radial density profile is given a density gradient according to 𝑛 𝑦 = 𝑛"𝑒𝑥𝑝(− 𝑦 − 𝑦" #/𝜎#) where 𝑛" =
• •
Early stage electron acceleration is dominated • by forward longitudinal motion
Electrons are accelerated by ponderomotive
force as well as waveguide modes
Later stage a very large return current develops.
Total E! (<10 THz) energy
𝑘! − 𝑘" converted to freq.
𝑥 (𝜇𝑚)
Cassegrain telescope
• Density, 𝑛 , is directly proportional to 𝐼 + 𝐼 (total intensity)
15𝑛$ Laser
𝑓 (𝑇𝐻𝑧)
Time (fs)
Laser frequency
𝑓 (𝑇𝐻𝑧)
" # $
• Rotation is obtained from intensity (both 𝐼# and 𝐼$)
Faraday Rotation
da = 2 𝑥 2.63.10−13l2n B ds . 180/p [°] e ||
Conclusion from Synthetic Diagnostics And
Challenge
n 𝐿% = 400 𝜇𝑚 n 𝐿& = ±60 𝜇𝑚
n 𝑛% = 8000 n 5th order
References
1. T. D. Arber, K. Bennett, C. S. Brady, A. Lawrence-Douglas, M.G. Ramsay, N.J. Sircombe, P. Gillies, R.G. Evans, H. Schmitz, A.R. Bell, and C. P Ridges, Plasma Phys. Controlled Fusion, 57, 113001 (2015)
2. RJ Smith, “Nonperturbative measurement of the local magnetic field using pulsed polarimetry for fusion reactor conditions (invited)a)” , RSI,79, 10E703 (2008);
3. Long wavelength Pulsed Polarimetry: RJ Smith, “Excess noise in Lidar Thomson scattering methods”, 15th International conf. on Laser Aided Plasma Diagnostics, Oct 13-19, 2011, Jeju, Korea
4. Yi, L., & Fülöp, T. (2019). Coherent Diffraction Radiation of Relativistic Terahertz Pulses from a Laser-Driven Microplasma Waveguide. Physical review letters, 123(9), 094801.
5. Yi, L., Pukhov, A., & Shen, B. (2016). Radiation from laser-microplasma-waveguide interactions in the ultra-intense regime. Physics of Plasmas, 23(7), 073110.
n 𝑛& = 4000 Laser parameters
particle shapes
150 𝜇𝑚 source is ideal for probing this equilibrium produces robust Faraday rotation of 10’s °.
Energy of source ~ 10 mJ
! • Polar component contains 99% of the radiation energy
n 𝐼 = 2×10#" W/cm# n 𝜏' = 50 𝑇"
n Spot size=3 𝜇𝑚
n Gaussian pulse •
𝑥 (𝜇𝑚)
Later stage polar electric field 𝐸 shows a well-developed •
Early stage energy increase is due to edge effects 50 mJ THz radiation is achievable as required for the pulsed polarimetry
n 5 J
n 57 TW
• •
envelope.
The radial component < 1% and very noisy
Conclusion from PIC Simulation
We can produce ~10 mJ energy pulse of ~150 𝝁𝒎 radiation using diffraction radiation at 1% efficiency
Energy (mJ)
𝑦 (𝜇𝑚)
𝑦 (𝜇𝑚)
𝑦 (𝜇𝑚)