High energy and high average power laser for plasma ... · PDF filerepetition probe laser system for plasma ... Multi-laser system 9 Two 2J x 10Hz lasers 1.6 J x 30Hz laser ... Advanced

High energy and high average power laser for plasma diagnostics

‘Efficient second harmonic generation with pico-seconds pulses from a chirping TRAM regenerative amplifier’

Ryo YasuharaNational institute for fusion science

Collaborators

1

National Institute for Fusion Science I. Yamada

Plasma Research Center, University of TsukubaM. Yoshikawa

Institute of laser engineering, Osaka universityJunji Kawanaka, Takahisa Jitsuno

Institute for laser technology Hiroaki Furuse, Shinji Motokoshi

Nagoya universityShin kajita, Masaya Sato

Contents

Introduction and laser Thomson scattering for plasma diagonostics

Development of high energy and high repetition probe laser system for plasma diagnostics

Laser induced damage in nuclear fusion application

2

About National institute for fusion scienceLHD：Large helical device

To conduct fusion-plasma confinement research in a steady-state and important research issues for plasma physics.

Our institute focused on the basic study about the magnetic confinement fusion and the inertial fusion. In this research, high energy and high average power laser is very important.

4

Laser Thomson scattering method• Thomson scattering diagnostics is one of the most reliable

methods for determining the local electron temperature and density in fusion plasmas.

Plasma

Scattering light

Laser

Collective optics

PolychrometorData acquisition system Wavelength [nm]In

tensi

ty o

f sc

atte

ring

light[

A.U

.]

Te=10keV

Te=500eV

Laser wavelenght:1064nm

LHD Thomson Scattering

• LHD TS has following features:– Multi-laser System

• Currently, we use 4 flash lamp pumped Nd:YAG lasers.• 2J / 10Hz Laser X 2• 1.6J /30Hz Laser X 1• 0.5J / 50Hz Lasers X 1

– Backward Scattering Configuration• Scattering angle at the plasma center = 167o .

– (5+1) Wavelength-CH Polychromators• 1-5 CHs detect Thomson scattering signals.• The 6th CH is installed for absolute Rayleigh calibration.

– FASTBUS Data Acquisition System

LHD Thomson Scattering

6

• The example of snap shots of Te (red) and ne (green) profilesLHD Thomson, 14th cycle

Shot#098888

Time Data (s)

0 500 1000 1500 2000 2500 30000

2

4

6Laser Power

0 1 2 3 4 5 60

500

1000

1500

Background subtracted

3.225(s) cnt=160

0

2

4

6

8

Te

(keV

)

3.233(s) cnt=161

3.266(s) cnt=162

3.275(s) cnt=163

3.300(s) cnt=164

3.325(s) cnt=165

3.333(s) cnt=166

3.366(s) cnt=167

3.375(s) cnt=168

0

2

4

6

8

Te

(keV

)

3.400(s) cnt=169

3.425(s) cnt=170

3.433(s) cnt=171

3.466(s) cnt=172

3.475(s) cnt=173

3.500(s) cnt=174

3.525(s) cnt=175

3.533(s) cnt=176

0

2

4

6

8

Te

(keV

)

3.566(s) cnt=177

3.575(s) cnt=178

3.600(s) cnt=179

3.625(s) cnt=180

3.633(s) cnt=181

3.666(s) cnt=182

3.675(s) cnt=183

3.700(s) cnt=184

0

2

4

6

8

Te

(keV

)

3.725(s) cnt=185

3.733(s) cnt=186

3.766(s) cnt=187

3.775(s) cnt=188

3.800(s) cnt=189

3.825(s) cnt=190

3.833(s) cnt=191

3.866(s) cnt=192

0

2

4

6

8

Te

(keV

)

3.875(s) cnt=193

2.5 3.0 3.5 4.0 4.5Major Radius (m)

3.900(s) cnt=194

2.5 3.0 3.5 4.0 4.5Major Radius (m)

3.925(s) cnt=195

2.5 3.0 3.5 4.0 4.5Major Radius (m)

3.933(s) cnt=196

2.5 3.0 3.5 4.0 4.5Major Radius (m)

3.966(s) cnt=197

2.5 3.0 3.5 4.0 4.5Major Radius (m)

3.975(s) cnt=198

2.5 3.0 3.5 4.0 4.5Major Radius (m)

4.000(s) cnt=199

2.5 3.0 3.5 4.0 4.5Major Radius (m)

Thomson scattering cross section is very small：6.65x10-25cm2

Development of a probe laser for the Thomson scattering system

Over 1 J in 10 ns lasers are needed for Thomson scattering system. Signal to noise ratio and repetition rate of TS are limited by a probe laser .

7

• Thomson scattering diagnostics is one of the most reliable methods but….

We are try to develop the advanced TS probe laser system by following methods.

Multiple laser system Multi-pass laser probing method Development of the new laser system

8

Development of a multi-laser system

Multi-laser system

9

Two 2J x 10Hz lasers

1.6 J x 30Hz laser

10Hz/2J10Hz/2J30Hz/1.6J

Laser operation mode1 ：For high time resolution

Laser operation mode2 ：For high S/N ratio

t

t

Co-axially polarization beam combining method

To minimize the multiple-laser error, we proposed the co-axially beam combining system which employs Pockel’s cells and the polarizers to combine each pair of orthogonally polarized laser beams.

PLInput 1

Input 2

QR

s

s

P

Output

Beam combiner

PC

Input 3

QR

s

P

Multiple laser

Scattering light

Collective optics

Polychromator

High rep. laser pulse

L1

L2

L3

L4

Ln

BC

BC

BC

Plasma

Electron density profiles at steady state plasma measured by multi-laser system

11

Shot#107509

2.5 3.0 3.5 4.0 4.5 5.0Major Radius (m)

0

2

4

6

8

Dens

ity [1

019/m

3 ]

LASER1, time=3.817 sLASER2, time=3.834 sLASER3, time=3.850 s

This figure shows good agreement between the ne profiles measured by coaxially laser 1 and 2.

In contrast, the ne profile measured by laser 3 is different. That has a different laser beam axis.

R. Yasuhara et al., PFR (2012)

12

Multi-pass probing system for Thomson scattering

A multi-pass Thomson scattering method

Thomson scattering light

Backward

Forward

Increasing scattering light

ｔ

Improvement of time resolution

Forward BackwardForward Backward

T TMinimization of ΔT

A multi-pass TS system allows the laser pulse to be focused several times onto the plasma, thus increasing the scattering photon number or increase the time resolution of the scattering light from the plasma.

New method: Polarization controlled multi-pass system

14

Features Image relay configuration

(To maintain the laser beam quality) Coaxial multi-beam pass Polarization switching

IP1 IP2

IP1'Mirror

Mirror

GAMMA10

Lens LensPolarizer

Polarization control device

Multi-pass TS system

：Additional optics for a multi-pass TS

Polychrometor

Plasma

Laser

Estimation of the multiplication of the scattering light

15

The multiplication of the scattering light as a function of a pass number from the result of the optical design.

At the 2 pass was the twice larger and the sixteenth pass configuration, scattering light was about six times larger than the single pass configuration.

GAMMA10 tandem mirror

16

Result of double pass

Line integrated density and Wp

The integrated scattering signal of double pass configuration is 2 times larger than the signal of single pass

configuration.

Scattering light intensity at a single and double pass

We measured the scattering signal from 1 pass and 2 pass system at equivalent 2 plasma shots.

Line integrate density and Wp is equivalentat the 2 plasma shots.

1 pass

2 pass

1 pass

2 pass

１pass

2 pass

R. Yasuhara et al., RSI (2012)

18

Development of the new laser system

Thomson scattering laser system development for a magnet confinement fusion reactor

→ One of candidates to minimize the number of ports is LIDER technique. By using this technique we can get the electron temperature and electron density from single port.

However, LIDER Thomson scattering system needs a pico-second and Joule class with high repetition rate laser system.

And green laser light is preferable because of the sensitivity of a silicon photo detector.

19

• At a magnet confinement fusion reactor, the number of ports on the vacuum vessel should be reduced.

20

GENBU laser

Main laser

OPCPA laser

Compressor

Amplifier 1(Cryo Yb:YAG)

Front end Expander

Compressor

Amplifier 2(Cryo Yb:YAG)

Compressor

GENBU = Generation of ENergetic Beam Ultimate

E = 30 Jt = 5 fs~8 ps (Variable)Ppeak = 0.04~6 PWf = 100 Hzl0 = 1030 nm = 600 nm

E = 50 JDt = 20 psf = 100 Hzl = 515nm

E = 1 kJt = 10 - 100 psf = 50 -100 Hz = 1030 nm

E = 200 JDt = 1 nsf = 100 Hzl = 1030 nm

OPCPA(3-stage)

White light generation

Expander

Frequency conversion

A part of GENBU system is suited for a probe laser of LIDER Thomson scattering system.

Cryogenic Yb:YAG regen. with CVBG

21

fiber-coupled LDQCW150W max., 700s, 100Hz)

CVBG8 mm x 8 mm x 25 mmt60 ps/nm ± 12 ps/nmc = 1030.8 nm = 0.87 nm

18mm60mm

60mm

Cryogenic Yb:YAG TRAM(Total-Reflection Active-Mirror)

Output power

22

Rep. rate = 100 Hz

Milli-joule pulse energy is obtained.J. Kawanaka et al., ASSP 2012

SHG experiment

BBO type 1 crystal（5mm）

Separator Power meter 1

Power meter 2

Result of experiment

Beam diameter 0.9 mm、BBO type1, Phase match angle θ=23.33、φ=90

Special thanks to Takashi Sekine

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.005 0.01

SHG

con

vers

ion

effic

ienc

y

Crystal length (m)

60MW121MW242MW245.7MW

We have demonstrated the 0.8 mJ, 10 Hz picosecond green laser pulses at 514.8 nm by using a 5 mm long BBO crystal after a chirping TRAM regenerative amplifier.

A second harmonic conversion efficiency of 53 % was achieved when the pump energy was 1.5 mJ in 480 ps at 1029.8 nm.

25

Laser induced damage on the metal and the silica glass for the nuclear fusion application.

Fused silica window in LHD

We use the fused silica windows in the laser beam line of TSas the vacuum window in LHD.Those windows are irradiated by plasma.

27

L. L Snead et al. Fusion Sci. Technol. 56 (2009) 1069.

Au, Cu, Fused silica and dielectric coated optics are proposed for final optics of IFE chamber.

It is very important to estimate the LIDT such optics in harsh environment.

IFE forum, Osaka （2006)

Metal based optics Fused silica based optics

Final optics of inertial fusion applications

28

Can we estimate the laser induced damage at the harsh environment?

・To understand the effect of each issue for LIDT

Reduction of LIDT

neutron

Multi-pulseX ray

plasma

LIDT at the severe environment

29

D

sample

Laser

Ion gunHe

PC

Single shot LIDT of fused silica irradiated by He plasma

Material Fused silicaGrade ED-A

（OH<200ppm)Roughness Roughness< 0.5nmDiameter 30mm

TOSOH

The sample was irradiated by the ion gun.

ConditionIon energy 5keV,Flux:1018 (He/m2s）,Fluence:1021 (He/m2)

30

Wavelength He irradiation LIDT

1064nmwithout 89.1 J/cm2

with 87.7 J/cm2

355nmwithout 24.9 J/cm2

with 19.7 J/cm2

Result of the measurement of LIDT

At the wave length of 1 micrometer, the value of LIDT with He irradiation and without irradiation is almost same.

At 355 nm LIDT of fused silica irradiated by He plasma is 20 % lower than without irradiation.

In our preliminary experiment, we can observe the significant reduction of the multi-pulse LIDT and the LIDT with plasma irradiation. We should collect more data of the LIDT at the harsh environment to realize the nuclear fusion application.

31

Summary

Advanced TS probe laser system of the multi-laser system and the multi-pass probing system is successfully demonstrated.

A second harmonic conversion efficiency of 53 % was achieved when the pump energy was 1.5 mJ in 480 ps by using GENBU pre-amplifier

To understand the effect of harsh environment on LIDT, we study about the multi-pulse effect on LIDT of cupper mirror and the LIDT of the fused silica with plasma irradiation.

In our preliminary experiment, we can observe the significant reduction of the multi-pulse LIDT and the LIDT with plasma irradiation.