A Nd:glass laser oscillator using rotating beam on the HT-7 Thomson Scattering Apparatus

1. A Nd:glass laser oscillator using rotating beam on the HT-7 Thomson Scattering Apparatus  L.Yang, J.Y.Zhao, Y.D.Li, Z.S.Fang, J.S.Mao, V.Sannikov* A. Gorshkov* ASIPP, Hefei, Anhui, 230031, P.R.China. * PRC Kurchatov Institute, Moscow 123182. RUSSIA

2. Abstract • Thomson scattering diagnostic has been an important and standard method for measuring electron temperature and density profiles on all modern tokamaks. When designing a Thomson Scattering system, the principal problems are getting sufficient scattering signal, suppression of stray light and suppression of plasma background light. Increasing the laser energy is clearly one of the methods which help to resolve these problems. In Thomson scattering diagnostic on our HT-7 Tokamak the solid state Nd:glass laser is used because it can be produced in large sizes to store high energy per unit volume. To usual solid state laser, the “thermal lensing” effect can’t be neglected. Many methods to resolve this problem have been reported. The most special way has obvious advantages by using rotating optical beam in the oscillator. In our ring cavity master oscillator, by using Dove prism placed in some angle, we can get the rotating optical beam. Comparing with other methods, this method is more simple and efficient. Further more, due to the dove prism, the resonant cavity of the master oscillator can pump and extract energy from as much of the laser medium as possible and keep reasonable beam quality.

3. Contents • 1 Introduction • 2 The working principles of the usual ring cavity and our ring oscillator. • 3 The stability of the resonator • 4 The analysis of output of the laser beam • 5 Summary

4. Introduction • For Thomson scattering diagnostic of high temporal evolution, we should study the thermal-lensing effect in our Nd:glass laser. • The reason of arousing thermal-lensing effect in solid state rod. • Different methods to reduce the “thermal lensing” effect in solid state laser rod have been reported. Rotating the cross-section of the oscillator beam in resonator is the unique one. Reference: 1 G.N.Steinberg; Phys.Rev.(A), 1971, 4, 1182 2 R.L.Carman et al.; Appl.Phys.Lett., 1969, 14, No.4, 136 3 Deng ximing et al.; Chinese laser, 1982, 10(5), 274-276

5. Input beam flashlamp s p The analysis of the thermo-lensing effect The different opinions about the reason for Thermal-lensing effect in solid state rod oscillator Input beam rod s p ►electrostrictive self-focusing effect ►To solid state rod laser, the refractive index of the rod will distribute in the form of parabola in poloidal direction when pumping exist. ►With the energy of laser beam arousing, the thermal-lensing effect will produce. rod 2f 2f Ref: Deng ximing et al.; Chinese laser, 1982, 10(5),274-276

6. Unique method to reduce thermal-lensing effect rod Main meaning: By making the input beam rotate along the laser axis, the energy distribution of the beam in the cross-section will become even, which can help to reduce the thermal-lensing effect. Reflective mirror Reflective mirror Rotating mirror Rotating axis Input beam Output beam Experiment have shown at lest two features of this kind of cavity: ► It will arise the damage threshold of the laser. ► The brightness of the output laser will increase obviously. ◆ It is effectively used in laser with 5-10ms in duration.

7. 0 L γ θ M3(R,S) M4(R1) The working principle of the oscillator(one) Due to the dove prism, the cross-section of the oscillating beam will rotate in the cavity which can help to reduce the thermo-lensing effect of the solid state rod. M2 M1 z With different R1, asymmetric output will produce. According to our master oscillator, the largest output is about two times of the least output. The schematic of the ring cavity When the oscillating beam is traveling in the cavity, it will deflect in the horizontal surface due to the structure of the ring cavity The self-deflection effect of the beam in the ring cavity

8. The working principle of the oscillator(two) 1 Y’ P γ X’ 0 Due to the dove prism, the oscillating beam will deflect in vertical direction.When the beam is shed on the surface of the dove prism, the output laser beam will rotate by angle 2γ. So, when the beam is rotating in the oscillator, the energy distribution on the polidal direction of the rod will become even, which help to reduce the thermal-lensing effect in the rod. Further, the polarization of the beam is decided by γ.

9. The spots of the output laser in different situations ~00 ~30--50 ~50<~100 With , the cavity is just like the usual ring cavity and one little bright laser spot will produce because only the population inversion in near field of the center in the rod will drop to the metastable level. With a little increasing of the , more and more ions are pumped to the high energy level and produce the laser pulse. So, the laser spot become larger and bright . When the angle is too large, a null hole will produce in the center of the laser spot because some of the deflecting effect of the dove prism.

10. a a b b c c d d Analysis of the stability of the resonator The optical matrix of the ring cavity is following : Two kinds of stabilities of the cavity ** condition of strict thermal stabilitycondition of weak thermal stability The cavity don’t fit for the condition of strict thermal stability. Due to the weak condition of which, the solution to the equation exist. So, the mode pattern and divergence of the beam in far field don’t sensitive to the thermal-lensing effect. Ref: Cao qing et al.;Acta physica sinica,1997,46(1)

11. The analysis of the output energy of the laser • When setting R1=0 and R1=1, we can see that the output energy of the laser(y0) will vary with different s. Considering , R=0.6 We can see that the output y0 with R1=1 is about two times the y0 with R1=0 during the whole range of s and the deviation of y0 with R1=0 is small. -3 x 10 120 7 6 110 5 4 100 3 90 y0 2 dyo/ds 1 80 0 -1 70 -2 60 -3 1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 s -3 x 10 s -3 x 10 Ref:Lin fucheng et al.; Chinese laser,1985,14(2),70-72

12. Summary • The master oscillator in our laser system consists of a ring resonator. Due to the dove prism placed in the cavity in some angle, the cross-section of the oscillating beam will rotate which will help to reduce the themostresses accumulation during the operation. When the reflection of the mirror M4 is changed, asymmetric output is obtained. Considering the use of laser in our diagnostic, we set . The high power laser pulse in one direction will produce during the burst. • Further, the Q-swich is down by plate of LiF-2 crystal with colored centers. When the angle between the axis of the optical beam and the bottom surface of the prism is changed, different output will produce. • The cavity is fit for the condition of weak thermal stability. The mode pattern and the divergence of the beam in far field don’t sensitive to the thermal-lensing effect.