Laser plasma diagnostics in rubidium vapor cell J.S.Bakos, - - PowerPoint PPT Presentation

Laser plasma diagnostics in rubidium vapor cell

J.S.Bakos, G.P.Djotyan, G.Demeter, P.N.Ignácz, M.Á.Kedves, B.Ráczkevi, Zs.Sörlei, J.Szigeti, K.Varga-Umbrich A.Czitrovszky, A. Nagy, P. Dombi, P. Rácz

HAS WIGNER RESEARCH CENTRE FOR PHYSICS

Motivation and issues found in measurements

• Plasma generation in Rb vapor by ultrashort laser

pulses

• Plasma diagnostics by CW diode lasers: plasma density

2

Why rubidium cell? Easily vaporized Rb Convenient spectral lines 780nm Vapor density simply controllable by temperature Why diode laser? Commercially available Cheap (CD writer 780nm) Simple to operate Easy frequency tuning

3

Another simple plasma source

Rb vapor source : getter @ double slit Rb vapor distribution above the slit

4

Direct way of plasma diagnostics: collecting charged particles

Langmuir flat probe:

• 1. Simple design
• 2. Ions/electrons come from

‘somewhere’ , calibration difficulties

Results of direct ion detection

5

Maximum laser intensity: 1011 W/cm2 Slope: ~ 2 ionization dependence on laser intensity

6

Indirect plasma diagnostics: plasma = ‘lack’ of neutral atoms

Population dynamics for a pair of resonant pulses Atomic processes: Decay time some 10 ns

7

Plasma diagnostics by CW diode lasers

Atomic Lorentz model: resonant absorption @ interferometry

8

Experimental layout

Parameters of the Ti:Sa laser Mean wavelength 806 nm Beam Diameter:9 mm (1/e2Gauss) Polarisation:Linear, vertical Repetition Rate 1 kHz Pulse duration (FWHM):35 fs Pulse 3.5 mJ Courtesy of

• A. Czitrovszky, P. Dombi,
• P. Rácz, A. Nagy, I. Márton

9

Experimental layout

10

Vapor cell, heating wires, reflector

Temperature distribution

Courtesy

• A. Bendefy (BME)

Spectroscopic observations

11

Detection of the radiation of the plasma by a fast spectrograph (Andor Mechelle 5000) High spectral resolution (0.05 nm accuracy) High temporal resolution with intensified camera (~ ns)

Spectrograph courtesy of L. Kocsányi (BME), and help with the measurements R. Bolla (WRCP)

Observed spectral lines of Rb

12

Time dependence of the spectral emission

13

Temperature: ~ 200 Co Ion relaxation mainly through D2 lines (and D1)

14

Transversal absorption measurements

• 1. Ionizing laser intensity
• 2. Probe laser detuning
• 3. Vapor density

Parameters:

15

Tipical transmission signals on microsec scale

Initial condition: atoms in the ground state (different) CW level: Positive peak @ Negative peak and relax. (New Focus 1591NF): 4.5 GHz Very fast peak: AC Stark shift 10 ns decay: atomic relaxation Slow (1-10 microsec) decay: plasma relaxation Decrease of transmission is attributed to reflection on the boundaries of the plasma channel.

Detuning:Rubidium frequency reference

16

17

Dependence of the fast peak maxima on the laser frequency

Slow relaxation component (negative peak) at different vapor densities

18

19

Transmission signal vs. vapor density

Signal oscillations ? Plasma freq.100 GHz Repeated reflections on the boundaries of the plasma channel

Summary of transmission signal detection

20

Cw level is different for detector 1 and detector 2:

• beam divergency, different coupling into the detector fiber
• condensed Rb on the windows surface
• different vapor temperature and density
• CW signal is absorbed close to the resonance lines
• Negative peak sygnal is missing far from the atomic lines
• ‘Plasma oscillations’

21

Plasma density measurements by longitudinal interferometry

22

Phase variation ) ( ) / 2 ( ) (

1

t n L t     

 

 

      

2 1 2 2 ) ( ) ( ) ( 2 1 2

) ( 2 1 ) (

j i j i j i j i i

p m fe N n     

] ) ( /[ 2 ) ( ) (

2 1 2 2 ) ( ) ( ) ( 2 1 2 2 1

 

 

     

j i j i j i j i i L p

p fe m t L t N      

Phase variation

)) ( cos( ) ( 2 ) ( ) (

1 int

t I t I I t I t I

ref tr ref tr erf

      

Refractive index Plasma density * length

Phase variation @ Doppler broadening

23

        d D n



  

          

2 2

) ( ) ( ) ( 1 ) (

Normalized detuning Comparative function:

) /(

2

   m Nfe  

2 2 /

1 ) (

 

  



 e D Absorption coefficient Doppler broadening

2 2 2 /

1 ) (             m Nfe n

) ( / ) ( ) (

) (

t N t N

D p p

     

Results: time dependent fringes

24

=2.0x1011 cm-3 7.8% 3.6 rad

N  N 

=2.4x1011 cm-3 10.8% 5.4 rad

N 

=1.0x1012 cm-3 8.5% 18.3 rad

N 

=1.1x 1012 cm-3 10.6% 24.8rad

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Plasma relaxation

)) ( cos( ) ( 2 ) ( ) (

1 int

t I t I I t I t I

ref tr ref tr erf

      

2

N dt dN   

N dt dN  1  

N   

3

N dt dN   

Pitaevski: Diffusion model 3 body recombination model

• P. Muggli MPP

/

N

t D

e N N

 



t N N N B 

3

1 

t N N e N N

N

t PM

  

 /

1 ) 1 (    



t N N N BP 

2 3

2 1 

Curve fitting for diffusion @ 3body model

26

/

N

t D

e N N

 



t N N N B 

3

1 

Curve fitting for mixed models

27

t N N e N N

N

t PM

  

 /

1 ) 1 (    



           

 

t N e N N

N

t BP D

  

 2 / 3

2 1 1 ) 1 (

Fit courtesy of M. Kedves

28

Interpretation of decay time

Knudsen regime: mean free path ~ characteristic length Intermediate state between molecular flow and viscous flow

Mean free path in Rb vapor:

Rb vapor cell below 120 Co : quasi collisionless flight of atoms: Probe beam channel is filled with neutral atoms out of the channel

N dt dN

N 0

1   

/

N

t

e N N

 



N d L 

2

2 1 

m x d

10

10 5





atomic diameter for Rb 4.5cm at 120 Co and 2x1013 cm-3 20.9cm at 95 Co and 4,3x1012 cm-3 Exponential decay Linear kinetic equation

) 1 ( /

/ L l C

e N N



 

Ratio of collisions on a characteristic length

cm l 1 

 L

 L

05 . /

0 

N NC

2 . /

0 

N NC

at 120 Co at 95 Co

The greater the density The greater the decay time

m T k v

B rms

3 

~ 330 m/s at 95 Co and 340 m/s at 120 Co

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Thank you for your attention