USE AS A PARTICLE DENSITY PROBE

In the SHG experiments described in the previous sections the second harmonic is generated only in the volume in which the laser beam and magnetic f i e l d cross. At r e l a t i v l e y low p a r t i c l e d e n s itie s (t h a t is den s itie s well below the f i r s t peak in the r e l a t i o n s h i p between SH power and p a r t i c l e density) the amount of SH generated in such a volume is

proportional to the square of the atomic d en s ity . Thus measurement of the SH power as the laser beam and a lo c a lis e d magnetic f i e l d are moved around would allow a three dimensional mapping of the atomic density to be

performed [ 1 8 ] .

For such a technique to be useful the magnetic f i e l d must be r e s t r i c t e d to a r e l a t i v e l y small f r a c t i o n of the length of the region occupied by the vapour. This was achieved using the permanent magnets described in section 4 . 3 . 2 , but mounting them in stacks tipped by conical polepieces. At the

r e l a t i v e l y low magnetic f i e l d produced at the centre of the polepieces (0.02 T ) , the SHG e f f i c i e n c y was p ro p o rtio n a l to the square of the magnetic f i e l d strength; the s p a t i a l d i s t r i b u t i o n of the SHG e f f i c i e n c y was thus c a lc u la te d to be as shown in f i g u r e 5 . 9 . 1 .

This magnet assembly was set up across the r e l a t i v e l y long heatpipe shown in fig u r e 4 . 4 . 3 ( e ) . The laser beam was focussed into the pipe with a 0.53 m focal length le ns, and the SH was detected as described p re v io u s ly . The peak SHG e f f i c i e n c y ocurred when the thermocouple on the outside

surface of the heatpipe read 330*C; the experiments were performed at 2 7 3 *C. The oven was moved r e l a t i v e to the apparatus, thus removing

problems such as the s p a t i a l l y non-uniform response of the p h o to m u ltip lie r tube which would have had to have been considered i f the laser beam had been moved.

10

5

0

U

0

Figure 5 . 9 . 1 . SHG e f f i c i e n c y as a functio n of p o s itio n along the heatpipe. Position 0 . 0 corresponds to a p o s itio n in the centre of the magnet's pole pie ces.

Even with the e f f o r t s made to minimise the s p a t i a l extent of the magnetic f i e l d , the SHG e f f i c i e n c y was g re a te r than h a lf maximum over 34 mm, so the sample length examined by t h i s technique was la rg e . The re s u lts fo r a scan along the axis of the pipe at a b u f f e r gas pressure of 0 . 0 2 mbar are shown

in f i g u r e 5 . 9 . 2 . The r e s t r i c t i o n of the sodium vapour to the c en tral region of the pipe is e v id e n t, but i t is not possible to determine a d e t a i l e d p a r t i c l e density d i s t r i b u t i o n due to the long sample length. However, i t can be seen th a t the pipe is not operatin g in a tru e heatpipe mode, as two peaks are seen in the SH power, corresponding to peaks in the sodium atom d en s ity .

The pipe was then moved so th a t the magnet was in the centre of the vapour zone. Horizontal and v e r t i c a l movement of the pipe produced the re s u lts shown in f i g u r e 5 . 9 . 3 . Although the SH is generated in a r e l a t i v e l y large length along the beam, the SHG occurs only across the width of the beam.