Fases del aprendizaje autorregulado

Sodium vapour, used as the nonlinear atomic medium for magnetic field induced SFM, was produced in heat-pipe ovens. Heat pipe ovens [87] are devices for producing uniform

buffer gas / vacuum line

window

. *, wick \

neater water cooling

Figure 3.6 : Schematic diagram of a heat-pipe oven.

The oven was made from a material such as stainless steel which did not chemically

react with the atomic vapour. The main oven body contained a supply of the alkali metal and had a fine stainless steel mesh lining which acted as a wick. Ultraviolet transmitting optical windows were mounted on water cooled flanges which protected the sealing O-rings against thermal damage. The central section of the oven was heated by an electrical heating cord (Electrothermal) which was driven by a temperature controller (FGH Controls Ltd., Type 5900-K-2-0-240 Vigilant with TRZ-10-240 triac power controller). The heat-pipe oven temperature was measured with a type K thermocouple (Comark) mounted in the wall of the

oven. The actual sodium vapour temperature was experimentally found to be -30 - 40°C less than the measured temperature on the thermocouple due to temperature gradients in the walls of the heat-pipe oven. A vacuunVgas handling system was connected so that the oven could be evacuated or buffer gas added.

In principle, the heat-pipe oven operated by heating an alkali metal (or other metal) to produce a vapour pressure which equaled the pressure of the added inert buffer gas. When this was achieved, the buffer gas was forced to the water cooled ends of the oven where it acted as a cold barrier to protect the oven windows against metal vapour contamination. The generated

temperatures to restrict the mean free path of the sodium atoms and prevent contamination and damage of the oven windows. However, this still produced a relatively uniform metal vapour zone [14].

The construction and design of the heat-pipe ovens used in this study is detailed in

reference 14. The oven illustrated in Figure 4.3.4 of reference 14 was mainly used for this SFM study. A new short heat-pipe oven, of a similar design to the one illustrated in Figure 4.3.3 (a) of reference 14, was constructed to restrict the length of the sodium vapour zone to the length of the electromagnet poles. This prevented large linear absorption of the single photon resonant laser beam in "dead" regions of the sodium vapour zone which were not subject to the transverse magnetic field.

A different type of sodium oven was designed and constructed for the experiments to attempt to induce the nonlinearity in the atomic vapour by optical pumping (VII) since the

sodium vapour was additionally required to be transversely illuminated, A schematic diagram of the Na oven/cell design is shown in Figure 3.7.

oven box

Y ////////////? \

glass

tube stainless steel quartz window flange

transverse optical access gas / vacuum

line

Figure 3.7 : Schematic diagram of the sodium cell used for the

optical pumping experiments described in Chapter VII.

chemically lined with an alkali vapour resistant barium borate coating (Glassbulbs), Most ordinary glasses such as Pyrex suffer damage from high temperature alkali metal vapours and become opaque. Owing to the fragile nature of the transparent resistant coating, the soda glass was difficult to work without damaging the coating and so air cooled stainless steel flanges mounted with O-rings were used to hold the quartz cell windows. The windows were displaced off-centie so that the longitudinal laser beam could pass close to the cell wall where the transverse laser beam entered the cell. Water cooling of the windows was not required since this cell was generally operated with a high pressure of buffer gas and this prevented sodium contamination of the windows. A gas/vacuum handling line was connected to one of the stainless steel flanges to allow gas to be added and the cell pumped out. The centre section of the cell was heated by an oven which was composed of Eureka whe non-inductively wound on a T shaped glass former which allowed transverse access to the wall of the cell. A type K tliermocouple was used to measure the cell temperature and was connected to a Digitron 3 BOOK temperature controller with the output voltage stepped down by a 240:15 15A tiansformer.

At an absolute temperature T, the sodium density N was given by [11]

N(m'^) = 9.66084x_10 exp [-12423.3/T + 17.3914] (3.1)

and the vapour pressure of the sodium vapour was

P(mbaiO = NT X 1.38x10 . (3.2)

The vapour also contained a proportion of sodium dimer molecules Na2, with the

percentage of dimers present in tlie atomic vapour at a given temperature being ~ 1% [88]. 3 .4 TRANSVERSE MAGNETIC FIELD

A large Newport type A electromagnet was used to produce the symmetry breaking transverse magnetic field. A power supply for this magnet was designed and constiucted. The

with a Hall probe (Scientifica and Cook).

3.5

ÛEEICS

Tliree main types of optical components were used to control the polarisation of the

laser beams: Gian Taylor linear polarisers, a zero order quarter wave plate for X - 590 nm (Vn) and Soleil-Babinet compensators as variable wave plates.

A non-polarisation sensitive beamsplitter cube (Melles Griot 03 BSC 009) was used to collinearly combine the two fundamental beams for the SFM experiments. To achieve higher input powers for SFM, this could be replaced by a knife edge mirror for one of the beams with a resultant slight non-collinearity in the overlap of the two fundamental beams.

A dichroic mirror was often placed at the output end of the sodium heat-pipe oven to separate most of the fundamental visible laser radiation from the generated ultr aviolet beam. The reflectivity of this mirror was -99% at 590 nm and the transmission -70% at 289 nm, the wavelength of the sum frequency beam.

CHAPTER 4

MEASUREMENT OF THE ZEEMAN SPLÏTTTNG