Infrared interferometry

T he u se of a n in te rfe ro m e te r w ith a lig h t source in th e f a r in fra re d allow s th e m e a s u re m e n t of th e p h a se sh ift c au se d by th e p resen c e of free electro n s in th e flow field. T h is tec h n iq u e allow s h ig h ly reso lv ed sp a tia l a n d tem p o ral m e a su re m e n ts, is n o n-invasive a n d th e ch an g e in th e refra ctiv e index is p u rely a fu nction of th e electro n n u m b e r den sity ; th e re b y allow ing s tr a ig h t fo rw ard c o rre latio n b e tw ee n th e frin g e sh ift an d th e electron po p u latio n p e r u n it volum e. (See Zalogin et al, 1980).

7. C 0 2 Laser Absorption (Inverse Bremsstrahlung)

W h en r a d ia tio n p a s s e s th r o u g h a p la s m a , th e r a d ia tio n w ill be ab so rb ed by th e in v e rse B re m s s tra h lu n g (free-free electro n tra n s itio n ) process. T he a m o u n t of a b so rp tio n w ill co rresp o n d to t h a t g iv en by

Chapter 4 : The experiments 108

L a m b e rt's law , t h a t is,

I = /„ e** ...(4.6)

w h ere k is th e sp e c tra l a b so rp tio n coefficient p e r u n it le n g th a n d x is th e p a th le n g th . T he ab so rp tio n coefficient a sso c iated w ith th e in v e rse B rem s S tra h lu n g process is given by (P en n er, 1959)

1.646 x 10 21

a

X

w h ere N e is th e n u m b e r d e n sity of free electrons; X is th e w a v ele n g th of th e in c id e n t r a d ia tio n a n d T is th e te m p e r a tu r e . In th e p r e s e n t in v estig atio n , th e expected v a lu e for th e electron p o p u latio n w as of th e o rd er of 1015 cm '3 a t a te m p e ra tu re of a p p ro x im a te ly 10,000 K. T he c a lc u la te d a b so rp tio n for th e s e d e n s itie s w as of th e o rd e r of 5% (for 10.6 p m la s e r ra d ia tio n ), b u t could in c re a s e d e p e n d in g on th e shock tu b e conditions. T h is p ercen tag e is v ery sm all an d th e d etectio n of such a m in u te ch an g e in la s e r in te n s ity (co n sid erin g t h a t th e m a x im u m voltage o u tp u t by th e in fra re d d e te cto r w as only 200 mV) w as deem ed too in a c c u ra te for th e in v erse B re m s stra h lu n g process to be u sed as th e io n isa tio n d iag n o stic.

C o n sid erin g th e foregoing th e n , th e in fra re d in te rfe ro m e try a n d S ta r k b ro a d e n in g of th e Hp lin e w ere chosen as th e io n is a tio n d iag n o stics. T he design a n d im p le m e n ta tio n of th e s e tec h n iq u e s is to be d e ta ile d in th e following sections.

Chapter 4 : The experiments 109

4.4 Infrared in terferom etry

4.4.1 The experimental arrangement

The infrared interferometric ionisation diagnostic, was chosen for several reasons. It allowed a high degree of spatial and temporal resolution, was non-intrusive and the change in the refractive index was purely a function of the electron number density. The contribution from the heavy particles to the refractive index was negligible when compared with that from the electrons. (Calculations showed the fringe shift due to heavy particles, at the experimental conditions and wavelengths, to be of the order of 1/20 of a fringe. This was not resolvable in the present experiment.) The experimental schematic employed is presented in figure 4.4. An overview of the experimental system is briefly described below. The various components that constitute the system are mentioned, but are discussed in further detail in the following sections.

A Michelson interferometer was constructed about the shock tube exit in order to determine the electron number density profile as the shock front exits the tube. As discussed in section 4.2.4, the interferometric system was constructed as close as possible to the shock tube exit to minimize the effects on the path length due to the presence of the Mach cone. To achieve the highest possible spatial resolution, it was necessary to have the smallest possible beam diameter in the test and reference arms. This was achieved by placing an aperture in the CO2

laser beam in the form of a circular iris; diffraction effects presented no problems. The C 02 laser beam could be seen, and therefore aligned in the optical system, using phosphorescent plates illuminated by an

To T im in g U n it S h o c k T ub e G old M irror P h o to d io d e T im in g S ta tio n P h o to d io d e T im in g S ta tio n

?

The schematic diagram detailing the experimental arrangment for the infrared interferometric experiments

Chapter 4 : The experiments 110

u ltra v io le t lam p . T he b e am d ia m e te r w as su fficien tly sm all acro ss th e shock tu b e ex it to give th e d e sire d s p a tia l re so lu tio n . T he M ichelson in te rfe ro m e te r em ployed w as ty p ical except for th e optics w h ich w ere specific to th e 10.6 pm C 0 2 la s e r rad ia tio n .

T h e i n f r a r e d d e te c to r s w e re d e s ig n e d s p e c ific a lly fo r p u ls e d a p p licatio n s. As such th e ir se n s itiv ity d e crea se d d ra m a tic a lly d u rin g CW a p p lic a tio n s , as th e te m p e r a tu re of th e ir a c tiv e a r e a in c re a s e d ra p id ly d u rin g ex p o su re. To e n s u re o p tim a l s e n s itiv ity d u r in g th e e x p e rim e n ta l r u n , a m e c h a n ic a l s h u t t e r w as p lac ed in th e in f r a r e d b e am (a lth o u g h p rio r to th e e x p e rim e n t th e shock tu b e blocked th e in fra re d b eam in th e te s t a rm as it w as in th e fo rw ard recoil position, it w as s till n e c e ssa ry to s h u tte r th e refe ren c e arm ). T h is s h u tte r w as opened by a trig g e r p u lse received from a photodiode. T he only lig h t allow ed to fall on th is photodiode w as a H e-N e la s e r w hich w as im ag ed onto its active a re a . The a lig n m e n t of th is H e-N e la s e r a n d photodiode w as such t h a t a b eam dum p, a tta c h e d to th e shock tu b e, o b stru c te d th e H e-N e b e am w h en in th e fo rw a rd recoil p o sitio n before th e sh o t, b u t allow ed th e b e am to fall on th e photodiode w h en th e tu b e h a d recoiled a p p ro x im a te ly 2 m m o u t of its to ta l recoil le n g th of 11 m m . T h is trig g e rin g sy ste m w as e s s e n tia l w h e n one co n sid ers th e tim e scales in v o lv e d in th e e x p e rim e n t. A ny m e c h a n ic a l s h u t t e r r e q u ir e s m illis e c o n d s to o p en . T h e sh o ck w av e h o w e v e r m oves in th e m icrosecond tim e scale. H ence th e s h u tte r could n o t be opened in tim e by a n y trig g e r asso ciated w ith th e p ro p ag a tio n of th e shock front. T he recoil how ever, occurs on a su fficien tly slow tim e scale, su ch t h a t th e s h u t t e r m ay be o p en ed b efo re th e shock e x its th e tu b e . T h e fu ll e x p e rim e n ta l sc h e m a tic of th e e le c tro n ic s s y ste m for th e in f r a r e d in te rfe ro m e tric e x p erim en ts is p re s e n te d in figure 4.5.

Main

Positive