If a ste ad y m ag n etic field (w hose dipole axis ru n s p a ra lle l to th e shock tu b e) can be ap p lied in to th e shock tu b e , th e n a n y d istu rb a n c e to th e ste a d y s ta te field can be sen sed u sin g a seco n d ary coil. T h ese ch an g es
Chapter 4 : The experiments 106
in th e field, a fte r som e calib ratio n , can give a m e a s u re of th e electrical co n d u ctiv ity d is trib u tio n b e h in d a shock front. To achieve h ig h s p a tia l r e s o lu tio n in a la rg e d ia m e te r tu b e , i t is n e c e s s a ry to p la c e th e in d u c tio n coil in sid e th e tu b e itself. T he probe th e n becom es r a t h e r in tr u s iv e (m u ch lik e th e e le c tro s ta tic probes). F u rth e rm o re , a s th e s p a tia l re s o lu tio n is a fu n c tio n o f th e w id th of th e m a g n e tic field, sp a tia l reso lu tio n a n d se n sitiv ity a re lo st in sm all d ia m e te r tu b es.
(See L in et al, 1962).
4. Line broadening due to the Linear Stark effect
S ta rk b ro a d e n in g , due to th e electric fields asso c iated w ith th e p la s m a ions a n d electro n s, p rovides a m eth o d for th e m e a s u re m e n t of ch arg ed p a rtic le d e n s itie s . O ne m a y u se m e a s u re d lin e p ro file s to d e riv e co n d itio n s in a n e m ittin g p la sm a . W h en c o n sid erin g h y d ro g e n a n d h y d ro g en like lin es, th e S ta rk b ro ad en in g AA,S, given by G riem (1964) as
AAS = C (N e, T ) N f 3 , ...(4.5)
m ay be em ployed, w h e re N e is th e electro n n u m b e r d e n sity in (cm*3) a n d C (N e, T) is a coefficient t h a t d ep en d s w eak ly on N e a n d T. V alu es for C(Ne, T) a re ta b u la te d by G riem for a ran g e of electro n d e n sities a n d te m p e ra tu re s for S ta r k b ro ad e n ed h y d ro g en a n d h ydrogenic lin e s. A t th e conditions of in te re s t, th e b ro ad en in g of th e H a , Hß a n d Hy lin es is of th e o rd e r of 20 n m , w hich can be e a sily reso lv ed u s in g e ith e r la s e r ab so rp tio n spectroscopic tec h n iq u e s or em ission spectroscopy.
5. Microwave transmission and reflection techniques
M icrow ave te c h n iq u e s em ploy th e w ay in w hich m icrow aves in te r a c t w ith io n ised gases. T he m icrow aves p a ssin g th ro u g h a n ionised m ed ia
Chapter 4 : The experiments 107
a re a tt e n u a t e d by a fa c to r a ; a b e in g a fu n c tio n of th e e le c tric a l co n d u ctiv ity a n d th e d ielec tric coefficient of th e gas. T he te c h n iq u e s su ffe r th e se rio u s d is a d v a n ta g e of e x tre m e ly poor s p a tia l re s o lu tio n , e sp ecially w ith in th e re la x a tio n zone in sm a ll d ia m e te r shock tu b es. A n o th e r se v e re lim ita tio n a r is e s w h e n one c o n sid e rs th e p la s m a freq u en cy cop. A n electro m ag n etic w ave w hose freq u en cy co is su ch t h a t co < cop, c a n n o t p ro p a g a te in to th e p la sm a . cop th e n , r e p r e s e n ts th e cu t-o ff freq u en cy for e le ctro m ag n e tic w ave p ro p ag a tio n . F o r electro n d e n sitie s g re a te r th a n a p p ro x im ate ly 1013 cm*3 (eq u iv a le n t to a p la sm a fre q u e n c y cop=1.98 x 1 0 11 H z), m icro w av es of w a v e le n g th X=0.01 m (co =1.88 x 1011 Hz) fail to p ro p ag a te due to th e ir ra p id a tte n u a tio n . T his lim its th e tech n iq u e to th e low ion a n d electro n n u m b e r d e n sity regim e. (See L in et a l, 1962, L in a n d K ivel, 1959, M a n h eim er-T im n a t a n d Low,
1959 a n d Gorelov, 1983).