CHARACTERISTICS OF A HIGH DENSITY DIRECTED PLASMA SOURCE

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Submitted on 1 Jan 1979

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CHARACTERISTICS OF A HIGH DENSITY DIRECTED PLASMA SOURCE

L. Bradley, E. Orham

To cite this version:

L. Bradley, E. Orham. CHARACTERISTICS OF A HIGH DENSITY DIRECTED PLASMA SOURCE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-307-C7-308.

�10.1051/jphyscol:19797151�. �jpa-00219126�

JOURNAL DE PHYSIQUE CoZloque C7, suppZ6ment au n07, Tome 40, ~ u i Z Z e t 1979, page C7- 307

CHARACTERISTICS OF A HIGH DENSITY DIRECTED PLASMA SOURCE

L.P. Bradley and E.L. Orharn.

University o f CaZifornia, Lawrence Livermore h b o r a t o r y , P.O. Box 5508, Livermore, CaZifomia 94550.

We have developed a plasma source t o produce 21 3

a plasma of 10 /cm p a r t i c l e density and project i t with a velocity of 2.4 cm/ps. This device will be used in t h e output s p a t i a l f i l t e r of Nova, a l a s e r f o r fusion, t o project a c r i t i c a l density plasma across the optical beam path and block t a r - get r e t r o r e f l e c t e d l i g h t . The object of t h i s paper i s t o describe the design of t h e source, present a numerical model f o r the plasma and discuss i n d e t a i l t h e experimental characterization of t h e pl asma.

A low inductance p a r a l l e l capacitor bank with UV illuminated spark gaps produces a current r i s i n g a t 5 x 10'' amp/sec and has 1 ns j i t t e r . Two groups of capacitors w i t h i n t h e bank a r e tuned t o optimally burst a wire, thereby producing a plasma, and t o magnetically propel the plasma. The wire is contained within a nozzle and has the electrodes extended in a r a i l gun configuration. In our appl i c a t t o n , t h i s geometry d i r e c t s t h e plasma across t h e beam path and minimizes vapor leakage toward t h e optics. A dump tank c o l l e c t s t h e bulk o f . t h e plasma, and baffles with an orthagonal 0.1 T magneti.~ fi e l d c o l l e c t the plasma debris and protect t h e optics. The pulser, plasma source, and c o l l e c t o r s were r e t r o f i t t e d i n t o a Shiva 20 cm aperture, f/10 s p a t i a l f i l t e r and t h e plasma was characterized. The experimental geometry i s shown i n Figure 1 w i t h expanded d e t a i l of the electrode region shown i n Figure 2.

The plasma velocity was measured with Faraday, cups and streak camera. The plasma divergence was recorded with witness p l a t e s and Faraday cups.

These data with t h e known geometry permit a

determination of t h e plasma density. A s e l f con- s i s t a n t Lagrangian numerical program modeled the pre and post burst behavior of the wire and includ- ed driving c i r c u i t parameters and magnetohydro- dynamic e f f e c t s . I t predicted the evolution of the plasma density and temperature. The code was use- ful f o r design and optimization of the nonlinear c i r c u i t and f o r interpretation of the data.

A p a r t i c u l a r representative configuration used a 125

rn

diameter aluminum wire sublimed by a 100 kA peak current. The measured plasma velocity was 2.4 cm/vs, a value s u f f i c i e n t l y f a s t t o i n j e c t the plasma a f t e r t h e t a r g e t directed l a s e r pulse passed by before the retropulse returned. The plasma density assuming s i n g l e ionization f o r a1 uminum exceeded 1 0 ~ ~ c m - ~ , a conservative value f o r blocking the 1.06 wn wavelength l i g h t . Ex- c e l l e n t agreement was found with these data and code predictions. A probe l a s e r was blocked as expected. The plasma volume i s calculated t o be s u f f i c i e n t l y large t o prevent optical burnthrough by a 10 kJ, 5 ns l a s e r pulse. Using a radiotracer technique, we determined t h a t l e s s than

10 2

10 atoms/cm were deposited on the optics per shot.

,Figure 1 : Plasma Shutter Experimental Configuration

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797151

Figure 2: Plasma Source Geometry

*Work perfomed under the auspices of the U.S.

Department of Energy by the Lawrence Livermore Laboratory under contract number W-7405-ENG-48.