Foil Reflectivity for Use in KrF Laser

Sarah Haynes, HB Woodlawn

Plasma Physics Branch, Naval Research Laboratory

John Sethian, Matthew Wolford, Patrick Burns (RSI), Drew Fielding (CTI)

Abstract An electron-beam transmission foil is needed to separate the electron beam generation region (diode) from the pressurized KrF gas. Low UV reflectivity of this foil can lessen or eliminate parasitics and amplified-spontaneous-emission (ASE) effects in the laser cavity. ASE diverts energy otherwise used in stimulated emission or laser energy.  Large amounts of ASE will reduce the efficiency of the laser driver in the fusion power plant beam line.

Measurements1. Specular reflectivity at 248 nm, using a Perkin Elmer

Instruments Lambda 800 UV-VIS Spectrophotometer. 2. Diffuse reflectivity at 254 nm, using an integrating sphere.3. Diffuse and specular reflectivity (full spectral range of a

Hg lamp) over a 62.4º total cone angle. 4. UV (254 nm) reflectivity as it is affected by angle of

incidence.5. UV (254 nm) reflectivity as affected by the direction of the

foil’s grain relative to the path of the beam.

Hg LAMP ORIEL 65160

H8025-254 HAMAMATSU UV POWER METER w/ C8026 DETECTOR HEAD

ANGULAR FOIL REFLECTIVITY MEASUREMENT

‘Hibachi’ Foils

The diode is separated from the KrF gas by a foil stretched across a ribbed ‘Hibachi’. In addition to low reflectivity in the UV, the ideal foil needs high tensile strength, oxidation resistance and electron-beam transparency.

Future ImprovementsA primarily 248nm wavelength light source would yield data more pertinent to use in KrF lasers than the 254nm Hg lamp used.A more complete data set including measurements of each type of foil both used (previously exposed to laser gas) and unused would be useful.

Specular Reflectivity (248nm)

0

10

20

30

40

50

60

70

Monel400

Al 5052 SS 304 Ti Gr. 4 Inconel600

*UsedMonel

*Used SS *UsedInconel

Foil

% Reflectivity at 248nm

Diffuse Reflectivity (Integrating Sphere)

0.0%

0.1%

0.2%

0.3%

0.4%

0.5%

0.6%

Al 5052Inconel 600Ti Grade 4Monel 400

Stainless Steel 304

5-22-06: SS6-9-08: Inconel6-16-08: Monel

5-16-08: Etched Ti

Kapton

Diffuse Reflectivity

62.4 Cone Angle Reflectivity (Beamsplitter)

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Al 5052

5-16-08: Etched Ti6-16-08: Monel

6/9/2008: Inconel

Gr. 4 Ti Kapton

Inconel 600

5-22-06: Stainless Steel

Monel 400

Stainless Steel 304

Reflected off Foil / Beam Split from Light Source

0%

5%

10%

15%

20%

25%

30%

35%

5/23/10

SS

6/10/12Inconel

Al TiMonel6/17/12

Etched Ti (F)Etched Ti (No F)

Kapton

Ni-Coated Inconel (Ni side)

Ni-Coated Inconel

Reflectivity

90450

Angle off grain

Averaged Over All Angles of Incidence

Conclusion

From each method of measurement, Aluminum had the highest reflectivity and Stainless Steel had the lowest reflectivity of the unused foils.