Model 248/310G Grazing Incidence Monochromator for the Soft X-ray and Extreme Ultraviolet Scanning or Multichannel detection (interchangeable) Broad Selection of Gratings Variable Angle of Incidence Precision Rowland circle accurate to ± 4 microns Precision continuously variable Slits Slits Equipped with Isolation Valves Optional Integral Filter Slide Rugged Construction, Compact Design, Operates in any Orientation

The most popular instrument for the 1 to 100-nm region is the 1m focal length grazing incidence spectrometer, McPherson Model 248/310. This instrument is packed with features simplifying measurement in the extreme ultraviolet (EUV, XUV) and soft x-ray spectrum. A similar version is available with a 2.2m focal length (Model 247.) Inquire for other focal lengths. The scanning monochromator selects wavelength by moving the exit slit along the Rowland circle, a circle equal to the grating radius.

The Model 248/310 is available as a scanning monochromator instrument, or as a multi channel spectrometer / spectrograph (using microchannel plate intensifier or CCD) and, when configured to operate in reverse, a variable source of XUV and soft x-ray emissions. All versions can be supplied with o-ring sealed vacuum or ultra high vacuum UHV all metal sealed compatibility. Equipped with microchannel plate or CCD this instrument is a powerful diagnostic tool used for fundamental plasma physics research (eg. at fusion / tokamak reactors or in high harmonic generation laser experiments.) With hollow cathode, Penning, or filament x-ray source this is a excellent source of calibration lines for detector, material or optic characterization.

When the Model 248/310G is used as a scanning monochromator best resolution and broadest wavelength coverage are achieved. The low profile exit slit assembly can scan within 50mm of the grating pole (center of the grating). Operating in these close confines, the user can scan spectra as short as 0.98nm using an 88degree angle of incidence.

A wide variety of gratings allow wavelength coverage from about 1nm to 310nm. With the coarse (133.6g/mm) grating calibration and alignment of this instrument can be completed at atmospheric pressure using ultraviolet emission lines (those from a Mercury light source, for example.) Kinematically mounted gratings insure that gratings repeat position when interchanged.

The grating chamber is stainless steel and equipped with a standard 4.5" metal sealed pump port. The grating chamber provides a kinematic grating mount, in vacuum adjustable focus micrometer to correct for slightly different radii of diffraction grating, aperture slide, entrance slit, exit bellows mounting port and flange and an optional filter insertion slide.

With the scanning system Model 789A-3 (36,000 steps /rev.) 0.0004" increments are made in chordal distance. This distance relates directly to wavelength and depends on the grating groove density and angle of incidence. The bilateral entrance slit (adjustable from 5 to 500um) is mounted to the wall of the grating chamber. An precision micrometer adjustment of slit position across the Rowland circle is provided. Ports are provided in the slit assembly for installing a vacuum gauge, air admittance valve and a roughing valve. To improve performance at wavelengths shorter than 30nm thin film metal filters block scatter from central image and longer wavelengths. To operate at the highest energies / lowest wavelengths VYNS type filters can be used.

Specifications

Focal Length	1-meter, Grazing Incidence design Monochromator / Spectrometer
f / No.	44
Grating Size	25 * 20-mm ruled area
Angle between entrance and exit beams	176 to 157-degrees
Wavelength Reproducibility	0.005 nm (with 1200 G/mm grating)
Vacuum	o-ring seal version for 10^-6 torr, optional all metal sealed versions for 10^-9 torr
Focal Plane	40-mm or 25-mm on scanning carriage for full coverage
Wavelength Range	refer to grating of interest for range

Performance of the Model 248/310G with various gratings:

Grating (G/mm)	3600	2400	2160	1800	1200	600	576	300	133.6
Wavelength Range from 1-nm to	10-nm	17-nm	19-nm	23-nm	35-nm	70-nm	73-nm	140-nm	310-nm
Grating Blaze Angle	Holo	Holo	1-deg	Holo	Holo	Holo	1-deg	2-deg	2-deg
	1-deg	2-deg	3-deg		1-deg	1-deg	2-deg
					2-deg 18-min	2-deg
Resolution (nm)	0.006	0.009	0.01	0.012	0.018	0.036	0.038	0.072	0.16
Dispersion (nm/mm)	0.025 to 0.09	0.03 to 0.15	0.03 to 0.15	0.04 to 0.18	0.05 to 0.27	0.08 to 0.34	0.08 to 0.34	0.16 to 0.7	0.4 to 1.6

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Modes of Operation

There are several ways the Grazing Incidence Spectrometer can be configured to best suit an application or experimental requirement. To diagnose transient plasmas the user will likely want (or need) to collect a range of data quickly and possibly with temporal resolution. This results in an instrument equipped with a microchannel plate intensifier with gating capabilities. In contrast, to monitor a narrow region of spectra (e.g. x-ray laser) a CCD can be used for higher resolution direct detection but with much longer time scales. To do reflectometry measurements or calibrations in the soft x-ray and extreme UV the instrument can be set up in reverse, to better facilitate operation with reflectometers and goniometers. At left, the McPherson Model 248/310G confirgured for direct detection of soft x-ray and EUV spectra using a windowless, back illuminated, uncoated CCD detector. Grazing Incidence optics are coupled to a CCD perpendicular to the exit beam of the instrument. Center of CCD is in best focus. The CCD may be scanned along the Rowland circle to position any wavelength at the center of the detector.

To collect intensified spectra in the soft x-ray and extreme UV a microchannel plate may be used on tangent to the Rowland circle. Up to 40-mm of spectra can be collected simultaneously this way. The photocathode materials and the MCP vacuum requirement preclude alignment while the instrument is at atmospheric pressure. The MCP mount however includes a full complement of adjustments, for focus, roll, angle, etc. permitting to optimize spectral interception while under vacuum. The MCP typically used at McPherson is 40 mm wide so several shots need to be taken to cover broad wavelength regions. Data readout is by fiber taper (60%) connected CCD or PDA array. The entire MCP assembly can be scanned from about 2-nm low limit (at MCP's edge) to about 250-nm (at MCP center) where CsI photo cathodes no longer respond.

Calibration and reflectance work can be performed with the Model 248/310G. Essentialy built in reverse, the light source is mounted on the scanning carriage. Tune the monochromator to a specific wavelength and have a good, fixed trajectory exit beam for calibration and reflectometry applications. The light sources used with the monochromator set up this way should be compact. Good choices include the hollow cathode Model 629 and the filament x-ray source, Model 642. The latter source has two equialent output beams that help facilitate absolute measurements.

Historical Fact McPherson designed and built the first commercial grazing incidence spectrometer for Columbia University in 1967. Until then a void between the operating range of crystal and Normal Incidence monochromators existed. The grazing incidence geometry pushed the limits of reflective optics, diffraction gratings and precision machining. A 10.6 meter focal length Grazing Incidence instrument was built for the National Institute of Standards and Technology which (NIST) uses large film plates for operation as a spectrograph. This instrument collects unsurpassed data to this day.