PLASMA DIAGNOSTICS
Part 1
VT 2014
Purpose:
To be able to use spectroscopic instrument to identify optical emission lines.
To be able to use the instrument to determine relative intensities of optical emission lines.
To be able to use spectroscopic databases and finding lists.
To determine the content of three plasma discharges.
Equipment:
1.
2.
3.
4.
5.
A 1m Jarrel-Ash Czerny-Turner spectrometer (now: a spectrograph!)
A CCD detector with control program
Discharge lamps
Optical components to image the lamps onto the entrance slit.
Spectroscopic databases and finding lists-
Introduction
There a numerous methods to get information out from a plasma. Since almost all plasmas emit
radiation from atoms and/or ions (why?), one of the most powerful non-intrusive methods is optical
emission spectroscopy. This exercise will show you some of the basic techniques.
Operation of the spectrograph + detector:
The spectrometer is originally a traditional Czerny-Turner plane-grating spectrograph, recently
converted from photographic detection to a CCD-spectrograph simply by replacing the old
photographic plate-holder with a CCD-detector.
The wavelength selection is performed by turning the grating with an electrical motor, or by a
manual handle. A 4-digit counter displays the approximate center wavelength. The motor is
connected to the grating turning mechanism by a gear-box with selectable gear ratio. Gear is
changed by pulling the inner wheel outwards and selecting the desired gear ratio. Try it!
The entrance slit (and also the exit slit, if used) is controlled by a micrometer screw in the front of the
instrument. Ideally, the recorded intensity would go to zero when the slits are closed, but due to
damage or due particles present on the slit wedges he slits seldom close completely. The slit width is
not very important in this exercise, but too wide a slit may cause neighboring spectral lined to blend
together, and a too narrow slit may prevent you from observing weak lines.
A thing to remember is that the recorded intensity is not a linear function of the slit width in general,
so when comparing intensities the slit width has to be left unchanged between exposures. The
recorded intensity is however a fairly linear function of exposure time, so when comparing very
strong line to week ones, the exposure time is the parameter to change. We think that our shutter if
fairly reliable down to opening tomes of 10-20 ms, but you may come to another conclusion!
Start-up procedure:
1: Turn on the computer and log in. The user name is lab and password ora (“Ora et labora!”)
2: Turn on the white box (shutter control) on top of the spectrograph. You may also turn on a little
cooling water for the detector.
3: Start the AndorMCD-program. Go to Files->Configuration files -> load and load the lab_2014-file.
4:Hardware -> cooler -> on. Set the temperature to -20 deg
5:Setup acquisition:
a. Select imaging
b. Select shutter time (> 0,01s)
c. Select Single scan
All other options should be set automatically if the proper configuration file is loaded.
Now is the time to make an exposure to make sure that everything is OK. You should hear a “click”
from the mechanical shutter.
Now is a convenient time to set up the optics: Position one of the lamps in a suitable place. Use the
lens and the lens formula to calculate the proper positions for imaging the lamp onto the entrance
slit. Try to find a line by moving the spectrograph setting.
As opposed to the photographic detection of past times the CCD detector has a very linear response,
provided you do not approach the full-well capacity of the pixels. This corresponds to a converted
digital number of about 60000, so you have to make sure at all times that the pixels are not
saturated. The best way to check this is first make recordings in image mode and check the maximum
intensity by moving the cursor to the most exposed parts of the image, i.e. that the maximum is
below say 50000.
Later, it is convenient to switch to another mode of recording (single track) in order to facilitate the
reduction of the spectrum to a 1-dimensional vector of intensities as a function of wavelength.
Switching to 1-dimensional mode:
6.Setup acquisition:
a. Select single track -> setup
b. Set center track ~512 (middle poition)
c. Set track height 100-400 (max is 511)
Close the image window and make an exposure. You will see window containing a 1-dimensional
spectrum.
To save data, use the EXPORT function. Export as ASCIIXY and load it into a program of you choice
(MATLAB, Excel, python,….) for further processing:
Intensity determination:
Strictly, the intensity of a spectral line is determined by the integrated signal over the line profile. If
the line profile is solely determined by the instrument profile, and thus the same for all emission lines,
the peak value may be used instead, assuming that the width is unaffected by the intensity. You decide
what measure to use.
To measure the intensity, determine the peak position and width, subtract background and integrate
over the profile. You should ideally use the same peak width for all the lines you are recording for a
proper comparison of intensities.
The instrument has a response curve (efficiency vs wavelength), which has previously been recorded
using a calibrated continous lamp. You will find copies of the calibration curve on the desk. Use it to
bring all your measured intensities on the same relative scale.
Results
Your report should describe what you did, how you did it, especially the intensity determination.
Include a list of the lines used for each of the three discharge lamps, together with the measured
intensities and intensities from the literature. Explain why the may differ.
Where any contaminants present in the discharges? If so, which?
Of course, you will have to tell what you think is the main element content of the three discharge
lamps!
Theory
For the equipment we use a 1-meter Czerny-Turner spectrometer. This uses a grating to disperse
the light. The grating equation is:
nλ = l = d(sinα + sinβ)
(1)
where n = order number, λ = wavelength, l = the path difference between contributions from
adjacent grooves, d = groove spacing for the grating, α = angle of incidence and β = angle of
diffraction. The grating equation is the condition that contibutions from all rulings interfere
constructively. Differentiating the grating equation leads to an equation for the dispersion (i.e. the
number of ˚A per mm the spectrum is spread out over). This is:
(2)
where R is the radius of the focusing mirror. The grating is illustrated in Figure 1.
Figure 1: Reflection grating with different diffraction orders. Figure from
http://www.andor.com/library/spectrographs/?app=339.
For a 1200 lines/mm (i.e. d = 1/1200) grating with a 1-meter radius and small angles the
dispersion is around 8 ˚A/mm. This number can be used to roughly calibrate the spectra. The CCD
chip has 1024 pixels in the direction of the spectrometer dispersion and the pixel size is around
24.4 μm (25 mm/1024, since the CCD is around 25 mm). Hence one spectral recording covers
about 200 ˚A (8x25 ˚A). These figures depend on the actual wavelength region under study as the
equation for the dispersion contains a cosβ.
Figure 3: A diagram of a Czerny-Turner monochromator. See text for more information.
In the common Czerny-Turner design, see Figure 3, the broad band illumination source (A) is aimed
at an entrance slit (B). The slit is placed at the effective focus of a curved mirror (the collimator C,
usually a spherical mirror) so that the light from the slit reflected from the mirror is collimated
(focused at infinity, i.e. is parallel). The collimated light is diffracted from the grating (D) and then
collected by another mirror (E) which refocuses the light, now dispersed, on the exit slit (F). At the
exit slit, the colors of the light are spread out (in the visible this shows the colors of the rainbow).
Because each color arrives at a separate point in the exit slit plane, there are a series of images of
the entrance slit focused on the plane. Because the entrance slit is finite in width, parts of nearby
images overlap. The light leaving the exit slit (G) contains the entire image of the entrance slit of
the selected color plus parts of the entrance slit images of nearby colors. A rotation of the grating
causes the band of colors to move relative to the exit slit, so that the desired entrance slit image is
centered on the exit slit.
Charge Coupled Devices, CCDs are multichannel silicon array detectors designed using standard
Metal Oxide Semiconductor (MOS) architecture. Each pixel in the CCD has three electrode gates of
varied potential, see Figure 4. When appropriate voltage potentials are applied to the different
electrodes, the electrostatic potentials in the electrodes produce zones of negative potentials
surrounding potential wells. Photoelectrons generated by the incident light are then collected and
stored in these potential wells.
Figure 4: The basic layout of a three-phase two-dimensional CCD. The sequence 1,2,3 on each
set of electrodes indicates the normal direction of charge transfer in the parallel and serial
registers. Figure from Mackay, 1986 in ARA&A 24.
The CCD chip is around 25 mm and has 1024x1034 pixels. For most of the spectral lines to be
studied, the CCD chip has to be cooled to low temperature to decrease thermal noise. This is done
internally and the temperature is set in the computer program (around -20 deg).