detector output. One fringe corresponds to a phase shift of 6~ 2n. The corresponding average electron density ne = L
1 JL
n e ( x) dx, is0
(3.2. 7) 2 2
when w fwp2(x) >> is imposed. Thus the electron density is a linear function of the phase shift or the number of output fringes when the micro1t1ave frequency satisfies the above condition,
(r};w~e(x))
» 1. The frequency of the mi crmvave interferometer used on the Cal tech tokamak is 60 GHz. and the maximum average electron density is about 7 x 1012 particles per cm3 , which corresponds to an electron plasma frequency of 24 GHz.If it is assumed that the density profile is a parabolic function of dis- tance, the relation between the peak density and the average density is npeak = (3/2)navg· So the peak density corresponding to our case is ap- proximately l x 1013 particles/cm3, 1t1hich gives an approximate electron plasma frequency of 36 GHz. Therefore, the assumption of (w 2 2 /wpe(x)) >> l is a good one even for the peak density.
The fringe counting for the microwave system on the Caltech tokamak has an uncertainty factor of ±l/4 fringe. The source of the uncertainty comes from the noise superimposed on the interference signal from the de- tector. The origin of the noise is not completely understood. Some of it may be due to actual fluctuation in the plasma density. By carefully
matching the fringes for the initial density buildup with the decay fringes, the time dependence of the plasma density can be determined fairly well.
e. Langmuir Probe Measurement
The conditions at the edge of the plasma are mild enough that Langmuir probes can be used to measure the local electron density and
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temperature. Data have been taken for the first 5 em into the plasma by R. Kubena [30] without any major probe damage. The results when extrapo 1 a ted agree fairly we 11 \'lith the density me as uremen ts from the microwave interferometer mentioned in Section 3.2c, and the electron temperature data from the plasma resistance measurement depicted in Section 3.2b.
3. 3 Summary of Plasma Parameters
From the diagnostics just described, the Caltech tokamak plasma has the follovJing characteristics:
Toroidal field:
Plasma current:
Line average elec- tron density:
Average electron temperature:
3 to 6 kG (4 kG on center) at R = 30 em and R = 60 em, respectively
15 kA (peak)
12 msec (duration) 7x 1012
to 1.5 x 1012 cm-3 (decays during the first two msec)
50 to 100 eV (assuming Zeff = 1 .5)
where R is the major radius of the torus.
3.4 Digital Data Acquisition System
All experimental data from the Caltech tokamak experiments, such as the signals from various diagnostics, the crystal detecte1:! r.f.
signals, etc., are recorded on a multi channel digital transient recorder which converts the various analog signals into digital data that are stored in its semiconductor memories. Each of the 16 channels of the
transient recorder has a 1024 word memory with 8 bits amplitude resolu- tion per word. Four of the channels have a one-microsecond per word clock rate, so the maxi mum frequency response with four-word reso 1 uti on is about 200 kHz. The rest of the channels have a clock rate of 5 microseconds per word, so the frequency response \'Jith four-word resol u- ti on is about 40 kHz.
The di gi ta 1 output signa 1 s from the transient recorder memories can then be used in several ways. Analog signals can be reconstructed with 0-A converters for continuous display on scope monitors after each plasma shot. The transient recorder can also drive an analog pen plotter, so that hard copies of the signal can be produced. If calculations need to be done with the data, the digital data can be written on magnetic tape for later processing at the Caltech central
computer facility (IBM 370, model 158).
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IV. EXPERIMENTAL SETUP FOR THE R.F. MEASUREMENTS 4.1 Experimental Arrangement for Transmission Measurement
The first step in the study of the magnetosonic cavity modes vias to observe them \vi th a receiving probe located 180° toroi dally from a transmitting antenna (Figure 4. 1).
A simple single-turn transmitting loop antenna made of tungsten vias first used (Figure 4.2). The race track shape antenna had the
dimension of 3.75" x 1". The design of the antenna Has governed by three factors. First, it must fit into a 4"x l"x 6" port. Second, to get good coupling with the plasma, the loop area should be maximized.
Finally, the antenna should be kept a~vay from the center region of the plasma where most of the damage to the antenna will occur. This made the shape long and narrovJ. R.F. signals are carried to the tungsten antenna by parallel copper wires enclosed in a glass-to-stainless steel transition tube. The stainless steel tube provides the mechanical feed- through from the outside into the vacuum chamber. The glass is to give electrical insulation for the antenna from the tokamak. The measured
resistance of the entire antenna structure is about 2 ohms at 10 MHz.
The antenna can be moved radially in and out of the plasma through a vacuum 0-ring seal. All transmission measurements are done with the antenna located no more than 1.25 inches into the vacuum cham-
ber in order to prevent any plasma damage to the antenna. This is the lm"l density region in the tokamak, according to Langmuir probe data,
(n < 5x lOll particles/cm2).
e