Power and phase monitoring system for the lower hybrid phased array heating system on ATC machine Page: 3 of 20
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tour waveguide phased array slow wave struc-
ture has been constructed to couple microwave energy
into plasma in the ATC Tokaac at Princeton. Theory
has indicated that the coupline of power into the
pla'-a column is a strong function of the !''ed
fourier spectrum at the antenna aperture.1
To uptini:e heating, and to verify theoretical
results, a precision amplitude and phase monitoring
system has been designed and constructed. The
system data output is routed to an I9?! 1800 com-
puter where the fourier spectrum in n1 space is
computed for discrete increments of time during an
RF ,ulse. Computer output data is used to update
the adjustment of transmission line parameters
The RF power source consists of four indcpon-
dent 50 kit klystrons driven from a comr'on exetter
with phase shifters and attenuators utilised in the
low level drive circuits to centre the klvstron
outputs. 502, 3-1/9 inch coax lines are run to the
ATC platform where transitions are made to R -975
waveguide. Triple-stub waveguide tuners are also
provided in the transmission loop to aid matching.
The system contains 8 bi-directional couplers,
one before and one *!Ler the tuner in each line.
Square law back diode detectors provide gross power
in and pcwer out neasurenents at rsch directional
coupler while quadrature detectors after the tuners
are used to measure amplitude and phase of the
forward and reverse voltage waves.
Henodyne quadrature detectors are constructed
from double balanced mixers and 900 hybrids. A
sample of the exciter signal is split and fed in
phase to each mixer thus providing a fixed reference
phase for all measurements.
A waveguide system is calibrated before instal-
latlon and all relative phase lengths from the
directional couplers are measured to a common ref-
erence flange with dowel pins insuring proper
mechanical alignment. A short at the reference
flange establishes a reflection coefficient of
p = -1 for each line.
Corcecrion constants a-e stored in a computer
system so that data is automatically scaled. A test
signal is applied before each R1 pulse which permits
computation of the gair and offset of each data
Data is stored in real time an a 32 channel
analog magnetic drum recorder. '. ?DP-8/1 is used as
a cata controller-buffer to transfer information
from the A/D converter on the drum output into the
IBM 1800 where processing takes pi..ce.
Interactive program cut.rol permits a choice of
observation time and displayed data via a CRT graphic
The end product of the monltorinc sys
gtaphic computer output of forward and rev*
power versus tine for each waveguide in tn
array (Fig. 1), and a series of plots of tI
spectrum of the radiation fields in front of
array in nI space parolee'. to the totoi
magnetic field on ATC). This output is e
assist system tuning (an involved process o
act ing adjustments) and to evaluate tlhory#
Although directional couplers, tuners A
detection equipment are located on the ATC *
to minimir: errors due to carrier frequency
temperature and load changes, a grounding P!
is created because of the stray ohmic heatih
in the vicinity. Considerable caution is ta
eliminate ground loops which would disturb I
mnuss. All detection electronics are en,101
an tFI shielded rack into which enters only
nals and 60 cycle power. Outputs are dl? feo
driven shielded twisted pairs.
RtF Svstem O uts
Batore tuner fmv.ald
Before tuner reverse
After tuner forward I
After tuner reverse 1
After tuner quadratul
The Data Aquisition Services group at I
built a 32 channel analog magnetic dru: trat
recording system (Fig. 2) with a 100 Klz bat
and a 50 ms track length. Its differential
are driven directly by the monitoring system
Since the drum is several hundred feet from
machine platform, the twisted pairs are prof
terminated on both ends. The analog signal
Fi modulated before recording. When read bh
they are demodulated, filtered and digitized
software selectable but maximum rate e: 1 p4
every 10 :s. One 20 ms experiment gererate
10 bit words when digitized at maximu: throl
The interface to the DAS-1800 computer (als
designed and constructed by DAS) cann,- pas
process this much data in-between eve-:s (9
apart) so a PDr-8/I, designated to the cont
data transfer, is used to block out a -:ch
software selectable, segment of data r-.rr a:
time span for transfer to the DAS-180".
A calibration pulse, generated c the
and applied to each signal input, is freshly
ed on the drum each cycle and is used co
gain and offset drift errors during ar- ysi
Since the drum is a relatively r:-:sy si
(50 mV rms noise at 100 KHz 8W), a pre:amma
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Reed, B.W. Power and phase monitoring system for the lower hybrid phased array heating system on ATC machine, article, January 1, 1975; New Jersey. (digital.library.unt.edu/ark:/67531/metadc869483/m1/3/: accessed November 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.