Fundamental mode rectangular waveguide system for electron-cyclotron resonant heating (ECRH) for tandem mirror experiment-upgrade (TMX-U) Page: 3 of 5

                                
The gyretsams have tailed in a rather porisdir
Vey ad the emost *st tarn the cte is operated
sel to be esndestai to the failure. The failure
etore mat alte in the breat en the air aide of
the isdos althea we have hd failures s the
vecoss side as well. ith now  eacouragement Use
LLL. Varits Associates agreed ts replace the
siegle-disk beryllia wisda with a doeble-disk
window sing alams with ICI5 (a 1 flerecarbes) as
caoolnt between the two wisdsw. This wide has
served the problem ef having to seed the tube bck
te Varias for repairs. Me have brekee twe of the
ma windwa. always the vida as the air side, and
they were repaired ee-site in less them four hours.
These two wisdomw were broken a the seams tube and
illustrate a preblas that occurred with the sew
wisdaw. the yrotros tended ta pet set
multiple-frequescy oscillations caeoing reduced
efficiency of the gyrotreas and hig refletion in
the wavegaide tranmission system ceased by poor
matches at frequescies ether than 28 Cis. This
problem has bea greatly reduced by modifying the
gyrotros interaction cavity. The tubes we are
presently receiving appear to be single-frequency
sad high-effiiency devices.
Arc Detectors
In an effort to protect the gyrotron window, an
arc detector we installed that looked directly at
the gyrotro window. This device, as delivered to
us. was not satisfactory. It tended to indicate
arcs when they were not present and if the
sensitivity was reduced to the point where it did
not do this, arcs ware not adequately indicated. In
an effort to protect the gyrotron, we devised a
system that used a directional coupler as a sensing
device rather than the light emitted by arcs. We
sensed the forward power in the directional coupler
and if this power dropped by more than 252, it
indicated an arc behind the coupler. We also sensed
the reflected power in the directional coupler and
if this increased by a significant amount, it
indicated an arc beyond the coupler. We installed
one of these directional couplers in each arm of the
eight-arm node converter giving 32 forward and 32
reflected power monitors for the four gyrotrons.
The performance of this system was quite satis-
factory and could be set to sense arcs in the system
with a wide range of sensitivity. It was very fast
and would tend to trip on spurious noise in the
system. We installed a filter in the post-detection
circuits to slow the system down to a 0.5-ms
response that would sufficiently limit the amount of
energy in an arc so that no damage would occur to
the system.
Mode Filter
The mode filter used was designed by Varian
Associates and consisted of a number of disks with a
2.5-in. internal diameter. etal disks and non-
conducting disks were alternated. The nonconducting
disks were made lossy by putting a water column
behind thea. This filter should highly attenuate
noncircular sodas. We used the filter in all of the
systems, however, many tests were run without the
filters and it was difficult if not impossible to
see any contribution to performance by adding the
filters.

tiasihie Sellawe
A flexible bellows. we instatled to prevent
emteesive farces frem being applied to the gyrstres
window. This belaeus, manufactured by CA Tech-
selegies, coeisted ef an electreferm copper better
with a 2.5-im. miser internal diameter and stainlese
steel ester bottom to give strength (Fig. 3). This
device performed very well and gave adequate pre-
taction to the gyretrom as wall e making the stem
mc, less sensitive ts vertical asllesst.
tight-Arm Node Cenerter
The eight-arm made converter has its isput in
2.5-in. circular waveguides and its outputs are
eigt 1942 rectangular wavegides. It is term-
inated in a water load in the 2.5-ia. weveguide.

Fig. 3.
The eight-arm mode converter was designed by
GA Technologies and subsequently modified by LLPL.
A description of this device is included in Brian
Felker's paper entitled "Design and Fabrication of
Circular and Rectangular Components for ECRH on
'Mx-U."2
The major electrical problems encountered in
this device have been by akdown at high-power
levels. Operation was reliable to 100 kW with
frequent breakdowns to 150 kW and unreliable
operation beyond that. This problem was sol-ed by
pressurizing the waveguide with sulfurhexofluoride
allowing us to operate to input powers of about 170
to 180 kW, where breakdowns in other parts of the
system prevented higher power operation.
Other problems that were encountered were mode
problems in the gyrotron interacting with the
eight-arm coupler. The coupler was designed to
operate with the TE02 mode in the 2.5-in. waveguide.

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