ECH on the MTX (Microwave Tokamak Experiment) Page: 4 of 6
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experiments will focus on multi-pulse heating, radial transport of FEL absorbed power.
and ECH of pellet-fuelled plasmas.
ECH Transinission
The transport of FEL output power to the tokamak is accomplished by six mirror
quasi-optical transrmssion. as shown in Fig. 1. The transmission svstein is window-less
and mirrors are enclosed within a 50 cm diameter evacuated pipe. To avoid the need
of an achromatic jog in the transpc-rt pipe for the e-beam which drives the FEL. two
additional mirrors (Jo and Jt) were added to the original design previously described
3 Mirrors Jo. .\2. and M4 are focussing optics, and the remaining mirrors are flats.
The dominant output mode of the wiggler is TEmt in WR 229 rectangular wavegnde
(5.S2 cm x 2 91 cm) Using the MTH code 4] the overall transnussion efficiency at
140 GHz is e91 Most of the loss is clipping of side-lobe power of the mode at the
first three nurrors For the second phase of experiments using the IMP wiggler the
waveguide mode will be TEi in 3.25 cm circular waveguide. To transport this mode
nurrors Jo and 1t must be modified. The final optic (M14) focusses the microwave beam
into an elliptical cross-section (about 6:1 ellipticity) for transnusion through the narrow
port of NITX (4 cm horizontal x 30 cm vertical x 22 cm duct length.)
39*4'
M2 M3
2053 cm
xatzh:fg 229 cm
, 427 cm
lacer J7 . M4
. ar:-M 39*4'
2C8 cm JO 242 cm
to entrance
7/0 c of narrow
Electron J 165 m duct .tTX
Bean FEL
:fput
Figure I MTX microwave transport. Design for 140 GHz (not to scale).
Non-linear Absorption
Because of the intense electric fields of the FEL beam (Em., 200 to 300 keV/cm),
the absorption is non-linear and reduced from the linear theory value 51. The expected
absorption was modelled by the following multi-step process. 1) the calculated beam
profile at the port entrance (MITH code) is decomposed by Fourier analysis into the
appropriate set of waveguide modes excited by the incident beam, 2) the waveguide
modes are propagated to the end of the 22 cm long duct, taking into account the
differential phase shift between modes, 3) the MTH code calculates the electric field at
the plasma using diffraction theory and the mode amplitudes and phases at the duct exit,
and 4) attenuation of the beam and heating of electrons as they pass through the beam2
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Stallard, B. W.; Byers, J. A.; Hooper, E. B.; Makowski, M. A.; Meassick, S.; Rice, B. W. et al. ECH on the MTX (Microwave Tokamak Experiment), article, April 1, 1989; [Livermore,] California. (https://digital.library.unt.edu/ark:/67531/metadc1110575/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.