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Straight Corrugated Waveguides

available in individual lengths up to seven feet cutaways of various corrugated waveguides APPLICATIONS Industrial processing systems using millimeter waves Electron cyclotron heating of fusion plasmas Plasma diagnostics: Narrowband: Alpha-particle scattering Wideband: Electron cyclotron emission Submillimeter laser cavities Antenna feeds FEATURES Extremely low loss (as low as 1% per 100 meters) Extremely large bandwidths in the HE11 mode (up to 8:1 or more) High power transmission (up to 1 MW in air; over 100 MW in vacuum) Waveguides are available in individual lengths up to 7 feet Waveguides are available in diameters from 31.75 mm to 88.9 mm Thick wall tubing allows for stable alignment Helicoflex® vacuum seals can be located between waveguides Waveguides are relatively light weight even in the largest size Can be used with quasi-optical equipment such as miter bends

STRAIGHT CORRUGATED WAVEGUIDES MADE AT GENERAL ATOMICS

diameter  (mm)	operating   frequency  (GHz)	mode	operating  power (MW)	total   fabricated  length (m)	application
88.90	80 - 170	HE11	0.5 - 1.0	265	ECH*
87.00	140	HE11	0.5 - 1.0	samples	ECH
63.50	82.6, 118	HE11	0.6	330	ECH (vacuum)
63.50	110	HE11	0.4	24	ECH (vacuum)
63.50	60	HE11	low	50	alpha-scattering diagnostic
63.50	75 - 300+	HE11	low	50	ECE† diagnostics
63.50	50 - 220	HE11	low	260	ECE diagnostics
63.50	28	HE11	0.2	10	ECH
44.45	11 - 12	TE01	100	9	accelerator waveguide bends (vacuum)
44.45	140	HE11	low	2	interferometer diagnostic
31.75	60	TE02	0.2	15	ECH waveguide bends
31.75	50 - 70	HE11	low	44	alpha-scattering diagnostic
31.75	110, 170	HE11	0.5 - 1.0	270	ECH (vacuum)
31.75	100 - 180	HE11	low	44	alpha-scattering diagnostic
31.75	245	HE11	low	8	far infrared lasers

* ECH: electron cyclotron heating    ^† ECE: electron cyclotron emission

LOW LOSS PROPERTIES OF THE HE11 MODE The waveguide ohmic (wall) loss is proportional to the square of the tangential magnetic field components at the wall. In circular waveguides, these components are Hz and Hphi. The electric and magnetic field components (Ez and Hz) along the waveguide axis become very small at high frequencies or large diameters. Waveguide fields can be viewed as superpositions of plane waves propagating at an angle psi relative to the waveguide axis. Ez and Hz at the center of the waveguide are proportional to sin (psi), which in turn is proportional to the ratio of the wavelength to the diameter of the waveguide.

Ez and Hz in the waveguide

COMPARISON OF HE₁₁ WITH TE₀₁ TRANSMISSION

• Ohmic (wall) loss for HE₁₁ is comparable to that for TE₀₁
• Mode conversion loss at miter bends for HE₁₁ is only about 35% that for TE₀₁
• HE₁₁ continuous curvature bends can be about half as long as for TE₀₁
• HE₁₁ is better seperated from competing modes, so propagation is less sensitive to wall distortions
• The HE₁₁ mode at open-ended corrugated waveguide couples directly to the fundamental free space Gaussian mode with less than 2% loss over wide bandwidths
• The far field radiation pattern from open-ended HE₁₁ waveguide is centrally peaked with low sidelobes. The TE₀₁ pattern is doughnut shaped.
• HE₁₁ can propagate in any polarization, from linear to circular. HE₁₁ linear propagation is nearly ideal, with negligible cross polarization.
• Quasi-optical polarizers and transmission filters can be made with gratings on miter bend mirrors in HE₁₁ transmission lines.

LARGE BANDWIDTH PROPERTIES OF THE HE₁₁ MODE

• The maximum low loss frequency is related to the corrugation width and period.
• The minimum low loss frequency is related to the corrugation depth.
• The upper frequency for guaranteed low loss is determined by the period p of the corrugations. If the wavelength is less than twice the period of the corrugations, Bragg reflections can occur due to the periodicity of the structure.
• In waveguides with larger diameter, E_z and H_z are smaller. Hence low loss can be achieved with a smaller reactance, X_z, from the corrugations. This reactance depends on the ratio of corrugation depth to the wavelength. Hence low loss can be achieved at lower frequency.
• In normal situations, the practical corrugation depth, d, is limited to about the corrugation width, w. For convenience in machining, w/p is typically about 0.7.
• The maximum wavelength is equal to the square root of the quantity, (0.5 times the diameter times the minimum wavelength).

  
  attenuation vs frequency data showing regions of Bragg scattering

• Installation of 21 meters of 63.5 mm corrugated waveguide and miter bends for a 75-350 GHz electron cyclotron emission diagnostic on the DIII-D tokamak at General Atomics increased the signal by a factor of 6 compared to a lens relay transmission previously used.

Contact Bob Olstad for additional information (858 455-4060)