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for strong and efficient beam-wave interaction. Also,sheet-beam interaction circuits are intrinsically overmoded,and fabrication errors or slight misalignmentsof the beam relative to the circuit can cause undesiredmodes to be excited. Thus, two major obstacles thatmust be overcome to achieve the full potential of sheetbeamamplifiers are 1) the formation and transport ofa sufficiently high perveance and uniform beam and2) the design and fabrication of suitable interactioncircuits, both of which must be executed with a highdegree of precision.Most sheet beam research has focused on magnetictransport, utilizing periodic permanent magnets[45], [46]. Such a configuration is relativelylight and compact, and stable beam transport can beachieved over a relatively long length. The key disadvantageof periodic permanent magnet focusing isthat it imposes a minimum beam voltage for stabilitythat is often far higher than desirable from systemsconsiderations. The alternative magnetic focusingscheme, employing a uniform or solenoidal field,can provide significantly higher field strength (andhence stronger focusing) from available permanentmagnets than periodic permanent magnet focusing,but it has usually been avoided because of potentialinstabilities [47].To determine the optimum transport technique for asheet beam, these stability considerations must be combinedwith the beam envelope equation, which determinesthe magnetic field strength required to balancethe outward space-charge and beam temperature forceson the beam. For relatively low-voltage (,20–25 kV)sheet beam devices, a solenoidal field can transport significantlyhigher current densities, thereby achievinghigher output power [48]. The price that must be paidis a somewhat heavier and larger magnet assembly.Whichever approach is used, the transport field mustbe carefully matched to the electron gun.25 cmPermanent GunMagnets (20 kv, 3.5 A)CollectorBeamTunnelPolePiecesEIK Circuit withInput/OutputGuidesInteractionCavitiesFigure 17. Layout of NRL’s 94 GHz sheet-beam extendedinteration klystron (EIK). The permanent magnet producesa uniform 8.5-kG field over the interaction volume.Most sheet-beam circuit configurations that havebeen considered for millimeter-wave amplifiers areeither like EIK, consisting of a number of discrete cavitystructures, or traveling-wave types, with a suitablestructure to slow the wave phase velocity to match theelectron velocity. While these configurations are generallyanalogous to corresponding well-developed pencil-beamstructures, their design and optimization arecomplicated and quite challenging. Unlike the beamtunnel in pencil-beam devices, which is below cut-offfor the operating frequency so that adjacent circuit elementsare isolated, a sheet beam tunnel is wide enoughto propagate the fundamental transverse electric (TE)mode, which can easily be excited and lead to oscillationand/or beam disruption if the midplane symmetryof the tunnel, circuit, and beam is destroyed by misalignment,fabrication errors, etc. Also, as the aspectratio of a sheet beam device increases, it becomes moredifficult to ensure uniformity and proper phasing ofthe beam and the mode field across the entire widthof the device.Recent progress in devising such sheet beamstructures, largely facilitated by more powerful computercodes and computing resources, is promising.These codes are enabling much more realistic designsto be evaluated and, with their optimization routinesto fine-tune parameters, make first-pass fabricationsuccess a reality. As an example, such simulationspredict that a 94-GHz, 20-kV, 3.5-A sheet-beam EIK,which has an interaction circuit ,1.5 cm long, cangenerate a peak output power .10 kW. This is at leasta factor of five larger than a pencil-beam device atthis frequency and operating voltage. A device basedon this simulation is being fabricated (Figure 17), andefforts are underway to scale such devices into theterahertz regime.Compact, High-Power Terahertz SourcesSince demonstrating the first operation of a vacuumelectronic terahertz source based on microfabricatedfolded-waveguide (FWG) technology [49], NorthropGrumman has continued the development of compact,high-power THz sources in support of several DARPATHz initiatives. Similar to classical vacuum electronicdevices, these THz sources consist of a thermionic electronsource, a slow-wave interaction circuit to amplifythe RF wave, and a depressed collector to recoverenergy from the spent beam for efficient source operation.A 10 kG Nd-Fe-B permanent magnet solenoid isused to magnetically confine the electron beam as itpasses through the slow-wave structure. Vacuum electronicsdevices based on FWG regenerative oscillatorcircuits have been successfully fabricated and operatedbetween 0.600–0.675 THz with RF power levelsover 50 mW at duty cycles up to 3% [50]. This is significantlyhigher than the power capability of existingsources operating at these frequencies.48 December 2009Authorized licensed use limited to: US Army Research Laboratory. Downloaded on December 2, 2009 at 13:24 from IEEE Xplore. Restrictions apply.
A FWG slow-wave circuit is a serpentine waveguide,folded back upon itself multiple times, with abeam tunnel through the center of the structure. TheFWG is designed such that the axial velocity of theRF wave in the waveguide can be made to matchthe group velocity of the electrons travelling downthe beam tunnel. In order to achieve efficient couplingof the RF wave to the electron beam at THz frequencies,the cross-sectional dimensions of the FWG are onthe order of tens of microns by hundreds of microns,which must not vary over the entire circuit length ofseveral centimeters.Another requirement is smooth walls to minimizethe waveguide attenuation due to the shallowskin depth at THz frequencies. Realization of circuitsmeeting these requirements has only been enabledby the use of modern microfabrication techniques,like deep reactive-ion etching of silicon coupled withcopper sputter deposition and electroplating. The planarnature of the FWG circuit is naturally amenableto these planar microfabrication processes, enablingbatch processing of multiple circuits on a single wafer.Additionally, FWG circuits do not rely on fragile structuressuch as gratings to achieve amplification, and,therefore, they are structurally more robust. Theyare fabricated from a solid block for better thermalmanagement. Finally, cofabricated tapers at each endof the serpentine waveguide provide a natural andeffective way for coupling power into and out of thecircuit. The FWGs used in the terahertz sources (Figure18) were fabricated in halves (mirror-images) bya two-level silicon deep reactive-ion etching process:One step to create the FWG itself and a second step tocreate the beam tunnel through the FWG structure.Each half was then metalized via copper electroplating,aligned, and bonded together, forming the completedFWG circuit.In addition to the careful design and accuratefabrication of the FWG interaction circuit, the electrostaticgun design, electron beam transport magnetics,and the collector design are crucial in orderto achieve compact, high-power devices that operatewith minimal beam intercept and high device efficiency.Focus and transport of a 4 mA beam throughthe 60 360 mm 2 beam tunnel (60 mm—roughly thediameter of an average human hair) is a technicalchallenge as mechanical offsets, edge-emitted electronsfrom the cathode, and localized transversemagnetic fields on the order of a few tens of Gauss(less than 1% of the overall axial field) can force thebeam off axis, leading to significant beam intercept.Precision fabrication of the electron gun assembly,accurate mechanical alignment of the gun with thebeam tunnel in the FWG circuit, and minimizationof the transverse magnetic fields have lead to a beamtransmission of 78% for a 4.8 mA beam at 10 kV in arecent prototype. Lastly, the design of a single-stage,graphite lens collector was optimized to recover92.4% of the energy from the spent electron beam.The THz source prior to final assembly is shown inFigure 19.After assembly and exhaust, the vacuum electronicsFWG THz source is mounted on the test stand in thebore of the permanent magnet solenoid. Beam focusand transmission are controlled by the alignment ofthe magnet, which is adjusted, relative to the source,with two three-axis translation stages. A cylindricalcopper waveguide serves as an overmoded, externaloutput waveguide to transmit RF output power toFigure 18. Folded-waveguide circuit half fabricated bydeep reactive-ion etching process.Figure 19. The terahertz source prior to final assembly.Frequency (GHz)700680660640620TheoryExperiment6009,000 9,250 9,500 9,750 10,000Beam Voltage (V)Figure 20. Terahertz source operating frequencies.December 2009 49Authorized licensed use limited to: US Army Research Laboratory. Downloaded on December 2, 2009 at 13:24 from IEEE Xplore. Restrictions apply.
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