Experiments have been conducted on a 15-17 GHz free electron maser (FEM) for producing a 500 MW output pulse with a phase stability appropriate for linear collider applications. The electron beam source was a 1 {mu}s, 800 kV, 5 kA, 6-cm-dia annular electron beam machine called BANSHEE. The beam interacted with the TM{sub 02} and TM{sub 03} mode Raman FEM amplifier in a corrugated cylindrical waveguide where the beam runs close to the interaction device walls to reduce the power density in the fields. This greatly reduced the kinetic energy loss caused by the beam potential depression associated with the space charge which was a significant advantage in comparison with conventional solid beam microwave tubes at the same beam current. The experiment was operated in a single shot mode with a large number of diagnostics to measure power, frequency and energy

Carlsten, B. E.

Earley, L. M.

Fazio, M. V.

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LA-UR- f i  w m Author(s): Submitted to: ANNULAR-BEAM, 17-GHZ FREE-ELECTRON MASER EXPERIMENT L. M. Earley, B. E. Carlsten, M. V. Fazio, C. M. Fortgang, P.  C. Haddock, and W. B. Haynes 1997 Particle Accelerator Conference DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Gmernment nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. Los Alamos N A T I O N A L  L A B O R A T O R Y  MSTRtBtmUN OF THIS DOCUMENT IS UNLIM 4~ 0 Los Atamos National Laboratory, an affirmative actiodequal oppwtunity employer, is operated by the University of California for the U.S. Department of Energy under contract W-7405-ENG-36. By acceptance of this article, the publisher recognizes that the U.S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or to allow others to do so, for US. Government purposes. Los Alamos National Laboratory requests that the publisher identify this article as work performed under the auspices of the U.S. Department of Energy. The Los Alamos National Laboratory strongly supports academic freedom and a researcher's right to publish; as an institution, however, the Laboratory does not endorse the viewpoint of a publication or guarantee its technical correctness. Form 836 (1W)  DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. 1 ANNULAR-BEAM, 17GHZ FREE-ELECTRON MASER EXPERIMENT L. M. Earley, B. E. Carlsten, M. V. Fazio, C. M. Fortgang, P. C. Haddock, W. B. Haynes Los Alamos National Laboratory, Los Alamos, N.M. 87545 Abstract Experiments have been conducted on a 15-17 GHz free electron maser (FEW for producing a 500MW output pulse with a phase stability appropriate for linear collider applications. The electron beam source was a 1 ps, 800 kV, 5 kA, 6-cm-dia annular electron beam machine called BANSHEE. The beam interacted with the T& and m3 mode Raman FEM amplifier in a corrugated cylindrical waveguide where the beam runs close to the interaction device walls to reduce the power density in the fields. This greatly reduced the kinetic energy loss caused by the beam potential depression associated with the space charge which was a significant advantage in comparison with conventional solid beam microwave tubes at the same beam current. The experiment was operated in a single shot mode with a large number of diagnostics to measure power, frequency and energy. 1 INTRODUCTION Future linear colliders require microwave power sources in the 10-30 GHz fiequency range with output powers of at least several hundred megawatts. The klystron has historically been the source of choice for accelerator applications. For output power levels above several hunderd megawatts, new approaches are needed for microwave power generation. One example is a microwave tube based on a large diameter annular electron beam instead of the small diameter solid beam used in a klystron [l]. More power can be transported in an annular beam because the space charge limiting current in an annular beam is higher than in a solid beam of the same voltage and current. Free-electron lasers and FEMs have demonstrated high peak power and extraction efficiencies. An FEL or FEM offers the possibitiy of a way to avoid this fundamental power density limitation by operating .in a higher-order mode in a large microwave structure. In 1992, Conde and Bekefi tested an FEM that. produced 61 MW at 33 Ghz at 27% efficiency [2]. This tube was driven by a 750 kV, 300 A, 30 ns, solid electron beam. FEM phase stability has been examined in detail by Carlsten [3] for an axial interaction FEM with an annular beam operating in the exponential growth regime. Fazio presented a background description of the development of the Los Alamos FEM [4]. 2 EXPERIMENTALSETUP The FEM experimental configuration is shown in Fig. 1. The FEM is run on the BANSHEE pulsed power electron beam machine which produces the 500-800 kV annular electron beam with a stainless steel field-emission cathode. The beam has a nominal 2.8 cm radius with a thickness of 4 mm. The beam drift pipe has a 3.6 cm mean radius. The input microwave pulse to this amplifier is supplied by a surplus military magnetron. The input pulse is adjustable from 0.3 to 3 ps at a peak power of 1OOkW. The magnetron fi-equency can be varied from 15.5 to 17.5 GHz. The magnetron output is coupled fiom WR- 62 rectangular waveguide through a six-way power divider and fed symetrically into the circular waveguide. This waveguide feed was designed to reduce the number of higher order modes which could couple into the FEM circular waveguide input section. The input section is followed by the rippled wall structure with 15 ripple periods. The ripple wavelength is 3.5 cm and a 7% ripple modulation. After the ripple structure is a section of cylindrical waveguide which acts as the beam dump. A pulsed soleniodal magnet operating at 0.5T is used to confine the beam fiom the cathode to 10 cm beyond the end of the rippled structure. The electrons spread after the end of the solenoid and are absorbed on the waveguide walls over an area large enough to avoid damage. A permanent magnet dipole is located another 10 Cin downstream to prevent any electrons from reaching the diagnostics. Two types of diagnostics are located in the circular waveguide after the beam dump section. The directional couplers measure the pulse shape, power and frequency while a calorimeter acts as a waveguide load and energy measuring device. 3 DIAGNOSTICS DESCRIPTION Diagnostics were developed for this experiment to operate in circular waveguide with a diameter of 7.2 cm for fiequencies between 12 and 18 GHz. The energy of each FEM pulse was measured using a five channel circular waveguide calorimeter. Similar calorimeters have been developed [5,6] for previous intense pulse microwave sources. A cross sectional view of the calorimeter is shown in Fig. 2. A novel multilayer absorber for the calorimeter was developed using Contex textile sheets fiom Milliken [SI. An I oj$imized multilayer absorber has an SI, of -2OdB when measured with an HP8510 network analyzer. The Contex sheets are available in a large range of resitivities fiom a few Q/Ci to hundreds of ksz/O. The multilayer absorber was an excellent match over a broad fiequency range since most of the modes of interest had characteristic impedances greater than 300 R due to the large ratio of the diameter of the waveguide to the wavelength. The first several sheets are high resistivity to reduce reflections. The resistivity is then lowered for each sheet. Table 1 shows the resistivities for the sheets in the final design of the calorimeter. Absorber number 172 394 596 11,12 7-10 Resistivity 28 kQ/O 16 W O  8.5 ksz/O 5OOWU 7052IO Table 1. Calorimeter load absorbing sheet resistivities in order fiom the input of the device. The calorimeter consists of a sealed air cell. One lucite window seals the vacuum on the input side and another window seals the air on the back side. A Setra model 239 pressure gauge measures the rise in cell pressure due to the increase in temperature of the absorbing sheets. The sensitivity of the pressure calorimeter was 70 mV/J for a volume of 200 an3. Most of the energy that is deposited on the absorbing sheets is located near the thermistors for the TW2 and TW3 modes. More than 50% of the energy is absorbed by the last two sheets. The last sheet is thermally isolated fiom the rest by a foam insulator. Thermistors are placed on the last sheet only. Four thermistors located every 90" around the circumference of the absorbing sheet, measure the temperature rise of the microwave absorber. The final design of the absorbing load used twelve sheets. The thermistors are connected to a bridge circuit which provides an output voltage proportional to the microwave energy of the FEM. The thermistor calorimeter had a sensitivity of 20 mV/J. The pulse characteristics of the FEM were measured using three circular waveguide loop directional couplers [7]. Each loop directional coupler was optimized for a particular mode either TW2 or TIL&. The measured signals fiom each of the loop couplers were divided into several paths. One path was sent to a crystal detector, another path went through a bandpass filter to a crystal detector and the third path went to a heterodyne fiequency measuring circuit. In this way signals from different modes and frequencies could be measured separately. The beam current was measured at the diode and at three locations along the beam path using Rogowski coils. The magnetron output signal was measured prior to the six way divider input section. Three B-dots measure microwave signals in the circular waveguide before and afier the rippled wall section. All circular waveguide diagnostics were tested and calibrated using a gated 100 W traveling wave tube (TWT). A circular waveguide test bed was constructed for these calibrations. Various modes could be launched in the test bed to measure the diagnostics response. All modes were verified using the liquid crystal sheets. Power coupled fiom the TWT into the circular waveguide was monitored with conventional microwave directional couplers. 4 EXPERIMENTALRESULTS The FEM experiment has been operated in the beam OFF condition to determine a baseline for the diagnostics. Both the calorimeter and loop coupler measure the coupled power fiom the magnetron into the circular waveguide. The coupled power varies with fkequency and mode. The six way coupler can excite both T N 2  and T N 3  modes from 15.5 to 17.5 GHz. Power coupled into the FEM structure varies fiom 2 to 25 kW. The drive mode was measured by removing the calorimeter and allowing energy to radiate out the open waveguide. Liquid crystal sheets with thick film carbon absorbers attached, monitor the radiated mode pattern in the waveguide. In this way we were able to photograph the mode pattern at a given fkequency and measure the signal fiom the loop coupler to determine power. The mode pattern on the liquid crystal sheet displayed the JI Bessel function for a given mode. The response of the carbon absorber on the liquid crystal sheet was power density, which corresponds to E2. Then, by installing the calorimeter we could measure the pulse energy of the magnetron for approximately 100 pulses and determine the peak power. Thus, we were very confident of our mode and power level of the drive signal to the FEM. Initial results fiom this FEM experiment indicate that the amplified signal is concealed by oscillations with a larger generated power. Measurements show oscillations at 11.2 and 16.4 GHz at power level of a few MWs. Our amplifier would need a minimum of 20 dB of gain for the output signal to equal the oscillator peak power. We are currently working to increase the coupled power fkom our magnetron which would increase the amplifier drive. The signals from the calorimeter five channels agree within 20%. The measured pulse energies were less than 1 J and the resolution for the calorimeter was approximately 50 mJ. . 5 CONCLUSION The initial FEM experiments have been completed. The initial results &om the diagnostics are excellent. However, we have not been able to measure the amplification process due to unwanted oscillations. Our plans include reducing the beam current below 1 kA and increasing our coupled drive power to 40 kW. The unwanted oscillations should be reduced to the point that amplification can be measured. In addition we will explore both TW2 and T b  modes to determine mode growth rates. * Figure 2. Cross sectional view of circular waveguide calorimeter. FOAM INSULATION THERMISTOR GAUGE Figure 1. Cross sectional view of the FEM experimental configuration. REFERENCES [l] P. Wilson, “RF Sources for 5-15 TeV Linear Colliders,” presented at 3rd ICFA Workshop on Pulsed RF Sources for Linear Colliders (RF96), April 8-12, 1996, Hyama, Japan. [2] M. E. Conde and G. Bekefi, “Amplification and superradiant emission from a 33.3 Ghz free electron laser with a reversed guide magnetic field,” IEEE Trans. Plasma Sei., 20,240 (1992). [3] B. E. Carlsten, “Enhanced phase stability for a raman free electron laser amplifier in the exponential growth regime,” Physics of PZmm, 2, 10, p.3880 (Oct. 95). [4] M. V. Fazio et. al., “The Development of an Annular-Beam High power Free-Electron Maser for Future Linear Colliders,” Roc. X W I  International Linear Accelerator Conference, pp. [5] L. M. Earley et. al., “Comprehensive approach for diagnosing intense single-pulse microwave sources,” Rev. Sci. Instrum. 57 (9), pp. 2283-93, Sept. 1986. [6] C. B. Wharton, L. M. Earley, and W. P. Ballard, “Calorimetric measurements of single-pulse high- power microwaves in oversized waveguides,” Rev. Sci. Instrum. 57 (9, pp. 855-8, May 1986. [7] L. M. Earley, W. P. Ballard and C. B. Wharton, “New Directional Couplers for Multimode Circular Waveguides Applied to Intense Pulsed Microwave Systems,” IEEE Trans. on Nuclear 1985. [SI H. H Kuhn, “Characterization and Application of Polypyrole-Coated Textiles,” Intrimically Conducting Polymers: An Emerging Technology, Kluwer Academic Publishers, pp. 25-34.1993 228-230, August 1996. Science, Vol. NS-32, NO. 5, pp. 2921-3, Octok I N P U T  SECTION I RIPPLED-WALL STRUCTURE COUPLER BEAM LOAD & DUMP CALORIMETER 6 PORT I N P U T  COUPLER 

Annular-beam, 17 GHz free-electron maser experiment

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Annular-beam, 17 GHz free-electron maser experiment

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