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34 GHz, 45 MW Pulsed Magnicon Oleg A. Nezhevenko*, Michael A. LaPointe*, Vyacheslav P. Yakovlev*, Jay L. Hirshfielcf, Gennady V. Serdobintsevt1i, Gennady I. Kuznetsov11, Boris Z. Persov11, and Alexander Fix* * Omega-P, Inc., New Haven, CT 06511, USA Yale University, New Haven, CT 06520, USA 1 Budker Institute of Nuclear Physics, Novosibirsk, 630090 Russia ^Institute of Applied Physics, Nizhny-Novgorod, 603600 Russia f Abstract. A high efficiency, high power magnicon at 34.272 GHz has been designed and built as a microwave source to develop RF technology for a future multi-TeV electron-positron linear collider. The tube is designed to provide a peak output power of ~45 MW in a 1 microsecond pulse, with a gain of 55 dB, using a 500 kV, 220 A, 1 mm-diameter electron beam. The status of the tube itself as well as the near-term experimental program is presented. INTRODUCTION The motivation for development of a millimeter-wave magnicon for a future normal conducting collider arises from an expectation of exploiting the customarilyassumed scaling (in rough proportion to frequency) of the dark current limit [1] for the maximum accelerating gradient that can be sustained by a copper accelerating structure. The maximum gradient will be constrained below the dark current limit by rf breakdown [1], and further limited in practice by surface fatigue due to pulsed heating that can affect structure lifetime [2]. Further experimental tests to determine the maximum achievable accelerating gradient under a variety of conditions must be carried out before the absolute limits will be known. The magnicon is an RF source, based on circular deflection of an electron beam, whose main features are high power and high efficiency [3,4]. These properties make the tube especially attractive for accelerator applications. Furthermore, since RF cavities in the magnicon are significantly larger than in a klystron at the same operating frequency, magnicons can be designed for higher peak and average power. The first magnicon developed at Budker INP was a fundamental harmonic amplifier at 915 MHz. It operated with an efficiency of 73% using a 300 kV, 12A electron beam [5]. The measured output power was 2.6 MW and the pulse width was 30 jisec. In experimental tests also at Budker INP [6], a second harmonic magnicon amplifier operating at 7.0 GHz achieved an output power of 55 MW in a 1.1 jisec pulse, and a repetition rate of 3 Hz, with a gain of 72 dB and an efficiency of 56%. CP647, Advanced Accelerator Concepts: Tenth Workshop, edited by C. E. Clayton and P. Muggli © 2002 American Institute of Physics 0-7354-0102-0/02/$19.00 433 This device is driven at 3.5 GHz and uses a 430 kV, 230 A beam with an area compression ratio of about 2300:1 [7]. These encouraging results prompted Omega-P, in close collaboration with the Naval Research Laboratory (NRL), to undertake design, construction and evaluation of an 11.424 GHz, 60 MW pulsed magnicon [8]. At present, the tube is conditioned for power levels of up to 15 and 25 MW for 1.0 and 0.2 jisec pulse widths respectively, and already has been used for high power tests of active RF pulse compressors and dielectric-lined accelerating structures [9]. 34 GHZ MAGNICON AMPLIFIER In scaling magnicon amplifiers to higher frequencies (consequently, smaller physical dimensions), a few design problems arise at high power due to the limitations imposed by cathode loading, breakdown field and pulse heating of the cavity walls. The concept of a third harmonic magnicon amplifier is introduced to overcome these limitations [10]. In general, a magnicon (similarly to a klystron) consists of four major components, namely: electron gun, magnet, RF system and beam collector. The resulting design parameters of this amplifier are given in Table 1. Table 1; 34.3 GHz magnicon parameters. Operating frequency, GHz Power, MW Pulse duration, us Repetition rate, Hz Efficiency, % Drive frequency, GHz Drive power, W Gain, dB Beam voltage, kV Beam current, A Beam diameter, mm Magnetic field, deflecting cavities, kG Magnetic field, output cavity, kG 34.272 44-48 1.5 10 41-45 11.424 150 54 500 215 0.8-1.0 13.0 22.5 The complete engineering design [11] of the 34.3 GHz magnicon is presented in Fig. 1. The gun design [12] calls for a cathode current density of 12 A/cm2, and a maximum surface electric field strength of 238 kV/cm on the focus electrode. Beam compression in this gun is only partially electrostatic (500:1). Higher electrostatic compression would lead to a higher electric field at the focus electrode, and would require a magnetic field of about 13 kG at the edge of the pole piece, leading to undesirable saturation in the iron. Thus a magnetic compression of about 2:1 occurs as the beam passes through the hole in the pole-piece and into a ~5 kG field, and a further factor of 3:1 occurs adiabatically as the magnetic field gradually rises to 13 kG. The resulting compression ratio of 3000:1 is comparable to the 2300:1 compression ratio for the 7 GHz magnicon [7,8]. It is found in the 34.3 GHz magnicon gun design that 95% of the current is within a diameter of 0.8-mm [12]. 434 Before assembling the magnicon tube, the gun and beam collector were assembled and been tested up to the design power of 100 MW in jisec pulses at the Yale Beam Physics Lab [12]. Initial conditioning up to ~515 kV was carried out without beam current. To achieve this, a matched load was connected to the primary of the pulse transformer. After cold conditioning, the gun was conditioned and tested hot up to -480 kV and -200 A. To reach 500 kV, small modifications in the modulator are required. The measured beam current is in excellent agreement with the design value, with differences within the measurement error or better than ±2%. This indicates that the critical gun dimensions are within required tolerances and consequently, that the electron optics should be close to the design. FIGURE 1 34.3 GHz magnicon amplifier tube: 1-electron gun, 2-RF system, 3-output waveguide (WR28), 4-WR90 waveguide, 5-superconducting coils, 6-iron yoke, 7- cryostat, 8-beam collector. 435 The RF system consists of seven cavities: one drive (#1) and three gain cavities (#24), two "penultimate" cavities (#5-6), and one output cavity (#7). The shapes and dimensions of the cavities are chosen to avoid monotron self-excitation of axisymmetric modes, and harmonic frequency modes [11]. All cavities of the deflection system are about 1.25 cm long and their diameters are about 3.0 cm. There are four WR90 waveguides built in the body of deflecting system. One of them is for the drive cavity, and the rest are for measurements in cavities #3, 5 and 6. These waveguides are also used for pumping. The length of the output cavity (3.15 cm) and its shape were optimized to achieve maximum efficiency, absence of parasitic oscillations, and acceptable surface electric fields [10]. The diameter of the output cavity is about 1.75 cm. Power extraction is by means of a set of four WR28 waveguides with an azimuthal separation A0=7i/2 that couple to both field polarizations [10]. The RF system is made as a brazed monoblock that allows baking up to 400° C. Each of the four magnicon output is loaded by a waveguide watercooled vacuum load preceded by a directional coupler. A superconducting solenoid (see Fig. 1) provides a magnetic field of 13 kG in the deflection system and 23 kG in the output cavity. The magnet coils consist of three independently driven sections for adjustment of the magnetic field profile. At present, the magnet is at the final stages of adjustment prior to installation. Presently, the magnicon is assembled, cold tested, baked out and is under the operating vacuum of ~5-10~9 Torr. The RF tests are expected to commence immediately after the magnet is installed. A photograph of the assembled magnicon is shown in Fig. 2. Experimental Program Operation of the 34.3 GHz magnicon will allow the establishment of a unique accelerator class test facility, which gives the possibility of providing a wide range of investigations including: a) Research on the breakdown of accelerating structures of different designs. One example could be a 19-cell structure [13] with minimized electric and magnetic field enhancement. The magnicon could test it up to an accelerating gradient of 300 MeV/m. b) Experimentation on metal fatigue caused by pulse heating, and consequently determination of accelerating structure longevity. The use of -2 MW of magnicon power will allow a temperature rise of approximately 500 C° to be reached at the surface of novel designed test cavity. c) Development of high power components to drive the test structures and cavities that were recently started [14]. The list includes the following: output window(s), (probably in an overmoded structure), mode converters 436 for for the output window(s), window(s), phase phase shifters, shifters, power power splitters splitters and and power power combiners, combiners, low-loss low-loss transmission transmission lines, lines, etc. etc. d) d) For For the the most most advanced advanced breakdown breakdown experiments experiments with with traveling-wave traveling-wave accelerating accelerating structures, structures, compression compression of of RF RF pulses pulses isis inevitably inevitably required required and and is is included included in in future future plans. plans. [15]. [15]. FIGURE FIGURE 2. 2. The The assembled assembled 34.3 34.3 GHz GHz magnicon. magnicon. ACKNOWLEDGMENTS ACKNOWLEDGMENTS This This work work was was supported supported by by the the Division Division of of High High Energy Energy Physics, Physics, U.S. U.S. Department Department of Energy. of Energy. 437 REFERENCES 1. Palmer, R. B., in Pulsed RF Sources for Linear Colliders, edited by R. C. Fernow, AIP Conf. Proc. No. 337 (AIP, New York, 1995), pp. 1-15. 2. O.A. Nezhevenko, "On the Limitation of Accelerating Gradient in Linear Colliders Due to the Pulse Heating", PAC97, Vancouver, 1997, p.3013. 3. O.A. Nezhevenko, "Gyrocons and Magnicons: Microwave Generators with Circular Deflection of the Electron Beam," IEEE Trans, Plasma Sc,, vol.22 pp. 765-772, October 1994. 4. O.A. Nezhevenko, "Recent Developments in High-Power Magnicons for Particle Accelerators," Physics of Plasmas, vol.7 pp. 2224-2231, May 2000. 5. M.M. Karliner, E.V. Kozyrev, I.G. Makarov, O.A. Nezhevenko, G.N. Ostreiko, B.Z. Persov, and G.V. 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