摘要:

RF breakdown is one of the major factors determining performance of high power rf components and rf sources. RF breakdown limits working power and produces irreversible surface damage. The breakdown limit depends on the rf circuit, structure geometry, and rf frequency. It is also a function of the input power, pulse width, and surface electric and magnetic fields. In this paper we discuss multi‐megawatt operation of X‐band rf structures at pulse width on the order of one microsecond. These structures are used in rf systems of high gradient accelerators. Recent experiments at Stanford Linear Accelerator Center (SLAC) have explored the functional dependence of breakdown limit on input power and pulse width. The experimental data covered accelerating structures and waveguides. Another breakdown limit of accelerating structures was associated with high magnetic fields found in waveguide‐to‐structure couplers. To understand and quantify these limits we simulated 3D structures with the electrodynamics code Ansoft HFSS and the Particle‐In‐Cell code MAGIC3D. Results of these simulations together with experimental data will be discussed in this paper. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635116

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Theory and experiments are reported of the use of a waveguide with a special helical corrugation of its inner surface to provide pulse compression of frequency modulated pulses. Such waveguide has some important advantages over a conventional smooth waveguide as a dispersive delay line for compression of frequency modulated microwave pulses. A compressor using a helically corrugated waveguide can provide generation of nanosecond multi‐gigawatt microwave pulses. Results are presented of experiments at kilowatt power levels that are in good agreement with the theory. The principle exploited in this type of pulse compression is well known and used widely in radar applications. It is based on the propagation of a quasi‐monochromatic wave packet which has a slow (on a period scale) frequency modulation through a dispersive medium, for which the wave group velocity is a function of frequency. For the novel microwave pulse compressor the wave dispersion was synthesized by coupling a TE2,1mode and a counter‐rotating TE1,1mode as the near and far from cutoff modes, respectively, on a three fold helical corrugation in the wall of a cylindrical copper waveguide. A frequency swept pulse of 1kW peak power was used as the input source to the helical waveguide compressor. Experimental optimization of the start and the end frequencies as well as the input pulse duration resulted in measured peak output powers of over 10kW and a maximum compression ratio of 10.9 (10.4dB) with an efficiency of 44&percent;. These results including the output pulse shape were in very good agreement with the simulations. The simulations predict that with further optimization of the frequency modulation the power compression ratio and efficiency can be increased to 18.7 and 65&percent; respectively. A frequency modulation similar to that used here can be realized at the falling edge of a microwave pulse produced from a high‐power (∼1 GW) BWO, indicating the potential of this type of compressor for producing multi‐gigawatt output pulses. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635117

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We will review the design of the dual‐mode X‐band rf system proposed for the Next Linear Collider (NLC). Recent experimental data are presented. The system is to produce 400 ns pulses with power levels up to 600 MW. A proof‐of‐principle experiment is being constructed at SLAC. Four 50 MW klystrons will power a fully dual‐moded resonance delay line pulse compression system. Both the transfer line and the delay lines are dual‐moded. The modes carried by the transfer line are controlled by the rf phases of the different klystrons. The modes in the delay lines are controlled by a set of mode converters at the input and the end of each delay line. By manipulating the modes in the transfer line, one can achieve either no pulse compression or a pulse compression ratio of 4. The total output power will be 200 MW for 1.6 microseconds or 600 MW for 400 nanoseconds. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635118

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Principles and preliminary design for a microwave active pulse compressor using an electrically‐controlled ferroelectric switch are presented. The design of an 11.4 GHz, 500 MW pulse compressor with a pulse width of about 40 nsec and a compression ratio of 10 is described. It is planned to test this compressor using the Omega‐P/NRL X‐band magnicon. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635119

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Results are presented from experiments carried out at the NRL X‐band magnicon facility on a two‐channel X‐band active rf pulse compressor that employs plasma switches. Evidence is shown to validate the basic goals of the project, which include: simultaneous firing of plasma switches in both channels of the rf circuit, operation at high power of a quasi‐optical 3‐dB hybrid directional coupler, coherent superposition of rf compressed pulses from both channels, and stable operation of the X‐band magnicon directly into the resonant load presented by the rf pulse compressor. For incident 1.2‐&mgr;sec pulses in the range 0.63–1.35 MW, compressed pulses of 5.7–11.3 MW were obtained in a first experimental campaign, corresponding to power gain ratios of 8.3–9.3. Following improvements in the experimental arrangement, 1.2‐&mgr;sec pulses of 5.0 MW could be applied during a second campaign, yielding compressed pulses up to 53 MW peak power with power gain ratios up to 10.4. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635120

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A novel method to obtain an electromagnetic signal delay is described. It is shown that the positive magnetic and electric coupling between impedance conductors produces an increase in the time delay. It is also shown that the increase in delay time is obtained without additional attenuation. This allows a reduction in electromagnetic losses, by a factor of several times, for a delay time. An approximate analysis of electromagnetic delay lines based on coupled impedance conductors with “spiral” and “meander” patterns allowed obtaining very simple expressions for the wave deceleration factor, wave impedance, and attenuation factor. The results of the analysis are confirmed by the results of measurements. It is shown that a delay line based on counter‐wound radial spirals can be successfully used for compression of electromagnetic pulses. Although the offered delay line was designed to operate with a relatively small signal, the analysis of the “coupling effect,” taking place in this delay line, might be useful in devices for compression of high‐power microwave pulses. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635121

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Electron Cyclotron Resonance Heating (ECRH) is the main heating method for the Wendelstein 7‐X Stellarator (W7‐X), which is the next step device in the stellarator line of IPP and is presently under construction in the Greifswald branch of IPP. The experiment aims at demonstrating the inherent steady state capability of stellarators at reactor relevant plasma parameters. W7‐X (major radius 5.5 m, minor radius 0.55 m) is equipped with a superconducting coil system operating at 3 T for steady state operation and a divertor for 10 MW steady state heat removal. A 10 MW ECRH plant with CW‐capability at 140 GHz is under construction to meet the scientific objectives. The microwave power is generated by 10 gyrotrons with 1 MW each. A European R&D program aiming at the development of a prototype gyrotron for W7‐X has been successfully terminated by fall of 2002. A prototype gyrotron with the same specifications was developed for W7‐X at CPI (USA). Test results and limitations are reported. The distinct microwave beams from each gyrotron are combined and transmitted to the W7‐X Stellarator ports by an open quasi‐optical transmission system with high transmission efficiency, which runs at normal pressure and consists of water cooled imaging mirrors. Cold tests of a full size, uncooled prototype line and the related RF‐diagnostics are presented. The microwave power is launched to the plasma through 10 synthetic diamond barrier windows and in‐vessel quasi‐optical plug‐in launchers, which allow an independent steering of each beam. The commissioning of the ECRH plant is well under way and the status is presented. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635122

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A gyrotron is under development to produce 1 MW CW output at 140 GHz. The design will be briefly reviewed and recent test results will be presented. A power output of 930 kW with pulse length of 5 ms was demonstrated. In that case the pulse length was limited by the capabilities of the existing power supply. Pulse lengths of 700 s at 500 kW were also demonstrated. This power level is determined by the long pulse limitation of the power supply. The paper will discuss more details of the operation including the effect of various parameters on power output and efficiency, calorimetric measurements of how the mm‐wave power is distributed throughout the gyrotron, and measurements of output frequency and vacuum level versus time for long‐pulse operation. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635123

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

This paper examines the RF heating of the contaminants on a diamond window. Both heating of an isolated internal graphite impurity, and of a thin film on the window surface, are discussed. Comparison with recent experiments is given. It is found that heating of isolated internal graphite impurities is unlikely to account for diamond window failure. Upon averaging over the entire window, the thin surface film in general absorbs approximately a fraction of one percent of the total incident power. Intense local heating on the surface contaminant is possible, however. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635124

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Nonlinear gain in a 34 GHz, three‐stage, frequency‐doubling gyro‐traveling wave tube (gyro‐TWT) has been studied experimentally and theoretically. The device consists of a thermionic electron gun, TE01→TE02fundamental gyro‐TWT input section, second harmonic TE03intermediate buncher section, and a second harmonic TE02→TE04complex output circuit. In this study, the complex harmonic transfer characteristics were experimentally measured and compared with calculations based on the assumption that the gyro‐amplifier gain can be described, in the narrowband sense, as a classical frequency doubling circuit. The results show that narrowband intermodulation gain is 6 dB higher than the carrier as predicted in the small signal limit, but as the device reaches saturation the nonlinear products become suppressed with respect to the carrier. Tests on the noise gain characteristics show that output noise consists of second harmonic shot noise spontaneously excited in the output circuits along with product modulation between external noise and the carrier. Good agreement between the experimental results and the calculations is demonstrated. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1635125

出版商:AIP    

年代:1903

数据来源: AIP