RF Beam Research Articles

A Raman spectroscopy study of InN

We report on the Raman analysis of InN films grown by molecular beam epitaxy (MBE), RF sputtering and remote-plasma enhanced chemical vapor deposition (RPE-CVD). Varying the excitation wavelength, information on the homogeneity of the InN films and the free carrier concentration is extracted. The use of UV Raman spectroscopy to probe excess nitrogen in the InN films via a Raman band located around 2300 cm −1 is illustrated. RF sputtered InN was found to contain significantly more excess nitrogen than MBE and RPE-CVD grown InN. The use of resonant Raman scattering spectroscopy to contribute information to the current controversy of the InN band gap is proposed and first results are presented. Resonant Raman scattering under 1.49 eV excitation suggests the presence of a critical point in the InN band structure within 200–300 meV of this laser excitation energy. Further investigations will be needed to clarify whether this is the band gap or a higher critical point in the band structure.

On-line SASE FEL gain optimization using COTRI imaging

Overlap of the particle and the photon beams in a self-amplified spontaneous emission (SASE) free-electron laser (FEL) is one of the keys to optimizing gain. We have now directly demonstrated an on-line method for gain improvement at 540 nm on the Advanced Photon Source FEL. This was achieved by steering the e-beam with correctors before an undulator based on the fringe symmetry in coherent optical transition radiation interference (COTRI) images observed after that undulator. For these conditions we determined that both the SASE and COTR image intensities were improved by about a factor of three in one 2.4-m-long undulator section. The initial tuning had been based on RF beam position monitor readings and the maximization of SASE image intensity in the cameras.

A full-DC injector for an energy-recovery linac

We propose a full-DC injector for an energy-recovery linac. The injector is based on a 2 MeV DC accelerator that is commercially available (Dynamitron type), combined with a photo cathode. A bunched beam from the DC accelerator is merged with a recirculating beam and injected into a superconducting buncher-booster, which accelerates the beam up to ∼20 MeV . The buncher-booster is operated in partial energy-recovery mode, in which we can reduce the capacity of the RF generators to 10 kW even for a high-average current operation, 100 mA . We present results of beam dynamics simulation and RF system optimization.

Study of millimeter wave high-power gyrotron for long pulse operation

The development of a high-power millimeter wave source, gyrotron, is under way for fusion application. A performance of ∼10 s oscillation has been attained at 1 MW level output for 170 GHz frequency. Although the pulse extension was interrupted by a sudden outgassing in the gyrotron, the cause was confirmed to be the local heating of the internal component due to stray RF deposition. To suppress the heating, the inner surface of the component (bellows for RF beam steering mirror) was coated with copper, which reduced the Ohmic loss to 1 10 of the original one. Moreover, forced water cooling for the bellows was incorporated. As a result, no sudden pressure increase was observed, and a quasi-steady-state oscillation of 100 s with 0.5 MW power level was demonstrated at 170 GHz. The temperature of the major components of the gyrotron stabilized, which indicates a prospect for a 1 MW-CW, 170 GHz gyrotron.

Development of a high-current microwave ion source for proton linac application systems

A microwave hydrogen ion source was developed to improve reliability, and to increase operation time of proton linac application systems. The ion source needs no filament in the discharge chamber, which leads to better reliability and less maintenance time. The developed source produced a maximum hydrogen ion beam current of 70 mA (high current density of 360 mA/cm2, beam energy of 30 keV) with a 5 mm diam extraction aperture and 1.2 kW microwave power. The proton fraction was increased with an increase in rf power and reached around 90% at 1 kW. Measured 90% beam normalized emittance was 0.4 π mm mrad. Rise times of rf power and beam current to 90% of the final values were about 30 and 35 μs, respectively, at a pulse operation mode with 400 μs pulse width and 100 Hz repetition rate. The dynamic range of beam currents was enlarged (3–63 mA) in the pulse mode with a modified rf wave form to assist ignition of microwave discharge. These performance parameters will be desirable for pulse operation accelerator applications like proton therapy systems. A long time operation stability (150 h) was confirmed with a beam current of 51 mA; change in the current was 2%.

Development of a dedicated ion injector for accelerator-based nanoprobe facilities

The paper discusses possible ways of increasing beam brightness in ion injectors. The argon/helium ion injector comprising a newly designed RF ion source and, a Wien filter has been designed for use in accelerator-based nanoprobe facilities. The phase set degradation due to aberrations in the injector ion-optic system was simulated with allowance for multipole and fringing fields. The RF ion sources with different permanent magnet systems were tested. Experiments were performed with argon and helium. A plasma density of up to 3×10 11 cm −3 and beam brightness of ∼100 A/(m 2 rad 2 eV) were obtained. The ion current density inside an extracting electrode in the source was 10 mA/cm 2 for an emission hole diameter of 0.6 mm. Measurements of the current value and emittance were performed with ion source testing equipment permitting measurements of the ion beam current, emittance, mass composition, and RF power input into the plasma.

High current ion beam RF acceleration and perspectives for an inertial fusion driver

The actual situation with respect to the use of an RF linac driver for heavy ion inertial fusion (HIF) is discussed. At present, there is no high current heavy ion linac under construction. However, in the course of linac projects for e−, p, d, or highly charged ions several developments were made, which may have some impact on the design of a HIF driver. Medium- and low-β superconducting structures suited for pulsed high current beam operation are actually designed and investigated at several laboratories. A superconducting 40 MeV, 125 mA cw linac for deuteron acceleration is designed for the Inertial Fusion Material Irradiation Facility (IFMIF). The Institute for Applied Physics (IAP) is developing a superconducting 350-MHz, 19-cell prototype CH-cavity for β = 0.1. The prototype cavity will be ready for tests in 2004. A superconducting main HIF driver linac would considerably reduce the power losses. Moreover, it would allow for an efficient linac operation at a higher duty factor.The 1.4-AMeV room-temperature High Current Injector HSI at Gesellschaft für Schwerionenforschung (GSI) has been in routine operation for more than 2 years now. With a mass-to-charge ratio of up to 65, a current limit of 15 mA for U4+, and an energy range from 2.2 AkeV up to 1.4 AMeV, this linac is suited to gain useful experience on the way toward the design of a HIF RF driver. The status and technical improvements of that A/q ≤ 65, 91-MV linac are reported. Beam dynamics calculations for Bi1+-beams show that powerful focusing elements at the linac front end are the bottleneck with respect to a further increase in beam current. Besides superconducting and pulsed wire quadrupoles, the potential of the Gabor-plasma lenses is investigated.

Efficiency enhancement in free-electron lasers by means of concurrent rf acceleration.

Efficiency enhancement in free-electron lasers (FELs) using rf beam acceleration in the wiggler is described. Since the beam tube is a waveguide, there are low and high frequency resonances. Injection of low frequency power can act as an inverse-FEL accelerator concurrently with high frequency power extraction. Simulation of a FEL using this technique shows that substantial efficiency enhancements are possible without significant increases in the beam energy spread, which facilitates the use of energy recovery schemes. The technique is applicable to amplifier and oscillator configurations.

On cyclotron emission from toroidal plasmas near the ECR heating frequency

High power electron cyclotron plasma heating in toroidal traps results in the generation of non-Maxwellian electron distribution functions. Electron cyclotron emission (ECE) spectra originating from such distributions in a close vicinity of the heating frequency are investigated analytically based on a simplified kinetic approach. An increase of the ECE level due to the quasi-linear degradation of re-absorption near the heating frequency is shown to be rather sensitive to the non-thermal distortions in the bulk electron distribution function. This effect is of peculiar interest for the diagnostics of quasi-linear effects in the case of low-field-side launch of the heating rf beam when absorbed electron cyclotron resonance (ECR) power is mainly deposited to the low-energetic electron fraction.

Power modulation capabilities of the 140 GHz/1 MW gyrotron for the stellarator Wendelstein 7-X

In current tokamaks and, in particular, in future larger devices such as ITER, the control of neo-classical tearing modes (NTM) is essential for achieving high performance in terms of the β limit. A commonly used scheme for NTM stabilization consists in driving a helical current at the resonance surface of interest with electron-cyclotron-current-drive. Depending on the ratio between the magnetic island size and the RF beam width, complete stabilization of the NTM will only be achieved with deep RF power modulation in phase with the mode. In the frame of the European development program of high power sources for ECRH applications between Forschungszentrum Karlsruhe, IPP Garching/Greifswald, EPFL Lausanne, IPF Stuttgart, CEA Cadarache and Thales Electron Devices, the modulation capabilities of the 140 GHz/1 MW gyrotron have been experimentally investigated. RF-power modulation depths higher than 80% at a frequency of 50 kHz with cathode modulation and 1.5 kHz with depression voltage modulation have been obtained. The limitations in frequency were given by the corresponding power supplies and not by the gyrotron itself. Detailed analysis of the collector loading with respect to the modulation scheme will be presented and the intrinsic gyrotron limitations for long-pulse operation with deep modulation will be discussed.

High J/sub c/ YBCO films on biaxially textured Ni with oxide buffer layers deposited using evaporation and sputtering techniques

Epitaxial buffer layers of CeO/sub 2/ and Yttria-stabilized ZrO/sub 2/ (YSZ) have been deposited on biaxially textured nickel substrates using thermal reactive evaporation, electron beam evaporation and rf sputtering. The buffer layers were characterized by XRD, SEM and optical microscopy. YBCO films were deposited by high-pressure dc sputtering on CeO/sub 2//YSZ/CeO/sub 2/ buffered substrates and the resulting superconducting properties were measured inductively. On exclusively evaporated buffer architectures critical current densities of 0.6 MA/cm/sup 2/ (77 K, H = 0 T) were measured. The buffers showed some cracks after YBCO deposition, which we considered to be responsible for the relatively low J/sub c/. Buffer architectures with evaporated CeO/sub 2/ followed by rf sputtering of YSZ and CeO/sub 2/ in oxygen atmosphere remained completely crack free after YBCO deposition. Critical current densities between 1.5 and 2.3 MA/cm/sup 2/ (77 K, H = 0 T) in 460 nm thick YBCO films were obtained reproducibly. The critical temperatures T/sub c/ ranged between 88 and 92 K and the typical width /spl Delta/T/sub c/ of the transition was 1.5 K.

High power experiments of remote steering launcher for electron cyclotron heating and current drive

High power (0.5 MW) radiation tests of a remote steering launcher for an electron cyclotron heating and current drive (EC H&CD) system were carried out to study the characteristic of the rf beam steering and transmission. The remote steering launcher consists of a 4.632 m corrugated square waveguide (45.7×45.7 mm 2) and a movable flat mirror which is placed at the input of the waveguide. Measurements were made in the range 0–10° at a frequency of 170 GHz. The measured radiation pattern is clearly a Gaussian-like distribution, which is in agreement with the theoretical prediction and the low power measurements. A steering capability of up to 10° was experimentally confirmed. Transmission and radiation of 0.5 MW 3.0 s and 0.2 MW 10.0 s pulses were successfully demonstrated over a steering angle range of 0–10°. Transmission losses in the launcher waveguide over 0–10° were measured calorimetrically to be less than 5%. No arcing was observed during the experiments and no damage to the waveguide was found after the experiments were finished. We have confirmed that remote steering is a possible alternative scheme for EC H&CD beam steering.

Advanced high-power gyrotrons

Gyrotrons at high frequency with high-output power are mainly developed for microwave heating and current drive in plasmas for thermonuclear fusion. For the stellarator Wendelstein 7-X, now under construction at IPP Greifswald, Germany, a 10-MW electron-cyclotron-resonance-heating (ECRH) system is foreseen. A 1-MW 140-GHz gyrotron with synthetic diamond window for continuous wave operation and with a single stage depressed collector for energy recovery and improvement of efficiency has been designed, constructed, and tested in collaboration with CRPP Lausanne and TED Ve/spl acute/lizy. It operates in the TE/sub 28,8/-cavity mode and provides a linearly polarized TEM/sub 0,0/ Gaussian RF beam. In short pulse operation at the design current of 40 A, an output power of 1-MW could be achieved for an accelerating voltage of 82 kV without depression voltage, an output power of 1.15 MW at an accelerating voltage of 84 kV with a depression voltage of 25 kV. These values correspond to an efficiency of 49%. After some problems with the RF-load, long-pulse operation was performed. The power measurements were done by the calibrated signal of the diode detector placed at the second mirror. Output powers of 1 MW could be achieved for 10 s, and an energy as high as 90 MJ per pulse has been produced with an output power of 0.64 MW. The pulselengths were mainly determined by the preset values. Only for the 100-s pulse at 0.74 MW, a limitation was found due to a pressure increase beyond about 10/sup -7/ hPa. The ITER task (task for the future international thermonuclear experimental reactor) on development of coaxial cavity gyrotrons ended in 2001. In accordance with the goal of the task, the potential of coaxial gyrotrons has been investigated and, as a result, data necessary for an industrial realization of a 2-MW CW 170-GHz tube have been obtained. In addition, first work on tube integration has been done. The results will be presented and discussed. By biasing the coaxial insert a fast (within 0.1 ms) frequency tuning has been demonstrated. In particular, a fast step tuning between the 165-GHz nominal mode and the azimuthal neighbors at 162.5 and 167.2 GHz have been performed. In addition, at the nominal mode a continuous frequency variation within the bandwidth of up to 70 MHz has been done.

Double beam RF MEMS switches for wireless applications

A double cantilever beam MEMS switch has been developed for wireless applications. The two beams are controlled by a single actuation electrode. This switch has low pull-in voltage of 15 V and fast switching speed of 60 /spl mu/s. An RF performance with >-50 dB isolation below 5 GHz and <-0.18 dB up to 30 GHz was observed.

A 1.2-MeV Two-Section Continuous Wave Linear Electron Accelerator with 50-mA Average Beam Current

A 1.2-MeV compact two-section continuous wave linear electron accelerator with an average beam current of 50 mA and beam power of 60 kW is described. A simulation of the beam dynamics, accelerating structure, and RF system was carried out. Measured beam parameters are close to the design objectives: 1.15 MeV, 44 mA, and 50.6 kW.

Control and RF-transmission in the ECW system on TEXTOR-94

A real-time and multitasking control system has been developed for the new ECW system on the TEXTOR tokamak. It allows the system to be remotely controlled by client/server application. A quasi-optical transmission line has been installed which uses confocal mirrors and can be used for different frequencies (>100 GHz). It is suitable for transmission of up to two RF beams from different sources to the plasma. The launcher is mounted in a main horizontal port and injects a focused beam with a spot size of 2 cm (at 110 GHz) near the plasma axis. The launcher is steerable independently in the toroidal and poloidal directions.

Performance of the 1 MW, 110 GHz, 10 s gyrotrons recently installed in the DIII–D ECH system

ECH power has proven capabilities for both heating and current drive in energetic plasmas. For the second phase of ECH power on DIII–D, there will be three 1 MW sources added to the existing three systems for a total power generating capacity of 5.1 MW. This upgrade is based on the use of the single disc chemical vapor deposition (CVD) diamond window, 1 MW diode gyrotron, recently developed by CPI. All gyrotrons are connected to the tokamak via a low-loss, windowless, evacuated transmission line system, using circular corrugated waveguide for propagation in the HE 11 mode. Each waveguide system incorporates an in-vessel two-mirror launcher. The newest launcher can steer the rf beam poloidally from the center to the outer edge of the plasma and toroidally for either co- or counter-current drive. An overview of the total system, its integration with the DIII–D tokamak, and recent results will be discussed. The various new aspects of the upgrade ranging from building modifications to the use of the new steerable launcher will also be addressed.

A Regulated Power Supply for the Filaments of A High Power Gyrotron

As part of the Electron Cyclotron Heating (ECH) Facility upgrade at the DIII–D National Fusion Facility, a regulated AC power supply for the filaments of a 110 GHz Gyrotron was designed, tested and integrated into the new ECH power system.1 A precision regulated filament power supply is essential for maintaining the stability of the cathode temperature and consequentially the gyrotron beam current and rf production.2 This paper provides a description of the design of the filament power supply, presentation of test results, and an assessment of its performance.

Design status of EC system in ITER — design of reliable RF beam launching system

The design of two types of EC-launchers which fulfil many functions requested to EC system, is described. One with toroidal RF beam steering is mainly used for main plasma heating and current drive. The other with poloidal RF beam steering is mainly used for neoclassical tearing mode stabilization. The RF beam is deflected by a steerable mirror. In this design, one mirror, steerable in a single direction, is assigned to one row of waveguides. A reliable RF beam launching system can be constructed by using a spiral pipe for cooling water and a simple push/pull shaft for mirror rotation without a cam and link. The design includes a mirror composed of a high melting temperature material plate and a SS pipe.

Initial results of electron cyclotron range of frequency (ECRF) operation and experiments in JT-60U

The 110 GHz 1 MW electron cyclotron range of frequency (ECRF) system was designed and constructed on JT-60U to locally heat and control the plasmas. The gyrotron has a diamond window to transmit RF power with Gaussian mode, which is easily transformed to HE 11 mode for the transmission line of the corrugated waveguide. The second diamond window is installed at the inlet of the antenna for a vacuum seal between the transmission line and the JT-60U tokamak. The total length of the transmission line from the gyrotron to the antenna is about 60 m including nine meter bends, The antenna has a focusing mirror and a flat steerable one to focus and to control the RF beam angle mainly in the poloidal direction. In the initial operation, the power of P EC∼0.75 MW for 2 s was successfully injected into plasma when the gyrotron generated the power up to 1 MW. The total transmission efficiency from the gyrotron to the plasma was about 75%. A controllability of local electron heating with the deposition width of =15 cm was well demonstrated by using the steerable mirror. A large downshift in the deposition position was observed at the high T e plasma. Strong central electron heating was obtained from 2.2 to 6.6 keV for P EC∼0.75 MW, 0.3 s at the optimized polarization. An effective electron heating was also obtained up to ∼10 keV during EC injection for ∼1.6 s in the high β p H-mode plasma produced by NBI.