mW CW Research Articles

Recent Tests on 117.5 GHz and 170 GHz Gyrotrons

Two megawatt-class gyrotrons at frequencies of 117.5 GHz and 170 GHz have recently been fabricated and tested at CPI. The 117.5 GHz gyrotron was designed to produce up to 1.8 MW for 10-second pulses, and will be used for electron cyclotron heating and current drive on the DIII-D tokamak at General Atomics. The 170 GHz gyrotron is specified as a 500 kW CW system, but has been designed with the goal of generating up to 1 MW CW. Oak Ridge National Laboratory will use the gyrotron in ITER ECH transmission line testing.

Remote-Steering Launchers for the ECRH system on the Stellarator W7-X

For electron cyclotron resonance heating of the stellarator W7-X at IPP Greifswald, a 140 GHz/10 MW cw millimeter wave system is in construction. Two out of 12 launchers will employ a remote-steering design. This paper describes the overall design of the two launchers, and design issues like input coupling structures, manufacturing of corrugated waveguides, optimization of the steering range, integration of vacuum windows, mitrebends and vacuum valves into the launchers, as well as tests of prototype parts.

Damage-free single-mode transmission of deep-UV light in hollow-core PCF.

Transmission of UV light with high beam quality and pointing stability is desirable for many experiments in atomic, molecular and optical physics. In particular, laser cooling and coherent manipulation of trapped ions with transitions in the UV require stable, single-mode light delivery. Transmitting even ~2 mW CW light at 280 nm through silica solid-core fibers has previously been found to cause transmission degradation after just a few hours due to optical damage. We show that photonic crystal fiber of the kagomé type can be used for effectively single-mode transmission with acceptable loss and bending sensitivity. No transmission degradation was observed even after >100 hours of operation with 15 mW CW input power. In addition it is shown that implementation of the fiber in a trapped ion experiment increases the coherence time of the internal state transfer due to an increase in beam pointing stability.

Diode-End-Pumped Midinfrared Multiwavelength Er:Pr:GGG Laser

We report a diode-end-pumped midinfrared multiwavelength Er:Pr:GGG laser, generating the output powers of 324 and 354 mW in CW and pulsed modes, respectively. The slope efficiencies of CW and pulsed modes are 15.18% and 16.06%, respectively. Compared with Er:GGG, the laser properties of Er:Pr:GGG are much better and the self-termination bottleneck is broken successfully. The transverse beam profile and spectra of Er:Pr:GGG laser are measured and the relationship between lasing wavelengths and operating temperature in the triwavelength emission state are also studied. The Er:Pr:GGG crystal is a potential midinfrared laser gain medium in medical and military application.

Mid-infrared fiber lasers at and beyond 35 μm using dual-wavelength pumping

We report the first, to the best of our knowledge, erbium-doped zirconium-fluoride-based glass fiber laser operating well beyond 3 μm with significant power. This fiber laser achieved 260 mW in CW at room temperature. The use of two different wavelength pump sources allows us to take advantage of the long-lived excited states that would normally cause a bottleneck, and this enables maximum incident optical-to-optical efficiency of 16% with respect to the total incident pump power. Both output power and efficiency are an order of magnitude improvement over similar lasers demonstrated previously. The fiber laser operating at 3.604 μm also exhibited the longest wavelength of operation obtained to date for a room temperature, nonsupercontinuum fiber laser.

Development of Advanced Gyrotrons in Europe

The German PMW project and the European EGYC consortium are driving gyrotron developments for the two major plasma fusion experiments in Europe using ECRH, Wendelstein W7-X at Greifswald and the international ITER at Cadarache. 1 MW CW, 140 GHz conventional cavity gyrotrons have been developed and are being delivered to W7-X whereas advanced 2 MW CW, 170 GHz coaxial-cavity gyrotron technology has been tested for ITER. Additionally, an 1 MW, 170 GHz conventional cavity design is under development for ITER. Furthermore, research work on gyrotron concepts for future fusion experiments, focusing on frequency-step tunable gyrotrons and multi-MW coaxial-type gyrotrons is ongoing at Europe, in particular at KIT. This paper is reporting some of the important results and the ongoing research work.

The Performance Test and the Improved Design of 28 GHz 1 MW Gyrotron

For the GAMMA10 tandem mirror, a 28 GHz 1 MW gyrotron has been developed. The output power was achieved 1 MW in short pulse test, and 0.54 MW / 2 s in long pulse test. Moreover, the temperature rise of sapphire single disk window installed in 28 GHz 1 MW gyrotron was measured. The temperature rise was 9 K at 0.45 MW / 2 s. On the basis of the results of the performance test, the designs of 28 GHz 1 MW gyrotron have been improved to enable the operation of 1.5 ~ 2.0 MW several sec. required in GAMMA10 and NSTX, and 0.4 MW CW required in QUEST.

400 mW low noise continuous-wave single-frequency Er,Yb:YAl_3(BO_3)_4 laser at 155 μm

We build up a LD end-pumped Er,Yb:YAB laser at 1.55 μm and improve the laser performance by end cooling the gain medium efficiently through a sapphire plate. 680 mW cw single transverse mode laser output was obtained with the slope efficiency of 16.3%. Using an etalon placed in the laser cavity, 400 mW cw single frequency 1.55 μm laser output was achieved with the slope efficiency of 11.8%. The laser power fluctuation was less than ± 1.3% in a given period of 1.5 hours. The 1.55 μm laser presents low noise properties, that the intensity and the phase noise reach the shot noise level for the analysis frequencies higher than 4 MHz and 5 MHz, respectively.

Recent achievements on tests of series gyrotrons for W7-X and planned extension at the KIT gyrotron test facility

Parasitic beam tunnel oscillations have been hampering the series production of gyrotrons for W7-X. This problem has now been overcome thanks to the introduction of a specially corrugated beam tunnel. Two gyrotrons equipped with the new beam tunnel have fully passed the acceptance tests. Despite excellent power capability, the expected efficiency has not yet been achieved, possibly due to the presence of parasitic oscillations suspected to be dynamic after-cavity-oscillations (ACI's) or due to insufficient electron beam quality. Both theoretical and experimental investigations on these topics are ongoing. On previous W7-X gyrotrons collector fatigue has been observed, not (yet) leading to any failures so far. The plastic deformation occurring on the collector has now been eliminated due to the strict use (on all gyrotrons) of a sweeping method which combines the conventional 7Hz solenoid sweeping technique with a 50Hz transverse-field sweep system.Starting in 2013, the gyrotron test facility at KIT will be enhanced, chiefly with a new 10MW DC modulator, capable of testing gyrotrons up to 4 MW CW output power with multi-stage-depressed collectors.

Advances in Red VCSEL Technology

Red VCSELs offer the benefits of improved performance and lower power consumption for medical and industrial sensing, faster printing and scanning, and lower cost, higher speed interconnects based upon plastic optical fiber (POF). However, materials challenges make it more difficult to achieve the desired performance than at the well-developed wavelength of 850 nm. This paper will describe the state of the art of red VCSEL performance and the results of development efforts to achieve improved output power and a broader temperature range of operation. It will also provide examples of the applications of red VCSELs and the benefits they offer. In addition, the packaging flexibility offered by VCSELs, and some examples of non-Hermetic package demonstrations will be discussed. Some of the red VCSEL performance demonstrations include output power of 14 mW CW at room temperature, a record maximum temperature of 115∘C for CW operation at an emission wavelength of 689 nm, time to 1% failure at room temperature of approximately 200,000 hours, lifetime in a 50∘C, 85% humidity environment in excess of 3500 hours, digital data rate of 3 Gbps, and peak pulsed array power of greater than 100 mW.

Gyrotrons for magnetic fusion applications at 110 GHz and 170 GHz

Two megawatt-class gyrotrons at frequencies of 110 GHz and 170 GHz have recently been fabricated at CPI. The 110 GHz gyrotron is designed to produce 1.2 MW of output power for 10-second pulses, and will be used for electron cyclotron heating and current drive on the DIII-D tokamak at General Atomics. This gyrotron has completed factory testing and has been delivered to General Atomics for installation and additional testing. The 170 GHz gyrotron, though specified as a 500 kW CW system, has been designed with the goal of generating up to 1 MW CW. Oak Ridge National Laboratory will use this gyrotron in ITER ECH transmission line testing. This gyrotron has been fabricated and is awaiting factory testing, Design features of each gyrotron are described, and test data for the 110 GHz gyrotron are presented. 1 Design and testing of a 1.2 MW, 110 GHz, 10-second gyrotron CPI's VGT-8115 gyrotron is specified to generate 1.2 MW of output power at 110 GHz, for pulse lengths up to 10 seconds. The gyrotron design is based on that of an earlier prototype (1), but incorporates several improvements, including a larger collector formed from a strengthened copper alloy, as well as a numerically-optimized dimpled-wall launcher and phase-correcting mirror system. The gyrotron operates in the magnetic field produced by a 43 kG superconducting magnet system (SCM) with three separately controllable axial coils (two for generating the desired field in the gyrotron cavity region, and one for adjusting the field at the cathode location) and two orthogonal sets of transverse field coils to ensure that the electron beam is centered in the cavity. The SCM cryostat maintains the necessary temperatures for the superconducting coils using a sealed refrigeration system that does not consume liquid cryogens. The gyrotron employs a single-anode magnetron injection gun to generate an annular electron beam. The electron beam interacts with the TE22,6,1 mode of the cylindrical interaction cavity, and this mode is converted to a Gaussian output beam using the optimized launcher and mirror system. The Gaussian beam passes through an edge- cooled chemical vapor deposition (CVD) diamond disc window. The depressed collector dissipates the residual power in the electron beam by using a combination of iron shielding to expand the size of the annular beam and active sweeping with a large collector coil to vary the beam strike point and lower the time-averaged power-density. A schematic of the gyrotron layout is shown in Figure 1.

Systematic Observation of Time-Dependent Phenomena in the RF Output Spectrum of High Power Gyrotrons

At KIT, high power gyrotrons of conventional cavity (e.g. 1 MW CW at 140 GHz for the stellarator Wendelstein 7-X) and coaxial cavity type (2 MW short-pulse at 170 GHz for ITER) for fusion applications are being developed and verified experimentally. Especially with respect to the problem of parasitic RF oscillations in the beam tunnel of some W7-X tubes, investigations of the gyrotron RF output spectrum have proved to be a valuable source of diagnostic information. 1 Transient effects in ms pulses, like frequency switching or intermittent lowfrequency modulation, have indicated that truly time-dependent measurements would be favorable over the existing exclusively spectrum-based system. 2 In this paper, an improved measurement system is presented, which employs a fast oscilloscope as receiver instead. Short-time Fourier transform (STFT) is applied to the time-domain signal, yielding time-variant spectra (“spectrograms”, “waterfall diagrams”) with frequency resolutions only limited by acquisition length. Typical reasonable resolutions are in the range of 100 kHz to 10 MHz with a memorylimited maximum acquisition length of 4 ms. A key feature of the system consists in the unambiguity of frequency measurement: The system receives through two parallel channels, each using a harmonic mixer (h = 9-12) to convert the signal from RF millimeter wave frequencies (full D-Band, 110-170 GHz) to IF (0-2 GHz). For an IF output signal of each individual mixer, injection side and receiving harmonic are not known. Using accordingly determined LO frequencies, this information is retrieved from the redundancy of the channels, yielding unambiguously reconstructed RF spectra with a total span of twice the receiver IF bandwidth, up to 4 GHz in our case. Using the system, which is still being improved continuously, various transient effects like cavity mode switching, parasitic oscillation competition, low-frequency modulation and general frequency dependence on external values (Ucath, Ibeam) have been documented and will be presented.

Progress on corrugated waveguide components suitable for ITER ECH&CD transmission lines

The 2 MW cw requirement for ITER EC transmission lines has led GA to design a number of new and modified components, namely power monitors and polarizers to meet this requirement. In addition, the ITER transmission lines may require sliding joints or bellows to accommodate the thermal expansion of the lines and/or vessel motion. The ex-vessel sections of the EC launchers require double seal waveguides, miter bends and possibly double seal sliding joints to assure tritium retention in this region. GA has developed designs for many of these components and some of them have already been tested at high power at the JAEA RF Test Stand. Thermal analyses of the standard GA power monitor miter bend show that our standard design is suitable for 1 MW cw operation when the H-field is in the plane of the miter bend. For 2 MW cw operation a modified design, which also requires H-plane orientation, has been developed and a prototype has been fabricated and is ready for testing. For long pulse/cw operation, a 2 MW calorimetric miter bend with thermally isolated mirror has been designed and a prototype has been fabricated. Since the mirror is thermally isolated, calorimetry on the mirror cooling water can provide a measure of absorbed power. Such a miter bend, when made in a double seal version, could be useful for monitoring total power at the end of an EC transmission line just before the in-vessel section of an EC launcher. A mode analyzer using an all metal water-cooled beam splitter is being developed for use in measuring in real time the HE11 and higher order mode content or total power in EC transmission lines. Such a high power diagnostic can be very useful in optimizing the alignment of the MOU output into a transmission line and in monitoring the HE11 mode purity at various locations in a transmission line. Prior to making a 63.5 mm 170 GHz version suitable for use on ITER, several prototype 31.75 mm 110 GHz versions have been fabricated and tested at DIII-D. A 170 GHz version is now being fabricated. This paper addresses the performance characteristics, design features and test results for the new and modified components being developed to assure low loss transmission and acceptable component stresses for 2 MW cw operation.

Influence of scattering on superluminescence in composites dye solution – nanoparticles

Spectral and energy luminescence characteristics of R6G dye solutions in ethanol with addition of Ag nanoparticle suspensions in different aggregate states are experimentally investigated. It is demonstrated that incorporation of non-aggregated and aggregated nanoparticles causes the superluminescence thresholds in R6G solutions to decrease. It is established that the optical properties of the laser beam propagation channel are transformed when low-power (20 mW) cw laser radiation passes through the suspension of nanoparticles. This is manifested through the occurrence of a region with enhanced nanoparticle density in the laser beam center, on which diffraction of laser radiation is observed.

Fabrication and laser behaviors of Nd:YAG ceramic microchips

Transparent Nd:YAG ceramic microchips were fabricated through the slip casting shaping directly from the slurry formed by the commercial Al 2O 3/Y 2O 3/Nd 2O 3 powders, and followed by the vacuum sintering procedure. Viscosity of the slurries, the phase evolution and the densification behavior were investigated. For the Al 2O 3/Y 2O 3/Nd 2O 3 compound slurries system, the optimal condition is 0.5 wt.% NH 4PAA dispersant and 30 wt.% solid loading at pH ≥ 8. The YAG phase started to form at 1250° C and pure YAG phase could be obtained at 1400° C. The optical in-line transmittance of the Nd:YAG ceramics with thickness of 2 mm was about 83.8% at 1064 nm and 82.5% at 400 nm, which hit the upper limit of the theoretically calculated value. For the 1.0 at.% Nd:YAG ceramic microchip, the slope efficiency was 43% for 1.0 at.% Nd:YAG ceramic pumped by 920 mW cw Ti:sapphire tunable laser, and the maximum laser output power 246 W was obtained for 2.0 at.% Nd:YAG ceramics pumped by 925 W LD.

Investigation of long wavelength green InGaN lasers on c‐plane GaN up to 529 nm continuous wave operation

AbstractWe present analysis of the experimentally determined gain coefficients g0 for green InGaN lasers with pulsed wavelengths from 495 to 526 nm. Variable facet coating as well as variable cavity length method are used to deduce the dependency of g0 on wavelength. From the experimental data we found that better laser performance at shorter wavelengths correlates with higher material gain. Therefore, lasers at 504 nm reach wall plug efficiencies (WPE) up to 5.3% at 50 mW cw output power, while lasers with 525 and 529 nm show a WPE up to 3 and 2.3%, respectively. The WPE for longer wavelengths is limited due to thermal rollover in cw operation.

RF modeling of the ITER-relevant lower hybrid antenna

In the frame of the EFDA task HCD-08-03-01, a 5 GHz Lower Hybrid system which should be able to deliver 20 MW CW on ITER and sustain the expected high heat fluxes has been reviewed. The design and overall dimensions of the key RF elements of the launcher and its subsystem has been updated from the 2001 design in collaboration with ITER organization. Modeling of the LH wave propagation and absorption into the plasma shows that the optimal parallel index must be chosen between 1.9 and 2.0 for the ITER steady-state scenario. The present study has been made with n || = 2.0 but can be adapted for n || = 1.9. Individual components have been studied separately giving confidence on the global RF design of the whole antenna.

5 MW CW supply system for the ITER gyrotrons Test Facility

ECH (Electron Cyclotron Heating) for ITER will deliver into the plasma 20 MW of RF power. The procurement of the RF sources will be shared equally between the three following partners: Europe, Japan and Russia. Moreover, Europe decided to develop a RF source capable of 2 MW CW of RF power, based on the design of a coaxial gyrotron with a depressed collector. In order to be able to develop and test these RF sources, a Test Facility (TF) has been built at the CRPP premises in Lausanne (CH). The present paper will first remind the main operation conditions considered to test safely a gyrotron. The power supplies parameters allowing to fulfill these conditions will be reviewed. The core of the paper content will describe the newly installed Main High Voltage Power Supply (MHVPS), to be connected to the gyrotron cathode and capable of −60 kV/80 A-CW. The principle, the characteristics, the on-site test results will be described at the light of the requirements imposed by the gyrotron testing. Particular aspects of the installation and commissioning on-site will be highlighted in comparison with the ITER environment. The synchronized operation of the MHVPS and the BPS (Body Power Supply) on dummy load, piloted through the TF remote control, will be presented and commented. Since the TF supply structure has been built integrating the particular conditions and requirements expected for ITER, a conclusion will summarize the performances obtained at the light of these criteria.

Ti:sapphire laser intracavity difference-frequency generation of 30 mW cw radiation around 45μm

A cw mid-IR coherent source based on difference-frequency generation is designed and characterized. For mid-IR generation, a periodically poled MgO:LiNbO(3) crystal is placed inside a compact Ti:sapphire laser cavity. This provides high-power pump radiation for the nonlinear process. Optical injection by an external-cavity diode laser ensures single-frequency operation of the Ti:sapphire laser, while signal radiation is provided by a fiber-amplified Nd:YAG laser. Mid-IR radiation can be generated with 3850-4540 nm tuning range, narrow linewidth, Cs-standard traceability, and TEM(00) spatial mode. 30 mW power is obtained at 4510 nm.

Optical Operation by Chromophores Featuring 4,5-Dicyanoimidazole Embedded within Poly(methyl methacrylate) Matrices

We have studied photoinduced absorption, birefringence, and optical second-harmonic generation in poly(methyl methacrylate) (PMMA) films doped by organic chromophores featuring 4,5-dicyanoimidazole in the weight content equal to 5%. The chromophores indicated as IM1-IM6 were synthesized from 2-bromo-1-methylimidazole-4,5-dicarbonitrile by either nucleophilic substitution or Suzuki-Miyaura cross-coupling reaction. The samples were obtained as films of several micrometers thickness by the spin-coating method on a quartz substrate. Measurements of the optically induced birefringence were done by the Senarmont method at wavelength 1150 nm, and photoinduced absorption was studied in the spectral range 250-700 nm under optical treatment by 300 mW cw 532 nm laser. Photoinduced optical effects were studied by bicolor 1064 and 532 nm coherent laser pulses. The maximal changes were observed for the ratio between fundamental and writing beam intensities equal to about 7:1. To interpret the observed experimental measurements, theoretical simulations of photoinduced optical properties were performed by quantum chemical computational methods.