CW Operation Research Articles

Thermal and Structural Analysis on Simulated Model of a Beam Dump

This paper deals with the design, thermal and structural analysis for stopping the proton beam. A conical beam dump is being designed to handle the thermal and structural stresses resulting due to high heat load and hydraulic pressure of coolant, while handling 600 KW proton beam power in CW operation at 20 MeV energy. Pressure drop and heat transfer calculations are done considering spiral channel cooling system. The heat flux has been calculated by using Bi-Gaussian distribution density function of particles in beam. Nickel is selected as the beam dump material because of its favorable properties. In order to understand the thermal performance of beam dump a finite element model has been developed to predict the steady state temperature and stress distributions within the beam stop material. Key Word: Beam dump design, FEM, Thermal and Structural stress.

High-power, stable single-mode CW operation of 1550 nm wavelength InP-based photonic-crystal surface-emitting lasers

1550 nm wavelength photonic-crystal surface-emitting lasers (PCSELs) are attractive for optical communication and eye-safe sensing applications. In this study, we present InP-based PCSELs featuring a double-lattice photonic-crystal structure designed for high-power single-mode operation at a wavelength of 1550 nm. These PCSELs demonstrate output powers exceeding 300 mW under continuous-wave conditions at 25 °C. Additionally, highly stable single-mode oscillation with a side-mode suppression ratio of over 60 dB is verified at temperatures from 15 °C to 60 °C. Measurement and simulation of photonic band structures reveal the impacts of the threshold gain margin and optical coupling coefficient on the single-mode stability.

Rf and thermal design studies of a 704.4 MHz, β≈0.2 , continuous-wave rf quadrupole accelerator for frequency jump

The radio-frequency quadrupole (RFQ) linac is often used as the first radio-frequency (rf) structure for both bunching and preaccelerating proton or heavy ion beams directly behind an ion source or a low-energy beam transport section. Typically, RFQ linacs operate at frequencies below 450 MHz and beam velocity ranges from β=0.01–0.08. Taking the MYRRHA project as an example, a 704.4 MHz continuous-wave (cw) RFQ linac has been recently proposed to realize an efficient fourfold frequency jump at β≈0.2. Due to its high frequency, small dimensions, and cw operation mode, careful design studies for the RFQ cavity structure including the water-cooling channels have been further performed, integrating the idea of using additive manufacturing technology for future construction. Systematic rf and thermal simulations and analysis demonstrated the reliability of the 704.4 MHz RFQ linac at cw operation and the frequency tuning ability via water-speed adjustments. This investigation shows that it could be a highly promising solution to perform an efficient frequency jump with a high-frequency RFQ accelerator at β≈0.2 for large-scale hadron linacs, which could potentially shorten the low- and medium-β parts of the whole facility. Published by the American Physical Society 2024

Continuous-wave GaAs/AlGaAs quantum cascade laser at 5.7 THz.

Design strategies for improving terahertz (THz) quantum cascade lasers (QCLs) in the 5-6 THz range are investigated numerically and experimentally, with the goal of overcoming the degradation in performance that occurs as the laser frequency approaches the Reststrahlen band. Two designs aimed at 5.4 THz were selected: one optimized for lower power dissipation and one optimized for better temperature performance. The active regions exhibited broadband gain, with the strongest modes lasing in the 5.3-5.6 THz range, but with other various modes observed ranging from 4.76 to 6.03 THz. Pulsed and continuous-wave (cw) operation is observed up to temperatures of 117 K and 68 K, respectively. In cw mode, the ridge laser has modes up to 5.71 THz - the highest reported frequency for a THz QCL in cw mode. The waveguide loss associated with the doped contact layers and metallization is identified as a critical limitation to performance above 5 THz.

Смена режимов генерации в эрбиевых волоконных лазерах

Operation features of the erbium fiber lasers with short cavities near two phase transitions have been established experimentally for the first time. The power-law behavior of the system parameters in the vicinity of the critical point in a wide range of pump power values was revealed. The parameters and operation regimes dependence on the cavity length and pump power, namely the switching from pulsed to CW operation, was investigated. The pulse duration and frequency dependencies with varying cavity length have been investigated, which made it possible to experimentally establish the universal dependence of these parameters on the lasing power for passively Q-switched erbium fiber lasers. The critical indices for pulsed operation of the erbium fiber lasers are determined weakly depending on the cavity parameters. The results of the study can be used for designing high-coherent fiber laser sources.

III–V gain region/Si waveguide hybrid lasers by chip-on-wafer hydrophilic bonding process using UV-ozone treatment

The direct bonding process of InP chips on a silicon-on-insulator (SOI) wafer is investigated using surface hydrophilization by UV-ozone treatment. The influence of the treatment on surface roughness is observed by atomic force microscopy, and found to be negligibly small. A high-quality III–V/Si bonding interface without crystal defects is verified in a scanning transmission electron microscope observation. The III–V layers remaining on the SOI wafer after the removal process of the InP substrate show uniform photoluminescence intensity over the whole region of the chip, indicating a bonding interface without the influence of lateral etching and peeling-off. From this bonding process, Fabry–Perot lasers with III–V gain and Si waveguide regions are fabricated, and cw operation is successfully achieved at a stage temperature from 20 °C to 85 °C. Stable operation is also confirmed from the changes of threshold current in the aging test (200 mA at 85 °C) after 2000 h.

Improved device performance of interband cascade lasers with hybrid cladding layers operating in the 3–4 µm wavelength region

In this work, we briefly review the development and status of interband cascade lasers (ICLs) as related to long-standing issues due to the InAs/AlSb superlattice cladding. By focusing on a hybrid cladding approach to alleviate these issues, we demonstrate substantially improved device performance of ICLs compared to earlier reported ICLs of a similar design in the 3–4 µm wavelength region. These improvements include a threshold current density for broad-area devices as low as 134 A/cm2 at 300 K and reduced threshold voltage with a peak voltage efficiency of 80%, which is more than 10% higher than that obtained from previously reported ICLs. Moreover, we have demonstrated continuous wave (cw) operation of a broad-area device up to 278 K, the highest cw operating temperature among epi-side up mounted broad-area type-II ICLs, implying improved thermal dissipation with the hybrid cladding approach. Additionally, by conducting a comparative study of ICLs with different GaSb layer thicknesses in the hole injector, we reveal and discuss an interesting correlation between the carrier transport, threshold voltage, and hole-induced absorption loss, which may help to guide device optimization for operation in a targeted temperature range.

Uniform/Selective Heating Microwave Oven Using High Efficiency GaN-on-GaN HEMT Power Amplifier

A high-efficiency uniform/selective heating microwave oven was developed. Because the power amplifier requires high-efficiency characteristics to function as a microwave source, a free-standing Gallium Nitride (GaN) substrate was applied in this study. By applying a harmonic tuning circuit, an output power of 71 W and PAE of 73% were achieved in pulsed operation, and an output power of 63 W and PAE of 69% were achieved in CW operation. Moreover, we fabricated a prototype PA module that consists of an oscillator, a driver amplifier, PA, and other RF circuits. The output power was controlled by pulse width modulation to maintain high efficiency regardless of output power. We evaluated the arrangement of antenna polarizations to isolate each antenna. By suppressing the interference of output from adjacent antennas, it is possible to irradiate the object on the top surface of the antenna, thereby demonstrating heating characteristics with small temperature unevenness. The prototype microwave oven successfully demonstrated uniform/selective heating.

Feedback control of thermal frequency drift for a normal conducting cyclotron under CW operation

Feedback control of thermal frequency drift for a normal conducting cyclotron under CW operation

Technical design of an IH-type buncher for KoBRA at RAON

To satisfy the beam requirement of the KoBRA experiment (0.5 nsec in 1σ), rebunching is required in the energy range up to 15 A MeV and for particles with an A/q range from 1 to 7. In cooperation with IBS and Hanmac Corporation, BEVATECH proposed a five-gap IH-buncher cavity which has excellent bunching properties from 5 to 15 MeV/u. Main challenges in this project are the wide energy range and CW operation at RF power levels up to 20 kW. The cavity will offer an effective voltage amplitude of up to 1 MV. To provide the required cooling channels and cooling efficiency, the stems and drifttubes of this cavity will be 3D printed. This paper will discuss the technical design approach and give a status on the development of the buncher cavity.

Long-Cavity M-Plane GaN-Based Vertical-Cavity Surface-Emitting Lasers with a Topside Monolithic Curved Mirror

We report long-cavity (60.5 λ) GaN-based vertical-cavity surface-emitting lasers with a topside monolithic GaN concave mirror, a buried tunnel junction current aperture, and a bottomside nanoporous GaN distributed Bragg reflector. Under pulsed operation, a VCSEL with a 9 µm aperture had a threshold current density of 6.6 kA/cm2, a differential efficiency of 0.7%, and a maximum output power of 290 µW for a lasing mode at 411 nm and a divergence angle of 8.4°. Under CW operation, the threshold current density increased to 7.3 kA/cm2, the differential efficiency decreased to 0.4%, and a peak output power of 130 µW was reached at a current density of 23 kA/cm2.

RF and thermal studies on conduction cooled Nb3Sn SRF cavity

Advancements in the development of Nb3Sn coatings for superconducting radio-frequency (SRF) cavities have enabled efficient RF operation at 4.2 K. This has made the use of new cooling methods possible, namely those based on conduction cooling from commercial cryocoolers. Using cryocoolers in place of liquid helium as a cooling source eliminates the need for expensive and complex cryogenic infrastructure, making SRF technology accessible to small-scale applications in fields such as medicine, industry, environmental sustainability and more. At Cornell University, we have developed a new cavity testing assembly which uses a Cryomech PT420-RM cryocooler to cool a 2.6 GHz Nb3Sn cavity. We have performed several rounds of RF and diagnostic testing using this new assembly. Our best results demonstrated stable CW operation at 10 MV/m, with the cavity remaining at 4.2 K or lower. This represents breakthrough performance for a conduction cooled cavity, in which accelerating gradients relevant to some industrial applications were achieved. Our analysis highlights the importance of reducing thermal gradients across the cavity during cooldown; different methods for achieving this were successfully developed and demonstrated. We also found close agreement regarding thermal behavior between experimental measurements and numerical simulations, validating our chosen conduction cooling methods and providing guidance for future improvements. These findings will serve as a foundation for designing a new cryocooler-based cryomodule which will provide beam energy gains on the order of 1 MeV for beam currents up to 100 mA.

Widely Tunable (2.47–2.64 µm) Hybrid Laser Based on GaSb/GaInAsSb Quantum‐Wells and a Low‐Loss Si3N4 Photonic Integrated Circuit

AbstractPhotonic integrated circuits fabricated using a Si3N4 waveguide platform exhibit low losses in a wide wavelength region extending from visible to beyond 2 µm. This feature is exploited to demonstrate a high‐performance integrated laser exhibiting broad wavelength tuneability near a 2.6 µm wavelength region. The laser is based on a Si3N4 photonic integrated circuit incorporating a tunable reflector and a AlGaInAsSb/GaSb quantum‐well gain element. A tuning range of 170 nm (2474–2644 nm) and single‐mode CW operation with a maximum power of 6.4 mW at room temperature are demonstrated. The performance is enabled by exploitation of several essential building blocks realized in Si3N4, namely low‐loss Y‐branches, inverse tapers, and a double‐ring resonator with a free spectral range of ≈160 nm. Moreover, the limits of wavelength coverage are explored using Si3N4 waveguides and show that the platform supports low propagation loss up to 3.5 µm. Finally, the possibility to achieve improved mode matching between Si3N4 and GaSb waveguides is analyzed, further enabling enhancing the performance of such a hybrid laser platform and supporting wavelength extension beyond a 3 µm range.

A new class of spatial harmonic magnetrons with potentials for CW and sub-THz operation

AbstractA strictly physically based design theory of a new class of Spatial Harmonic Magnetrons (SHM) is thoroughly derived which leads to analytically evaluable expressions. Thus two advantages are obtained: (1) The design parameters appear grouped into two separate categories – one just containing geometrical and material parameters, the other one exclusively containing beam current-related ones, which are in product-form determining performance. The influence of each parameter can thus easily be discovered and investigated. (2) Numerical efforts for any new design will be reduced by at least one order of magnitude. Subsequently and based on feature (1) it is derived that loading the anode structure by a suitably selected meta-material will increase output power and efficiency, will pave the way to CW operation, and can extend oscillation frequency well into the sub-THz range. Finally, it is shown that the Rising Sun Magnetron is a first step toward a meta-material loaded SHM offering the same but quantitatively reduced features and less stringent requirements to fabrication technology.

Theoretical Analysis of Continuous-Wave Mid-Infrared Optical Vortex Source Generated by Singly Resonant Optical Parametric Oscillator

Due to the important application in the study of vibrational circular dichroism and helical dichroism of chiral molecules, the tunable vortex beam at mid-infrared region has attracted increasing attention. Based on orbital angular momentum (OAM) conservation in nonlinear interactions, the vortex pumped singly resonant optical parametric oscillator (SRO) is recognized as a versatile source of coherent vortex radiation providing high power and broad wavelength coverage from a single device. However, the low parametric gain and high oscillation threshold under continuous wave (cw) pumping has so far been the most challenging factor in generating cw tunable vortex beams. To predict the output characteristic of vortex pumped SRO, a theoretical model describing the vortex pumped SRO is needed. In this study, the theoretical model describing the vortex pumped SRO is set up under collimated Gaussian beam approximation. Output characteristics of different SROs are simulated numerically. By proper selection of pump scheme (such as double-pass pumping scheme), the vortex pumped mid-infrared SRO can oscillate at a relatively low pump power. By controlling the gain (mode overlap ratio between the pump and resonant wave in the nonlinear crystal) and loss (employing a spot-defect mirror with different defect size as the output coupler) of the resonant signal mode in the SRO, the OAM of the pump beam can be directionally transferred to a specific down converted beam. The transfer mechanism of the OAM among the pump light and the down-converted beams and factors affecting the transfer are studied. Our study provides the guidelines for the design and optimization of vortex pumped SRO under cw operation.

Continuous wave operation of terahertz metasurface quantum-cascade VECSEL with a long intra-cryostat cavity

We report continuous wave (cw) operation of a terahertz quantum-cascade vertical-external-cavity surface-emitting laser with an external cavity length of approximately 30 mm, benefited by an intra-cryostat focusing cavity. Compared to previous plano–plano cavities, an off-axis paraboloid mirror is introduced into the external cavity as a focusing element to reduce the diffraction loss and to enable cw lasing using small-area metasurfaces and long cavity lengths. The device shows lasing operation in the cw mode up to 111 K, and cw output power up to 11.5 mW at 77 K (0.5% wall-plug efficiency). A circular, directive beam pattern is collected, and free-running linewidths on the order of tens of kHz are measured over tens of seconds.

CW and Q-switched operations of a Tm3+:YAP laser at 1892–1994 nm In-band fiber-laser pumped at 1670 nm

Tm3+:YAP lasers in-band fiber-laser pumped at 1670 nm with a linear or L-shaped cavity were studied in CW, actively and passively Q-switched regimes. High-efficiency CW and repetitively-pulsed operations were registered at three spectral regions: 1987–1994 nm, 1935–1941 nm and 1892–1897 nm. The transition between these three spectral regions was achieved by controlling the pump power and the output mirror in a laser cavity. The highest CW output power that was achieved without degrading the beam quality was 12.2 W at the 52% slope efficiency with respect to the incident pump. Acousto-optically Q-switched operation with the average power of 9.8 W at the 20 kHz pulse repetition rate (PRR) and 6 W at 3 kHz PRR was achieved, the pulse energy was scaled up to 2.7 mJ at 0.5 kHz PRR. The passive Q-switch operation at 1935–1938 nm was obtained using a Cr2+:ZnSe saturable absorber; the pulse energy reached 150 µJ at the 20 kHz PRR

Mitigating Valence Intersubband Absorption in Interband Cascade Lasers

AbstractInterband cascade lasers (ICLs), especially valued for their low power consumption, are particularly appealing for portable, compact, and battery‐driven trace gas sensors. However, their performance notably degrades outside the 3–4 µm region. Here, a solution to overcome current performance limitations is presented. Simulation results based on the eight‐band k· p method employing a generalized momentum matrix element model identify resonant intersubband absorption in the valence band as the causative underlying mechanism. Experimentally, a direct dependence of this resonant absorption on the thickness of the GaInxSb hole‐quantum well (h‐QW) is confirmed. This is reflected in the improvement of the laser's characteristic temperature T0, threshold current density Jth, slope efficiency η, and output power. Extracted waveguide losses from length‐dependent measurements substantiate the key role of the valence intersubband absorption. While the performance improvement is experimentally verified at 4.35 µm, the simulation results additionally show how to mitigate undesired absorption at longer wavelengths, paving the way towards high‐performance continuous‐wave (cw) operation above 6 µm.

High power solid state RF amplifiers for SARAF Phase II Linac

Compact narrow-band high power Solid State Power Amplifiers (SSPA) capable of delivering up to 10/20 kW CW RF power at 176 MHz were developed at SNRC for SARAF Linac Phase II. The 10/20 kW amplifiers were built by combining RF power from two/four 5 kW high power amplifiers using 3 dB 0–90° High Power Quad Hybrid Couplers (HPQHC). In their turn, the 5 kW amplifiers were combined from eight LDMOS water cooled amplifiers modules equipped with internal circulators and RF loads. The total power gain of the 10/20 kW amplifiers (including the driver stages) is 70/73 dB. These amplifiers have the ability to withstand with continuous wave full reflection power (VSWR at nominal rating > 100:1) for a long time without the use of an external circulator. Unconditional stability for CW and pulsed operation, reliability, linearity, high efficiency, simple design and compact size are the main features we sought for this amplifier. The underlying design principles, indigenous development and test results are presented.

Reverse-Taper Mid-Infrared Quantum Cascade Lasers for Coherent Power Scaling

We present a reverse-taper quantum cascade laser (QCL) emitting at 4.6 μm, a novel-geometry device that can scale the output power while maintaining good beam quality. Buried-ridge waveguides with tapered and straight regions were formed by ICP etching and HVPE regrowth – the tapered region scales the output power, while the emitting facet is located at the narrow-end taper section, which provides mode filtering by suppressing high-order spatial modes. Beam profiles were observed under quasi-continuous-wave (QCW)/CW operation and beam quality (M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) measurements along with beam-stability measurements were performed – a small degree of collimated-beam centroid movement (<0.46 mrad, peak-to-peak) was observed, along with M <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> values close to 1 up to ∼1 W QCW power. Devices of shorter cavity lengths were also investigated, indicating that the output power scales with the core-region volume but results in a small increase in angular deviation.