Комбинированные окна для газовых лазеров высокой мощности
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-01-01 |
| Journal | Журнал технической физики |
| Authors | М.В. Рогожин, В.Е. Рогалин, М.И. Крымский |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This study models and validates a novel combined output window design for multi-kilowatt CO2 gas lasers utilizing unstable resonators, significantly enhancing power handling and beam quality compared to traditional solid windows.
- Core Achievement: The maximum permissible output power (Pmax) was increased by over 3x, rising from 130 kW (solid CVD diamond) to 440 kW (combined window with Phase Change Material (PCM) cooling).
- Design Innovation: The window uses a two-component structure: a transparent peripheral ring made of polycrystalline Chemical Vapor Deposition (CVD) diamond and an opaque central disk made of copper.
- Thermal Management: The central copper disk, which is not exposed to the laser beam (due to the ring profile of the unstable resonator), is coupled to a PCM cryo-accumulator (heat sink) for enhanced, localized cooling.
- Beam Quality Improvement: The enhanced thermal management minimizes thermal lensing effects, increasing the axial intensity of the beam in the far field (5 km distance) from 460 W/cm2 to 650 W/cm2.
- Economic Benefit: By replacing the central, unused portion of the expensive CVD diamond with cheaper copper, significant material cost savings are achieved, ranging from $45,216 to over $2 million, depending on window diameter and material ratio.
- Operating Conditions: The modeling focused on long-pulse (60 s) operation in the multi-kilowatt range, simulating continuous wave (CW) thermal conditions.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Laser Wavelength (λ) | 10.6 | µm | Primary operating wavelength for CO2 lasers. |
| Max Output Power (Pmax) | 440 | kW | Combined window with PCM cooling (H = 6 cm). |
| Pmax (Solid Window) | 130 | kW | Limit due to mechanical strength failure. |
| Pmax Increase Factor | >3 | - | Achieved by combined design + PCM cooling. |
| Far-Field Intensity (I5km) | 650 | W/cm2 | Combined window at P = 440 kW. |
| Far-Field Intensity (I5km) | 460 | W/cm2 | Solid window at P = 130 kW. |
| Window Outer Diameter (D) | 200 | mm | Total window size. |
| Window Inner Diameter (D) | 80 | mm | Diameter of the opaque central copper disk. |
| CVD Diamond Thermal Conductivity (λ1) | 2000 | W/mK | Peripheral transparent ring material. |
| Copper Thermal Conductivity (λ3) | 400 | W/mK | Central opaque disk material. |
| Diamond Absorption Coefficient (α) | 5 x 10-2 | cm-1 | Used for thermal modeling at 10.6 µm. |
| Peripheral Cooling Temperature (TL) | 7 | °C | Liquid cooling applied to the outer edge. |
| PCM Material | Glycerin | - | Used in the cryo-accumulator. |
| PCM Phase Transition Temperature (Tph) | 18 | °C | Melting point of Glycerin. |
| Optimal PCM Heat Sink Thickness (H) | 6 | cm | Thickness yielding maximum Pmax. |
| Simulation Time (t) | 60 | s | Duration of the simulated high-power pulse. |
| Indium Thermal Conductivity | 81.8 | W/mK | Material used for the plastic vacuum gasket. |
Key Methodologies
Section titled “Key Methodologies”The study employed a three-part mathematical model (thermal, mechanical, and optical) to simulate the behavior of the combined window under intense laser irradiation.
- Thermal Modeling (Heat Equation): The heat conduction equation was solved in cylindrical coordinates, accounting for the different thermal properties of the peripheral CVD diamond (index 1) and the central copper disk (index 3).
- Boundary Conditions:
- Irradiation Profile: The laser intensity profile was modeled as a ring (annular profile), characteristic of an unstable resonator, ensuring the central disk received no direct beam energy.
- Thermal Contact: A dense thermal contact was assumed between the diamond ring and the copper disk, neglecting the thermal resistance of the thin Indium gasket.
- Peripheral Cooling: The outer edge of the diamond ring was maintained at a constant temperature (TL = 7 °C) via liquid cooling.
- PCM Cryo-Accumulator Integration: The thermal model for the central copper disk included a Phase Change Material (PCM) heat sink (Glycerin, Tph = 18 °C). The phase transition was modeled using the Dirac delta function approximation within a small temperature interval (the “smearing” region) to account for latent heat absorption.
- Mechanical Stress Calculation: Mechanical stress fields were calculated independently for the central and peripheral parts, assuming the Indium vacuum gasket acts as a plastic buffer, compensating for thermal expansion differences and preventing stress transfer between the two materials.
- Optical Performance Modeling: The far-field intensity (I5km) was calculated using a Gaussian beam approximation, incorporating the focal length (F) induced by the thermal lensing effect in the transparent CVD diamond ring.
Commercial Applications
Section titled “Commercial Applications”The combined window technology is highly relevant for applications requiring extremely high-power laser output with stringent beam quality requirements, particularly in industrial and defense sectors.
- High-Power Industrial Lasers: Used in heavy-duty cutting, welding, and surface treatment applications where multi-kilowatt CO2 lasers are employed (e.g., continuous-wave or long-pulse operation).
- Directed Energy Systems (Defense): Essential for systems requiring maximum power output and minimal beam divergence (low thermal lensing) over long distances, often utilizing unstable resonator designs.
- Specialized Gas Lasers: The methodology is transferable to other high-power gas lasers operating in the mid-infrared, such as CO, HF, DF, iodine, and alkali vapor lasers, which also rely on materials with excellent thermal properties.
- CVD Diamond Optics: Promotes the use of large-aperture polycrystalline CVD diamond, leveraging its superior thermal conductivity (2000 W/mK) and high optical damage threshold (107 W/cm2).
- Cost-Sensitive High-Power Optics: Provides a cost-effective solution for large-aperture systems by minimizing the volume of expensive optical-grade CVD diamond required.
View Original Abstract
Based on the previously developed mathematical model of the behavior of the multi-kilowatt laser window with an unstable cavity, the case of a two-component output window is considered. The two-component window consists of a transparent polycrystalline diamond ring and a central opaque area separated by a plastic vacuum gasket. The central opaque area is equipped with a cryoaccumulator to reduce heat load. Numerical calculations of thermomechanical processes are performed for such windows used in high-power CO2 lasers. Mathematical model used for the calculations consists of three parts - thermophysical, mechanical and optical. The advantages of using a two-component design with a cryoaccumulator under the conditions of a gas laser operating in the multi-kilowatt power range are demonstrated. The dependences of the maximum output power, temperature distribution and mechanical stresses versus the thickness of the window are obtained. The optimal conditions providing maximum radiation strength and minimum beam divergence are considered.