In some general aspects, a protection apparatus includes a buffer generator and a heating apparatus. The protection apparatus can protect a buffer flow guide in an extreme ultraviolet (EUV) light source and provide for improved flow stability of a buffer gas within a chamber of the EUV light source. The buffer generator is configured to interact a buffer with a surface of a substrate (a substrate surface) that is positioned inside a chamber of the extreme ultraviolet (EUV) light source. The heating apparatus is in thermal communication with the substrate and is configured to increase a temperature of the substrate and the substrate surface. Implementations can include one or more of the following features. For example, the heating apparatus can be configured to increase the temperature of the substrate and the substrate surface to a value at which any temperature gradient at any location of the substrate surface remains below 10% of an average temperature of the substrate. The heating apparatus can be configured to increase the temperature of the substrate and the substrate surface to a value that is greater than a melting point of a target material that travels through the EUV light source chamber. The substrate can be a buffer flow guide positioned adjacent to an optical element that interacts with Tight within the chamber of the EUV light source. The optical element can be an EUV collector mirror and the buffer flow guide can be positioned at an opening of the EUV collector mirror. The buffer flow guide can include a conically-shaped portion that extends from the EUV collector mirror toward a focal region of the EUV collector mirror. The optical element can interact with light within the EUV light source chamber. The optical element can be a mirror configured to reflect EUV light or a window configured to pass EUV light. The protection apparatus can further include a control apparatus in communication with the heating apparatus, the control apparatus configured to adjust the heating apparatus to actively maintain the temperature of the substrate and the substrate surface. The protection apparatus can also include a temperature metrology device configured to measure a temperature of the substrate. The control apparatus can be in communication with the temperature metrology device such that it actively maintains the temperature of the substrate and the substrate surface by comparing the measured temperature to a target temperature. The temperature metrology device can include one or more of a thermocouple, an infrared camera, a thermometer, or a laser reading. The buffer generator can include a buffer flow generator and the buffer can include a buffer fluid such that the buffer flow generator is configured to pass the buffer fluid across the substrate surface. The buffer fluid can be a gas that includes hydrogen. The buffer flow generator can pass the buffer fluid across the substrate surface by advectively transporting the buffer fluid across the substrate surface. The buffer flow generator can pass the buffer fluid across the substrate surface by reducing an amount of target material debris deposited on the substrate surface. And, the heating apparatus can be configured to maintain the effectiveness of the buffer flow generator in reducing the amount of target material debris that is deposited on the substrate surface. The heating apparatus can be configured to increase the temperature of the substrate and the substrate surface to a value that is greater than a temperature at which at least some of the buffer fluid dissociates. The heating apparatus can be configured to increase the temperature of the substrate and the substrate surface to a value greater than 600 ℃. The heating apparatus can include a resistive heating element, a light source, a friction device, or a thermoelectric device in thermal communication with the substrate. The protection apparatus can also include an insulating device associated with the substrat
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