Tiny Torrid Torches: Seeing the light your eyes can't from tiny, million degree plasmas

What is it about?

This paper describes the construction and use of a special detector that lets us see x-rays emitted from tiny, million degree laser-produced plasmas, or 'Tiny Torrid Torches'. Everything you can see has a colour like green grass, a blue sky or the golden sands of a beach. There are also things that produce colours that our eyes aren't equipped to see. 'Soft x-rays' and 'extreme ultraviolet (EUV) light' are general terms which describe colours, just like green, blue and gold that our eyes can't detect. These soft x-rays and EUV light are not just invisible in the conventional manner, but they are also invisible in the sense that they underpin many cutting edge technologies that you probably use today. It's likely that the device you're reading this article on contains state of the art microchips that ultimately make it function. These are in part made by shooting a high powered laser at tin, which becomes a 'torch' emitting EUV light that is collected to make the smallest microchips today. Soft x-rays on the other hand can be used for applications like high resolution (dye-free) cell imaging. These soft x-rays can be produced by shooting a molybdenum target with a high powered laser, forming a soft x-ray torch. In both cases, whether a tin or molybdenum target, the laser pulse is crammed into a tiny area (nominally less than the width of a human hair) and a short period of time (a few billionths of a second). The target absorbs the laser power and heats up so much that it ionises, forming a tiny, million degree glowing plasma, alternatively labelled under the catchy alias as a 'Tiny Torrid Torch' (small scorching sun didn't make the cut). Depending on the material and laser power, these torches can release a portion of the absorbed laser energy as soft x-ray or EUV light, which scientists and engineers can collect and use as light sources like those mentioned above. That all sounds great, but if these soft x-rays from these plasmas are invisible to the naked eye, how do we detect them? How do we determine what colour light is being produced, and how much is being generated? This paper details the design and construction of a compact, affordable detector that lets us 'see' soft x-rays (the kind I mentioned can be used for cell imaging) emitted from these torches that our eyes can't. This device is a spectrometer, which just separates the light emitted from these laser-produced plasmas onto different parts of an off-the-shelf camera. The camera is designed in such a way that it can convert the x-rays into a digital signal, and we can tell what colour x-rays these tiny torrid torches are emitting. A key motivator of building our spectrometer as a small, compact device is that we can perform spectroscopy in gas. In laser plasma light sources, a huge challenge is debris. When we shoot our target with a laser, we dump a load of energy into the target, which responds by ejecting ions and small chunks of the target material. This is Newton's third law; every action has an equal and opposite reaction! This debris can damage optics, electronics and other parts of the light sources that are required to make them function. Gas can act as a buffer, which slows and stops debris before it damages anything. A trade off of using a buffer gas is that the x-rays get absorbed by the gas over long distances. We elected to build a spectrometer that, at the sacrifice of resolution, lets us position it close to the plasma meaning we can successfully perform spectroscopy in gas without the x-rays being attenuated, and at the fraction of the cost of conventional soft x-ray and EUV spectrometers. We achieved this using 3D printed components, commercially available cameras and a simple optic (a transmission grating).

Featured Image

Photo by Dynamic Wang on Unsplash

Photo by Dynamic Wang on Unsplash

Why is it important?

Crucially, learning what x-rays we're producing in the lab lets us know what application our plasmas can be used for. The colour of light produced is not only dependent on the material you pick, like tin or molybdenum, but but also the laser colour, the laser energy, the pulse length and the laser spot size. Each of these parameters can impact what x-rays the plasma / torch will emit. In this paper for example, we look at soft x-ray emission from copper, zinc, tin molybdenum and iron plasmas. Given the large choice of experimental setup, building a device that lets you characterise the light emitted from different setups is essential when building a light source. This paper tells you how to make such a device that is relatively simple to construct, affordable and has reasonable resolution.

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