Mirror-like Shine and Starlight

Published in "NCFertigung," Issue 06|2015, by Hubert Winkler
(courtesy of Schlütersche Verlagsgesellschaft, Hanover)

In the specialized laboratory of the Laboratory Astrophysics Division at the University of Kassel, the materials from which our stars were formed are created in a small vacuum chamber. The highly precise, geometrically accurate mirror systems made of aluminum with a high reflectivity required for this process are manufactured using a ball mill from Horn—equipped with a monocrystalline diamond cutting edge.

Scientists at the University of Kassel are creating matter that normally exists only in space, thereby gaining insights into the formation and demise of stars. Sophisticated laser technology and precise mirror systems help the researchers bring their ideas and theories to life. “The universe is contained within the small vacuum chamber at the Institute of Physics.” There, Prof. Dr. Thomas Giesen and his research team create conditions similar to those that prevail during the various phases of star formation. Prof. Giesen explains the principle: “Under vacuum and at temperatures of minus 250°C—just above absolute zero—scientists can produce molecules in the metal chamber under conditions that are otherwise found only in space.” Powerful laser pulses are used to simulate the high-energy processes that occur during the so-called embryonic phase of stars. The laser pulses are directed at a material, such as high-purity graphite. If operated continuously, the power consumption for a single laser pulse would be equivalent to that of the entire city of Kassel. Fortunately, however, we researchers only need this high energy for one-billionth of a second. That’s enough to break down the carbon-based graphite into its atomic components.”

From 10,000 to -250°C in milliseconds

“The challenge now is to cool the 10,000°C—which prevail during that extremely high-energy moment—down to minus 250°C in a fraction of a second. “We achieve this,” says Giesen, “by firing a supersonic jet of helium gas into the vacuum chamber. The sudden expansion of the gas in the vacuum causes the temperature to drop in the blink of an eye. In this atmosphere, individual atoms can rearrange themselves, just as they do in space. This results in the formation of compounds that are completely different from those formed under terrestrial conditions. In the case of carbon, this process produces 100 different types of molecules.” Terahertz spectroscopy is used to analyze these molecules. The terahertz spectral range lies between the far-infrared and microwave regions and covers frequencies from 300 GHz to 10 THz, i.e., wavelengths between 1 mm and 30 μm. THz beams are passed through the gas being analyzed and detected by a sensor. Each gas has different absorption factors and absorption patterns. The individual gases differ in their absorbed frequency spectra. In the laboratory, THz beams of different frequencies pass through the gas being analyzed, thereby generating a unique “fingerprint” of the gas under investigation.

Unsuitable cutters

To do this, a terahertz beam is directed through an opening in a flat mirror inside the vacuum chamber; the beam is then reflected back to the flat mirror by a concave mirror. The more often the beam is reflected back and forth within the mirror system—and the more frequently it passes through the gas cloud—the clearer the signals become. Finally, a detector behind the opening in the flat mirror captures the beam. To make the invisible terahertz beam visible within the mirror system, a visible green laser beam is superimposed on it. All specialized terahertz optics used to date had shortcomings. Only metallic mirrors brought the researchers closer to their goal. However, previous attempts at milling and polishing did not yield usable surface qualities. The milling marks were too deep, and the polishing process created additional geometric inaccuracies. As a result, there were too few usable reflection passes in the mirror system.

On the Right Track with MKD Milling Cutters

Through an article in “World of Tools,” the in-house magazine of tooling specialist Horn, Ingo Schulz, the department head at the university’s precision engineering workshops, came across the topic of high-gloss milling with MKD tools. The initial trials already showed that using these MKD milling cutters was the right approach. To improve the results, the surface resolution in the OpenMind-Hypermill CAM program was refined to 0.0005 mm. The Heidenhain control system on the Hermle C20U processed this resolution without any problems.

Resolution: 5.1 million data points

In addition to MKD, CVD-D diamond will also be available soon

Unlike in the preliminary operations, where milling is performed with coolant due to the potential formation of built-up edges, coolant is not required when milling with MKD inserts, as diamond materials do not exhibit a tendency to adhere. This is also why Ingo Schulz will rely on diamond for preliminary operations in the future—specifically, CVD-D diamond. It is also hoped that, particularly during the critical profile semi-finishing stage, the light cut and low cutting pressure will further improve geometric accuracy and surface finish prior to high-gloss finishing.

More Accurate Fingerprints with a 300-mm Mirror

Using MKD cutting technology from Horn, the team in Kassel is also manufacturing flat mirrors and, in the future, a concave mirror with a 300-mm diameter—a project requiring over 20 million data points. However, the computer system must first be further upgraded to handle this. With this precise, high-gloss mirror geometry, the team can now reliably achieve 20 to 30 reflection cycles in the mirror system, providing an even more accurate basis for analysis. Each type of molecule leaves behind a characteristic image. “We’re talking about a molecular fingerprint of the molecule—it’s like a unique DNA,” explains Prof. Giesen. In the next step, the scientists will check whether the molecule’s fingerprint is also present in the light that space emits toward Earth. The signals are compared using the highly sensitive IRAM radio telescope in the Sierra Nevada, which has a diameter of 30 meters. If the fingerprints from Kassel are also found there, this will prove that these molecules also exist in distant space. Although these are only small pieces of the puzzle, they can be assembled into a larger picture. The scientists in Kassel are working to complete this picture in international collaboration with research groups from the U.S., France, and experts from Japan.

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