Deep inside Jinping Mountain, beneath two kilometers of rock, a proton beam slams into a cloud of gas. Previously, a thin foil stood between the accelerator and the target, like glass in a window. It slowed particles, smeared their energy, and obscured the rare nuclear reactions that power stars. Now the window is gone, yet the gas does not scatter away: it is held by an entire system of pumps, valves, and sensors.
The gas in the target is like water in a fountain, circulating in a loop. Pumps draw it out of the chamber, push it through filters, and return it back. Pressure stays stable to within one percent for eight hours straight, even as the beam continuously strikes the gas. Meanwhile, on the accelerator side, the vacuum remains nearly cosmic: about one ten-billionth of an atmosphere. Such a contrast is possible because pumping proceeds in stages, like locks on a canal.
The beam heats the gas, which expands like air over hot asphalt. The more powerful the beam, the fewer atoms remain in its path. At eight hundred milliwatts per square centimeter, the density drops by a quarter. This is important to know, otherwise the measured reaction rate would come out too low. The engineers calculated the correction and can now account for it in every experiment.
Another device, a copper calorimeter, acts like a scale for the invisible beam. It maintains a constant temperature, and when the beam delivers heat, it reduces its own heating. The difference in heater power before and after shows how much energy the beam carries. The precision is such that it can detect a spoonful of cold water poured into a boiling kettle.
Now it is possible to measure reactions with expensive isotopic gases, such as helium-3 or neon-22. The gas is not lost but circulates in a loop, and every proton in the beam is counted. This is a path to precise data on how neutrinos are born in the Sun and where neutrons in massive stars come from. The window that once blocked the view has finally been removed, and now the very flame of stellar reactions is visible.
The calorimeter measures beam power to within tenths of a watt, even though the beam itself carries hundreds of watts. That is like noticing that a kettle has cooled by half a degree when you add a spoonful of cold water.