General atomics breakthrough enables greater control of fusion energy

October 27, 2016
Members of the DIII-D Neutral Beam Group in front of a beam housing for two of the eight beamlines. Credit: General Atomics

Researchers working at the DIII-D National Fusion Facility at General Atomics (GA) have created an important new tool for controlling fusion plasmas that are hotter than the sun.

Energy and momentum in DIII-D's magnetically contained plasma is delivered by large neutral-particle beams systems, and GA's recent demonstration of precise control of injected power and torque is a first. Scientists are now able to pre-program these inputs over the duration of plasma discharges (called "shots"). GA led the development effort in collaboration with scientists from the University of California-Irvine and Princeton Plasma Physics Laboratory.

Previously, these inputs were tailored using on/off modulation of neutral beams, resulting in large perturbations, i.e. power swings. The new method allows separate and continuous specification of power and torque, including the important capability of maintaining a fixed injected power level while varying torque.

Changing the way this system operates is a significant effort, considering the size and complexity of each beam system; there are four truck-sized housings for eight total beams at DIII-D (Figure 1). The neutral beam system injects up to 20 megawatts of power, approximately the power used by 15,000 homes.

Spectrograms of measured beam ion loss. Both plasma shots feature the same total beam power, but the shot shown on the right utilizes a beam voltage program that greatly reduces the amplitude of coherent plasma waves. Credit: D.C. Pace, et al., Nucl. Fusion 57, 014001 (2017)

In the past, neutral beams have operated by accelerating ions through a high voltage (approximately 90,000 volts, compared to the 120 volts of a typical household power outlet) that is fixed in time, and then passing them through a chamber of dense gas where they neutralize and fly into the magnetized plasma. High acceleration voltage is necessary to maximize the velocity of the resulting neutral atom and beam heating power.

Experiments in recent years have shown that the velocity of the beam particles can produce or amplify electromagnetic plasma waves that kick those beam particles out of the plasma and into the walls of the tokamak. This presents a dilemma because high beam power is necessary to reach fusion temperatures, but the beam particle loss reduces the temperature and can lead to costly damage along the tokamak walls.

The solution is to vary the beam's high voltage over time, thereby reducing beam particle losses due to plasma waves while maximizing input beam power. As the plasma is heated, the behavior of the changes such that beam particles of different velocities interact with the waves. Now, the DIII-D neutral beams can be given pre-programmed voltage profiles that minimize wave-particle interactions. This keeps the beam particles in the plasma and allows the beam voltage to increase to higher levels that maximize the input heating power. An example of reduced plasma wave activity is shown in the plots below (Figure 2), where similar conditions produce very different waves based on the time evolution of the beam voltage.

"This project involved two years of engineers and physicists working hard to create something new, and it's wonderful to see it working successfully on DIII-D," said Dr. David Pace, a physicist who led the project for the GA Energy Group, "Now we get to focus on the next exciting step, which is demonstrating all the ways these variable voltage beams can improve magnetic fusion in machines across the world."

Initial results will be presented by Tim Scoville, head of the Neutral Beam Group at DIII-D, at the annual meeting of the American Physical Society Division of Plasma Physics, Oct. 31 - Nov. 4. This work is supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences, at the DIII-D facility operated by GA.

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Dennis Jasbey
1 / 5 (2) Nov 20, 2016
So here is the Nth fusion breakthrough of the year: Altering the voltage on beam injectors!!

If this thing is a "breakthrough", then the world's fusion programs need 300 more years of breakthroughs before anyone will see a fusion power reactor.

The ability to change the voltage on neutral beam injectors is like the ability to vary the speed of your car with the accelerator to avoid collisions.

For the last 40 years, the beam accelerator was only ON-OFF, but now it is variable!
Oh Wow! It took only forty years to introduce this feature.

Like all fusioneers, the GA crowd think that Consuming power is more important than producing it. This article boasts about INJECTING 20 Megawatts of beam power.

And how much fusion power is produced? The answer (when you can find it elsewhere) is less than 100 watts-- not a whole lot to control !
5 / 5 (1) Dec 07, 2016
You are correct that press releases can give a false impression of the magnitude of the advance.

The analogy between beam voltage and the speed of a car is incorrect because this has nothing to do with collisions (still a neat analogy!). Changing the beam voltage changes the velocity of the ions, which prevents them from interacting with plasma waves. The waves then lose their ability to kick ions out of the plasma. This is like changing the speed of your car to avoid stoplights :)

The engineering advance is appreciable. While it's already difficult to adjust a MW power system, it is also necessary to account for the changing behavior of the ion stream, which can reduce beam focus and lower the power reaching the plasma.

Another misconception in your comment is that the plasma control only applies to fusion power output. Control is needed for the entire plasma, and the plasma contains MJ of energy even in cases with no fusion output.
not rated yet Dec 07, 2016

Like all fusioneers, the GA crowd think that Consuming power is more important than producing it. This article boasts about INJECTING 20 Megawatts of beam power.

And how much fusion power is produced? The answer (when you can find it elsewhere) is less than 100 watts-- not a whole lot to control !

Your comment is polemic and specious. No existing fusion reactor has ever been designed and built to produce net output power (Q>1). That is not to say there has been no progress in nuclear fusion. Existing experimental fusion reactors are analogous to a 1/10th or 1/100th scale model aircraft in a wind tunnel where one can learn a lot and make accurate predictions about the stability, performance, fuel consumption, range, payload etc. of an aircraft before you actually build the thing. But only an idiot would look at a scale model aircraft is a wind tunnel and complain that a 10 foot craft can't fly 500 passengers half way around the world.

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