Tungsten isotope helps study how to armor future fusion reactors

Zeke Unterberg and his group on the Division of Power’s Oak Ridge Nationwide Laboratory are presently working with the main candidate: tungsten, which has the very best melting level and lowest vapor strain of all metals on the periodic desk, in addition to very excessive tensile energy — properties that make it well-suited to take abuse for lengthy durations of time. They’re targeted on understanding how tungsten would work inside a fusion reactor, a tool that heats gentle atoms to temperatures hotter than the solar’s core in order that they fuse and launch power. Hydrogen fuel in a fusion reactor is transformed into hydrogen plasma — a state of matter that consists of partially ionized fuel — that’s then confined in a small area by robust magnetic fields or lasers.

“You do not need to put one thing in your reactor that solely lasts a few days,” mentioned Unterberg, a senior analysis scientist in ORNL’s Fusion Power Division. “You need to have ample lifetime. We put tungsten in areas the place we anticipate there shall be very excessive plasma bombardment.”

In 2016, Unterberg and the group started conducting experiments within the tokamak, a fusion reactor that makes use of magnetic-fields to include a hoop of plasma, on the DIII-D Nationwide Fusion Facility, a DOE Workplace of Science consumer facility in San Diego. They wished to know whether or not tungsten might be used to armor the tokamak’s vacuum chamber — defending it from speedy destruction attributable to the consequences of plasma — with out closely contaminating the plasma itself. This contamination, if not sufficiently managed, may finally extinguish the fusion response.

“We had been attempting to find out what areas within the chamber can be notably dangerous: the place the tungsten was most definitely to generate impurities that may contaminate the plasma,” Unterberg mentioned.

To seek out that, the researchers used an enriched isotope of tungsten, W-182, together with the unmodified isotope, to hint the erosion, transport and redeposition of tungsten from throughout the divertor. Wanting on the motion of tungsten throughout the divertor — an space throughout the vacuum chamber designed to divert plasma and impurities — gave them a clearer image of the way it erodes from surfaces throughout the tokamak and interacts with the plasma. The enriched tungsten isotope has the identical bodily and chemical properties as common tungsten. The experiments at DIII-D used small metallic inserts coated with the enriched isotope positioned near, however not at, the very best warmth flux zone, an space within the vessel sometimes referred to as the divertor far-target area. Individually, at a divertor area with the very best fluxes, the strike-point, researchers used inserts with the unmodified isotope. The rest of the DIII-D chamber is armored with graphite.

This setup allowed the researchers to gather samples on particular probes quickly inserted within the chamber for measuring impurity move to and from the vessel armor, which may give them a extra exact concept of the place the tungsten that had leaked away from the divertor into the chamber had originated.

“Utilizing the enriched isotope gave us a novel fingerprint,” Unterberg mentioned.

It was the primary such experiment performed in a fusion system. One purpose was to find out the very best supplies and placement for these supplies for chamber armoring, whereas holding impurities attributable to plasma-material interactions largely contained to the divertor and never contaminating the magnet-confined core plasma used to supply fusion.

One complication with the design and operation of divertors is impurity contamination within the plasma attributable to edge-localized modes, or ELMs. A few of these quick, high-energy occasions, akin to photo voltaic flares, can harm or destroy vessel parts equivalent to divertor plates. The frequency of the ELMs, the occasions per second these occasions happen, is an indicator of the quantity of power launched from the plasma to the wall. Excessive-frequency ELMs can launch low quantities of plasma per eruption, but when the ELMs are much less frequent, the plasma and power launched per eruption is excessive, with a higher likelihood for harm. Latest analysis has checked out methods to manage and improve the frequency of ELMs, equivalent to with pellet injection or further magnetic fields at very small magnitudes.

Unterberg’s group discovered, as they anticipated, that having the tungsten removed from the high-flux strike-point vastly elevated the likelihood of contamination when uncovered to low-frequency ELMs which have increased power content material and floor contact per occasion. Moreover, the group discovered that this divertor far-target area was extra vulnerable to contamination the SOL although it typically has decrease fluxes than the strike-point. These seemingly counterintuitive outcomes are being confirmed by ongoing divertor modeling efforts in relation to this venture and future experiments on DIII-D.

This venture concerned a group of specialists from throughout North America, together with collaborators from Princeton Plasma Physics Laboratory, Lawrence Livermore Nationwide Laboratory, Sandia Nationwide Laboratories, ORNL, Basic Atomics, Auburn College, the College of California at San Diego, the College of Toronto, the College of Tennessee — Knoxville, and the College of Wisconsin-Madison, because it supplied a major instrument for plasma-material interplay analysis. DOE’s Workplace of Science (Fusion Power Sciences) supplied assist for the examine.

The group revealed analysis on-line earlier this 12 months within the journal Nuclear Fusion.

The analysis may instantly profit the Joint European Torus, or JET, and ITER, now below building in Cadarache, France, each of which use tungsten armor for the divertor.

“However we’re taking a look at issues past ITER and JET — we’re wanting on the fusion reactors of the longer term,” Unterberg mentioned. “The place is it finest to place tungsten, and the place must you not put tungsten? Our final purpose is to armor our fusion reactors, after they come, in a wise method.”

Unterberg mentioned ORNL’s distinctive Secure Isotopes Group, which developed and examined the enriched isotope coating earlier than placing it in a kind helpful for the experiment, made the analysis doable. That isotope wouldn’t have been accessible wherever however from the Nationwide Isotope Growth Heart at ORNL, which maintains a stockpile of virtually each factor isotopically separated, he mentioned.

“ORNL has distinctive experience and explicit needs for any such analysis,” Unterberg mentioned. “We now have a protracted legacy of creating isotopes and utilizing these in all types of analysis in numerous purposes world wide.”

As well as, ORNL manages US ITER.

Subsequent, the group will have a look at how placing tungsten into in a different way formed divertors would possibly have an effect on contamination of the core. Totally different divertor geometries may decrease the consequences of plasma-material interactions on the core plasma, they’ve theorized. Understanding the very best form for a divertor — a obligatory part for a magnetic-confined plasma system — would put scientists one step nearer to a viable plasma reactor.

“If we, as a society, say we wish nuclear power to occur, and we need to transfer to the subsequent stage,” Unterberg mentioned, “fusion can be the holy grail.”