# How do ions and electrons follow directions in a stellarator

1. Oct 12, 2016

### anonymous24

Hello,
I'm a high school student interested in fusion. I've done a bit research online but I got a bit confused how does stellarator like W7-X drives electrons to run in the same direction without plasma current. I know in a tokamak, the plasma current makes electrons follow a certain direction like in wires, correct me if I'm wrong, thank you.

2. Oct 12, 2016

### mathman

3. Oct 12, 2016

### Astronuc

Staff Emeritus
Is the question related to this statement: " However, our former Director, Jürgen Nührenberg, discovered a hidden symmetry characteristic of plasmas in the 1980s which makes it possible to also confine a plasma without plasma current?"

Indeed, but I believe the question relates to a plasma current, or the necessity of having a current to direct or confine a plasma.

In a plasma, typically one of hydrogen isotopes D or T, the electrons and nuclei are separated. Since they are heated, they have velocities in all directions, but a component of the velocity is parallel with the magnetic field and a component of the velocity is perpendicular with the magnetic field lines. The parallel component means the electrons and ions follow with or opposite the magnetic field direction, while the transverse or perpendicular component causes electrons and ions to move in a circle about the magnetic field lines, or in the magnetic field. The stronger the magnetic field, the tighter the circle. The combination of circular transverse motion and parallel or anti-parallel motion produces a spiral trajectories. The circular motion helps keeps the ions/electrons in the plasma.

Currents can be induced in a plasma, much the same way that currents are induced in transformers, with the plasma acting as one of the coils. A change in the magnetic field dΦ/dt will induce a electromotive force on a charged particle causing it to accelerate.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/farlaw.html#c2

4. Oct 12, 2016

### Staff: Mentor

As far as I understood the concept, and especially the complex arrangement of outer magnetic fields, the whole construction is only about the confinement. As long as it is hot and confined, there should no acceleration take place, or at least doesn't matter.

The basic difference between the two models is, that in a tokamak you have to permanently adjust the magnetic fields due to the current to keep the confinement, whereas the stellarator uses a homogeneous magnetic field at the cost of a really complicated arrangement.

5. Oct 12, 2016

### the_wolfman

Hi anonymous24,

There are a couple of things going in. First charged particles, electrons and ions, gyrate around magnetic fields like beads on a string. If you've had a physics course, then you might have learned about something called Lorentz force. The Lorentz force is what causes this behavior. The charged particles move freely parallel to the magnetic field, but the Lorentz force bends their motion perpendicular to the magnetic field into circular orbits. This idea is the basis for magnetic confinement.

Second, you might also know that currents produce magnetic fields. In magnetic confinement we distinguish between two different types of currents: internal and external. External currents refer to the currents running through magnets outside of the plasma. However, plasmas are great conductors and internal (plasma) currents are currents that run through the plasma itself. All magnetic confinement devices use some combination of external and internal currents to confine the plasma.

Tokamaks, by design, use both internal and external currents to confine the plasma. However, stellarators do not need internal currents to generate the confining magnetic field. Instead they use a more complex set of external magnets to generate the confining magnetic field. So W7-X does not need to drive current. In fact, it is specifically designed to minimize currents the naturally form in plasmas when you heat them up.

6. Oct 13, 2016

### anonymous24

Thank all of you for the input, but I still don't quite understand
perhaps I should make it clearer. I understand that tokamak uses plasma current to create poloidal magnetic fields which confine it better but creates instability in plasma, which is why stellarator gets rid of it. The part I don't understand is how electrons and ions run in opposite direction if there is no current conducted, wouldn't some of them run to left and some to the right? I understand the circular part but not the parallel direction. Thank you

7. Oct 13, 2016

### the_wolfman

The electron and ions don't flow opposite directions. This would give rise to a current. Both the electrons and ions will have a random distribution of velocities. But their average velocities will be the same. You're got the correct idea when you say that some to them would "run to left and some to the right." Perhaps you could explain you line of thought in more detail? It sounds like you still have some misconception, but it's at more basic level.

The electrons and ion will also have a random distribution of velocities in tokamak too. The net current is due to the difference between the average velocity of all the ions and the average velocity of the electrons. However some electrons will be moving slower than (or in the opposite direction to) the average electron velocity. This will be balanced by other electrons that are moving faster than the average velocity. The same is true for ions.

8. Oct 13, 2016

### anonymous24

Thank you for the reply. What I meant was I thought in tokamak, the electrons and ions move in random directions at first, and after the plasma current is induced, all the electrons will move in one direction and ions the others. Is it correct? Or the electrons and ions don't have to move in opposite direction for it to work?

9. Oct 13, 2016

### Astronuc

Staff Emeritus
In an induced electric field, positive ions, or nuclei, and electrons move in opposite directions.

10. Oct 13, 2016

### the_wolfman

The current is a plasma is due to the difference between the average ion flow and the average electron flow. For a hydrogen plasma without impurities the current density is given by the relation $\vec J = ne\left( \vec V_i - \vec V_e\right)$. Note that electron and ion's don't have to flow in opposite directions in order for there to be a current. The only thing that has to be true is that the average flow velocities have to be different. The actual dynamics of the current and flows in a magnetically confined plasma are complex and they depend on many factors, not just the induced electric field. The generalization that the electrons and ions flow in opposite directions is not true in general.

The actual velocity of individual electrons and ions is still random after you apply the current. The temperature is actually a measure of this. The hotter the plasma the greater the spread in the individual velocities. Applying the current shifts the average of the random distribution, but does not change the fact velocities are randomly distributed.

11. Oct 13, 2016

### anonymous24

Oh Oh I see, thank you so much for clearing my confusion :D

12. Dec 9, 2016

### 1oldman2

I see W7-X is having some initial success, sounds promising.
http://www.nature.com/articles/ncomms13493
http://www.space.com/34960-star-in-a-jar-fusion-reactor-works.html
In a study published in the latest edition of the journal Nature Communications, researchers confirmed that Germany's Wendelstein 7-X (W7-X) fusion energy device is on track and working as planned. The space-age system, known as a stellerator, generated its first batch of hydrogen plasma when it was first fired up earlier this year. The new tests basically give scientists the green light to proceed to the next stage of the process.

Incorporated into terrestrial power plants, this "star in a jar" technology would essentially provide Earth with limitless clean energy, forever. And according to new reports out of Europe this week, we just took another big step toward making it happen.

Last edited: Dec 9, 2016