# Presentation: 230870

## Achieving flux amplification with CHI on an ST

- Outline:
- Simple picture of dynamo action
- Description of why flux amplification happened on HIT-II
- Summary
- Something to try on NSTX?

## Generalized Ohm’s law including Hall terms is needed to analyze the dynamo.

- The essence of the pressureless, perfectly conducting generalized Ohm’s law with Hall physics is that the electron fluid is tied to the magnetic field.
- Resistivity allows the slippage between magnetic field and the electron fluid.
- The pressureless generalized Ohms law is found by a _Lorentz_ transformation from the inertial electron fluid frame (where **E** = **j)** to the Lab frame yielding:

## Dynamo mechanism may come from fluctuation modes coupling the driven and confinement regions 

- Time averaging the generalized Ohm’s law including the Hall terms yields [Ji 99]
-  For simplicity, assume ion fluctuation currents are small
- Fluctuation magnetic structure alone provides the coupling between driven and confinement regions and our intuitive pictures of field line tension and pressure are valid for this fluctuating field.

## Intuitive picture of relaxation dynamo mode emerges

- confinement
- region

-                                        anti-current drive

- driven   region

-                                  current drive

- •   Mode crossection (black) in plane of equilibrium field (red, blue)

- •Assumes electron flow is in the direction of field
-  External drive must overcome anti-current drive
- Current drive can maintain closed flux current
- Sheared electron flow due to velocity gradient distorts mode resulting in cross-field current drive stress.
- Practically any turbulence will cause relaxation CD.
- Turbulence does not have to open flux surfaces

**Intuitive picture of relaxation dynamo mode emerges**

- Direction of electron flow

## Key to flux amplification is preventing current drive on open, vertical field

- When driving a plasma with CHI, relaxation current drive can happen in two places.
- In the lower () (= oj/B) closed flux plasma. (the desired effect)
- In the lower () regions near the wall, especially on the open vertical field lines. (the undesired effect)
- This can be controlled by realizing that helicity leaves the region that is driven unstable by CHI via the weak field side.
- Driving a region unstable next to the outer shell produces the undesired effect.
- Driving a region unstable near the central column produces the desired effect.

## Dynamo from n=1 mode gives current drive near the outer wall with no flux amplification (undesired effect)

- Discharge has a well repeating n=1 mode so that a rigid rotation analysis reveals the toroidal mode structure.
-  Dynamo gives current drive in the vertical field and anti-current drive in the CHI driven region.
- Current drive is on the weak field side of the electrode-driven high current region, which is the vertical field near the wall.
- No flux amplification

**Dynamo from n=1 mode gives current drive near the outer wall with no flux amplification (undesired effect)**

**Toroidal angle [radians]**
**Poloidal flux at innermost probe (10cm)**

**Time [ms]**
**Shot # 26070**

**Fluctuation flux at 3ms, (10μWb spacing)**

- Direction of electron flow

- Vertical field region

- CHI driven region

## Driving current next to the central column may be most effective for CHI on an ST.

- Injector current path

- Wall current path

- Bank current path (bad curvature towards ST)

-  Ibank = Iinjector+ Iwall
-  Wall current is scraped off the bank current until the injector current just holds the bubble. (Iinj is independent of Ibank observed)
-  Outer shell is close enough to stabilize n=1 so there is no wasted dynamo on the outer shell. (no n=1 observed)
-  Bank current has the weak field side next to the closed flux region and dynamo drives close flux current giving flux amplification.      (Ip= Ibank tok /inj  observed.)
- Enough turbulence is generated when
-  ***inj**d *> 1/3. Also need *d <* *a/2. *
- Buffer field prevents absorber arcs.

## Internal probes show poloidal flux generation

**Internal probes show poloidal flux generation**

## Further evidence that the “weak field side” discovery is correct 

- In the 160kA transient CHI conditions the bubble kisses the absorber and the plasma current achieved is consistent with total relaxation not wrap-up
- Wrap-up: Ip = Iinjq = IinjITF/Ip = 4kA*2088/180 =46kA
- Total relax:  Ip = Iinjtok/inj= 4kA*1600/35 =182kA
- In the Grennady gun experiments on HIT
- Injection near the central column was successful (one or 2 guns)
- Injection in the outer shell failed to do anything (many guns)

## Summary

- Dynamo current drive may be thought of as an effective electron viscosity across the separatrix, which can drive closed-flux current without opening flux surfaces.
- Apparent conditions for flux amplification are the following:
- ST needs to be on the weak field side of the CHI driven current column.
- Want to avoid instability in the driven current channel when its weak field side is towards the wall.
- Need sufficient turbulence so dynamo is strong enough to drive current. (***inj**d* > 1/3 in HIT-II) (May simply be a requirement for pushing the bank current out the top.)
- Need a volume where the closed flux can exist. (*d <* *a/2.*)
- The “weak-field-side” discovery also explains other results.

##  Possible high flux startup on NSTX

- Initial flux

- Start CHI

- Reverse vertical field

- Allow flux amplification

## Goal is to achieve high Ip/ITF by wall and rotational stabilization 

- Motivation
- Scientific curiosity: How low can we make ITF?
- Lower ITF lowers neoclassical effects and narrows the banana orbits: may improve ST confinement.
- If ITF can be brought to zero then the ST becomes a very attractive reactor concept: Central column is only needed for startup and might be retractable and not suffer radiation damage.
- Learn something about the effects of rotation and NBI on spheromak performance, both stability and confinement.

## Stable ramp down of ITF might be possible

- Start with a good low-beta ST at around 1 MA current.
- Spin it up with NBI to a little less than Mach 1 with respect to the Alfven speed.
- Ramp down ITF
- Keep beta low
- Stay a little less than Mach 1
- Keep the separatrix as close to the wall as possible, everywhere.
- Try to keep Ip at or near 1MA
- During the ramp down measure:
- Confinement time
- Density, Ti, Te, and Zeff.
- Rotational profile
- Magnetics for EFIT
- Fast camera
- Any other measurements