# Presentation: 620824

## Investigation of current drive with CHI during an ohmic discharge in ST’s

- D. Mueller, Princeton University
- Brian A. Nelson, University of Washington.

## Coaxial Helicity Injection (CHI) has been used on the National Spherical Torus Experiment (NSTX), the Helicity Injected Torus (HIT) and HIT-II to initiate plasma and to drive up to 400 kA of toroidal current.  
We plan  to investigate if the CHI systems can be used to add current to an existing plasma.  
This may be an attractive method to add edge current and modify the stability characteristics of the discharge. 
	(also to modify the particle and energy transport in a spherical torus) 
	divertor biasing experiments have been successful in modifying particle and energy transport in  the scrape-off layer in tokamaks.  
Target lower-single-null ohmic plasmas have been developed on both NSTX and HIT-II.  
Technical difficulties have, so far, limited the successful addition of current from the CHI systems on NSTX.

- We plan  to investigate if the CHI systems can be used to add current to an existing plasma.
- This may be an attractive method to add edge current and modify the stability characteristics of the discharge.
- 	(also to modify the particle and energy transport in a spherical torus)
- 	divertor biasing experiments have been successful in modifying particle and energy transport in  the scrape-off layer in tokamaks.
- Target lower-single-null ohmic plasmas have been developed on both NSTX and HIT-II.
- Technical difficulties have, so far, limited the successful addition of current from the CHI systems on NSTX.

## Experiment

- During the plasma current (Ip) flattop apply voltage to the injector electrodes using Capacitor Banks or  power supplies.
- Investigate change in Ip with Iinjector and dependence on plasma configuration.
- Characterize toroidal current to evaluate the potential for sustainment of the  current, and edge current drive for stability control.

## Target plasma for CHI

- Flux plot of lower single null target plasma in HIT-II
- Similar plasma targets have been  made in NSTX

## Application of CHI increases IP

- VCHI applied using Capacitor banks
- Note increase in Ip  and eventual collapse during CHI
- Large density and radiated power increases during CHI

- Rtan =42.3 cm

- With CHI

- Without CHI

## The electron temperature decreases as the density increases during CHI

- The density increase during CHI is several-fold near the edge of the plasma.
- The electron temperature is lower during the higher density CHI phase, but still over 100 eV centrally (Note large error bars on the low density ohmic phase points)
- Radius of outer midplane wall is 50.5 cm, radius of inner wall is  10 cm.

- With CHI

- Without CHI

- Te

- ne

- ne

- Te

- With CHI

- Without CHI

## CHI can increase Ip at low Iinj with low inj 

- Taylor minimum energy state **B** =  **B**, with  = 0J/B a global constant, integrating **J** and **B** over a surface with current I and flux  gives  = 0I/.
- Helicity  K  **** d  where **A** is the magnetic  vector potential and **B** is the magnetic flux density  and d is the elemental volume.  For a tokamak this is approximately the product of toroidal and poloidal fluxes, K ~ Tp, for fixed T , K Ip    So all current drive techniques must sustain helicity.
- For a transformer, K = 2 Vloop  T
- For electrodes Vinj is applied to the magnetic flux, inj, penetrating the electrodes, thus helicity is injected at the rate, dKinj/dt  = 2 Vinj inj
- An effective loop voltage  Veff can be defined as the loop voltage to obtain the same helicity injection rate:
- 	  Veff = Vinj inj / T
- Then the efficiency  POH/PCHI = Ip Veff / Iinj Vinj = tok/inj  ; inj > tok  is required.  This assumes dissipation occurs in the tokamak, not in the injector.  Dissipation in the injector lowers the efficiency.

## For inj < tok  there is no increase in Ip

-   Use of power supplies  limit  Iinj to  < 2.8 kA.
-  Note drop in Ip with density and Prad increase.

- With CHI

- Without CHI

- Rtan =42.3 cm

- Increase in Prad

- Increased ne ~ 4 X 1013

- dIp/dt ~ -12.5 kA/ms

## Reducing inj raises  inj above tok results in an increase in Ip

- Decreasing inj raises inj
-  Ip rises and then falls, but is above what might be expected due to higher resistivity, dIp/dt from after CHI (blue) or from low inj shot (magenta).

- With CHI

- Without CHI

- Rtan =42.3 cm

- Increased ne ~ 4 X 1013

- Increased Prad

- dIp/dt ~ -12.5 kA/ms

- ~ -7 kA/ms

## Te, ne for low inj, inj > tok shots

- Central Te is still ~ 100 eV during CHI even at the lower injection  power
- Edge Te is ~ unchanged despite large ne increase at the edge
- Central Te is  lower during CHI -> more resistive

- With CHI

- Without CHI

- Te

- ne

- ne

- Te

- With CHI

- Without CHI

## Gas puffing fueled density rise similar to CHI with IGBT

- Ip falls with increasing density and radiated power.
- Typical value of the Greenwald density limit is ~1 1014/cm3 at 120 kA for the shots shown.

- Gas puff

- No gas puff

- Rtan =42.3 cm

- Increased Prad

- dIp/dt ~ -7 kA/ms

- Increased ne ~ 3 X 1013

## During gas fueled ohmic discharge the density increase is largest near the edge

- During gas fueling:
- 	  Density rises near the edge
- 	  Edge Te falls
-  Ip drops during the density rise, since Vloop is constant this implies an  increase in the plasma resistivity

- With gas puff

- Without gas puff

- Te

- ne

- ne

- Te

- With gas puff

- Without gas puff

## CHI can be used to supplement OH  current drive

- CHI increases Ip when inj > tok , but the increased density with CHI raises the resistive dissipation and can result in lower Ip.
- Reproduce the low inj shots on NSTX.
- The density ( and radiated power) increase during CHI must be controlled if this  is to become a useful tool for current drive.
- Need EFIT analysis of current profile changes before, during and after CHI.
- Eventually use NBI  and MSE to evaluate J(r).
- 		Will  require modification of the Ground Fault system to avoid 	spurious trips.