# Presentation: 795744

##                                            Supported by   

**22****nd**** IAEA Fusion Energy Conference**
**October 13-18, 2008, Geneva**

***College W&M***
***Colorado Sch Mines***
***Columbia U***
***Comp-X***
***General Atomics***
***INEL***
***Johns Hopkins U***
***LANL***
***LLNL***
***Lodestar***
***MIT***
***Nova Photonics***
***New York U***
***Old Dominion U***
***ORNL***
***PPPL***
***PSI***
***Princeton U***
***SNL***
***Think Tank, Inc.***
***UC Davis***
***UC Irvine***
***UCLA***
***UCSD***
***U Colorado***
***U Maryland***
***U Rochester***
***U Washington***
***U Wisconsin***

***Culham Sci Ctr***
***U St. Andrews***
***York U***
***Chubu U***
***Fukui U***
***Hiroshima U***
***Hyogo U***
***Kyoto U***
***Kyushu U***
***Kyushu Tokai U***
***NIFS***
***Niigata U***
***U Tokyo***
***JAEA***
***Hebrew U***
***Ioffe Inst***
***RRC Kurchatov Inst***
***TRINITI***
***KBSI***
***KAIST***
***POSTECH***
***ASIPP***
***ENEA, Frascati***
***CEA, Cadarache***
***IPP, J******ü******lich***
***IPP, Garching***
***ASCR, Czech Rep***
***U Quebec***

**                                           ****Supported by   **

**R. Raman 1),T.R. Jarboe 1), B.A. Nelson 1), D. Mueller 2)****, ****M.G. Bell 2), R. Bell 1), D. Gates 2), R. Kaita 2), H. Kugel 2), B. LeBlanc 2), **
**J. Menard 2), M. Ono 2), S. Paul 2), L. Roquemore 2), V. Soukhanovskii 3), S. Sabbagh 4), R. Maqueda 5), **
**R. Maingi 6),   M. Nagata 7)**
**1) University of Washington, Seattle, WA, USA**
**2) Princeton Plasma Physics Laboratory, Princeton, NJ, USA**
**3) Lawrence Livermore National Laboratory, Livermore, NJ, USA**
**4) Columbia University, New York, NY, USA**
**5) Nova Photonics, Princeton, NJ, USA**
**6) Oak Ridge National Laboratory, Oak Ridge, TN, USA**
**7) University of Hyogo, Himeji, Japan**

## Motivation For Solenoid-Free Plasma Start-up 

**Motivation For Solenoid-Free Plasma Start-up **

- The development of methods for solenoid-free current initiation would improve the prospects of the low aspect-ratio Spherical Torus as a CTF and fusion reactor
- Could also aid ARIES-AT design
- Of the three large tokamaks in the US (DIII-D, NSTX, C-MOD) only NSTX is engaged in solenoid-free plasma start-up research
- Transient Coaxial Helicity Injection (CHI) created plasmas in toroidal equilibrium carrying significant plasma current on HIT-II at Univ. of Washington
- Method has now produced 160 kA closed-flux current in NSTX
- World record for non-inductively generated start-up current in ST or tokamak

## Transient CHI: Axisymmetric reconnection leads to formation of closed flux surfaces

***Transient***** CHI: Axisymmetric reconnection leads to formation of closed flux surfaces**

**CHI for an ST: ****T.R. Jarboe, Fusion Technology, 15 (1989) 7**
**Transient CHI: ****R. Raman, T.R. Jarboe, B.A. Nelson, et al., PRL 90, (2003) 075005-1 **

- Demonstration of closed flux current generation
- Aided by gas and EC-Pi injection from below divertor plate region
- Demonstration of coupling to induction (2008)
- Aided by staged capacitor bank capability

## Simultaneous Requirements for Transient CHI

**Simultaneous Requirements for Transient CHI**

**** *****T.R. Jarboe Fusion Tech. 15, 7 (1989)**

- Bubble burst current*:
- 		= injector flux
- 		= flux foot print width
- 		= current in TF coil
- Time needed to displace toroidal flux
- For  typical voltage at the injector after breakdown ~500V  need ~1 ms to displace 600 mWb
- Energy for peak toroidal current:
- Exceed Energy for ionization and heating to 20eV (~50eV/D)
- For 2 Torr.L injected, need ~2kJ

## NSTX Plasma is ~ 30 x Plasma Volume of HIT-II

**NSTX Plasma is ~ 30 x Plasma Volume of HIT-II**

**Concept exploration device HIT-II **
-   Built for developing CHI
-   Many Close fitting fast acting PF coils
-   4 kV CHI capacitor bank

**Proof-of-Principle NSTX device **
-   Built with conventional tokamak components
-   Few PF coils
-   1.7 kV CHI capacitor bank

## Very high current multiplication (~70) aided by higher Toroidal Field:   Ip = Iinj(Tor Pol)                    

**Very high current multiplication (~70) aided by higher Toroidal Field:   I****p**** = I****inj****(********Tor ********Pol****)                    **

- 2006 discharges operated at higher toroidal field and injector flux
- Record 160kA non-inductively generated closed flux current in ST or Tokamak produced in NSTX

- Used LRDFIT reconstructions

**R. Raman, B.A. Nelson, M.G. Bell et al., PRL 97, 175002 (2006) **

**LRDFIT (J. Menard)**

## Discharges Without Absorber Arc Have High Current Multiplication Ratios (Ip/Iinj ~ 70)

**Discharges Without Absorber Arc Have High Current Multiplication Ratios (I****p****/I****inj**** ~ 70)**

## Fast Camera Fish-eye Movie of CHI Started Discharge

**Fast Camera Fish-eye Movie of CHI Started Discharge**

- Note:
-  CHI discharge evolution from the lower divertor plate region
-  Discharge contacting upper divertor region (Absorber arc)
- - Detachment from the injector region
- - Closed flux equilibrium decaying and shrinking in size

## CHI started discharge couples to induction and transitions to an H-mode demonstrating compatibility with high-performance plasma operation

**CHI started discharge couples to induction and transitions to an H-mode ****demonstrating compatibility with high-performance plasma operation**

- Te & Ne from Thomson
- Ti from CHERS
-  Central Te reaches 800eV
-  Central Ti > 700eV
- Note the broad density
- profile during H-mode phase

- Discharge is under full plasma equilibrium position control
- Loop voltage is preprogrammed
- Projected plasma current for CTF >2.5 MA  [Ip = Iinj(TorPol)]*
- Based on 50 kA injected current (Injector current densities achieved on HIT-II)
- Current multiplication of 50 (achieved in NSTX)

**CHERS:      R. Bell **
**Thomson:  B. LeBlanc**

***T.R. Jarboe, Fusion Technology, 15 (1989) 7**

## CHI started discharges use <15 kJ of capacitor bank energy to generate 100kA start-up plasma

**Discharge 128406 with center stack gas injection  **
**and higher NB power transitions to an H-mode**

**After the transition to H-mode in discharge 128406, T****e**** reaches 1 keV**

## Discharges produced after Li divertor plate conditioning are more reproducible and reach higher currents

**-  Improved performance after coupling to induction is similar to that seen on **
**   ****HIT-II with Ti gettering **
**-  The NB cryo-pumps were not used during either shot**

## Need auxiliary heating or metal divertor plates to compensate for increased radiated power with more capacitors

**Need auxiliary heating or metal divertor plates to compensate for increased radiated power with more capacitors**

**128400: 5mF (7.6kJ)**
**128401: 10mF (15.3kJ)**
**129402: 15mF (22.8kJ)**

**Plasma Current**

- Low-Z impurity radiation increases with more capacitors
- High Te in spheromaks (500eV) obtained with metal electrodes
- Test with partial metal outer divertor plates during FY09
- Upper divertor (UD) radiation also increases with more capacitors
- Need to reduce absorber arcs
- Absorber field nulling coils to be used during FY09
- Assess benefits 	of partial metal plates + Absorber coils
- Discharge clean divertor with high current DC power supply
- Use 350kW ECH during FY11

## In HIT-II nearly all CHI produced closed flux current is retained in the subsequent inductive ramp

**In HIT-II nearly all CHI produced closed flux current is retained in the subsequent inductive ramp**

- All three discharges have the identical loop voltage programming
- Coupling current increases as injected flux is increased
- Ip ramp-up begins after input power exceeds radiated power
- Auxiliary heating would ease requirements on current ramp-up system
- Radiated power can be decreased by using W or Mo target plates
- Start-up plasma (inductive or CHI) is cold (few 10s of eV)
- Reduce Low-Z line radiation

***HIT-II Results***

**R. Raman, T.R. Jarboe, R.G. O’Neill, et al., NF 45 (2005) L15-L19**
**R. Raman, T.R. Jarboe, W.T. Hamp, et al., PoP 14 (2007) 022504**

## NSTX has Demonstrated a Viable Plasma Startup Method for the ST

**NSTX has Demonstrated a Viable Plasma Startup Method for the ST**

- 160 kA closed flux current generation in NSTX validates capability of CHI for high current generation in ST*	*
*Modest requirements for increasing the CHI startup current to ~400kA*
*350 kW ECH to heat the CHI plasma*
*Metal divertor plates to reduce low-Z impurities*
*~20% increase in the capacitor bank voltage*
- Successful coupling of CHI started discharges to inductive ramp-up & transition to an H-mode demonstrates compatibility with high-performance plasma operation
- NSTX improvements over HIT-II
- demonstration of the process in a vessel volume thirty times larger than HIT-II on a size scale more comparable to a reactor,
- a remarkable multiplication factor of 70 between the injected current and the achieved toroidal current, compared to six in previous experiments,
- results were obtained on a machine designed with mainly conventional components and systems,
- favorable scaling with increasing machine size.