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22-27 October 2018
Mahatma Mandir Conference Centre
Asia/Kolkata timezone
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Advancing Local Helicity Injection for Non-Solenoidal Tokamak Startup

25 Oct 2018, 14:00
4h 45m
Mahatma Mandir Conference Centre

Mahatma Mandir Conference Centre

Gandhinagar (nearest Airport: Ahmedabad), India
Poster P6 Posters

Speaker

Dr Michael Bongard (University of Wisconsin-Madison)

Description

Robust non-solenoidal startup methods may simplify the cost and complexity of next-step burning plasma devices, and especially STs, by removing the need for a solenoid. Experiments on the $A\sim1$ Pegasus ST are advancing the physics and technology basis of Local Helicity Injection (LHI). LHI creates high-$I_p$ tokamak plasmas without a solenoid by injecting helicity with small current sources in the plasma edge. Its hardware can be withdrawn before a fusion plasma enters a nuclear burn phase. Flexible injector placement offers tradeoffs between physics and engineering goals. They are tested with LHI systems on the low-field-side (LFS) and the high-field-side (HFS) of Pegasus, producing plasmas predominantly driven by non-solenoidal induction and DC helicity drive ($V_{LHI}\sim B_{inj}A_{inj}V_{inj}$), respectively. Record LHI plasmas with $I_p = 0.2$ MA, $T_e > 100$ eV, $n_e\sim10^{19}$ m-3, and $Z_{eff} < 2.5$ are attained. A predictive 0D power-balance model describes experimental $I_p(t)$ and partitions the active current drive sources. It uses improved inductance models that have been extended to $A\sim1$. The analysis confirms the dominance of induction in LFS LHI and DC helicity drive in HFS LHI. Model projections for NSTX-U suggest MA-class LHI startup may be feasible with a modest LFS system. An advanced port-mounted LHI system is being deployed on Pegasus to test this path. Studies of HFS scenarios find favourable, positive scalings of $I_p$ with $V_{LHI}$ and $T_e$ with $B_T$. If they hold at higher $B_T$, LHI may directly offer useful targets for RF and NBI current drive. High-frequency MHD activity plays a strong role in LHI current drive, in addition to $n=1$ modes previously found in NIMROD simulation and experiment. A new regime of reduced MHD activity was discovered where the $n=1$ activity is suppressed. In this regime, high-frequency activity increases, LHI CD efficiency improves, and long-pulse plasmas are sustained with $V_{IND}\sim0$. LHI facilitates access to the favourable low-$A$ ST regime with non-solenoidal sustainment, high $\kappa$, low $\ell_i$, and high $\beta_t$. Low $B_T$ LHI operation has led to record $\beta_t=100$%, high $\beta_N$, and a minimum-$|B|$ well that may positively affect turbulence, transport, and fast particle confinement. Discharges at highest $\beta_t$ disrupt at the ideal no-wall MHD limit.
Country or International Organization United States of America
Paper Number EX/P6-34

Primary author

Dr Michael Bongard (University of Wisconsin-Madison)

Co-authors

Mr Alexander Rhodes (University of Wisconsin-Madison) Ms Carolyn Schaefer (University of Wisconsin-Madison) Mr Christopher Pierren (University of Wisconsin-Madison) Mr Cuauhtemoc Rodriguez Sanchez (University of Wisconsin-Madison) Mr Grant Bodner (University of Wisconsin-Madison) Ms Jessica Pachicano (University of Wisconsin-Madison) Dr Joshua Reusch (University of Wisconsin-Madison) Dr Justin Perry (University of Wisconsin-Madison) Mr Justin Weberski (University of Wisconsin-Madison) Dr Marcus Burke (University of Wisconsin-Madison) Mr Nathan Richner (University of Wisconsin-Madison) Prof. Raymond Fonck (University of Wisconsin-Madison)

Presentation Materials

Paper