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 Its hardware can be withdrawn before a fusion plasma enters a nuclear burn phase.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)
