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1 Coupling solenoid-free Coaxial Helicity Injection started discharges to induction in NSTX R. Raman 1), B.A. Nelson 1), D. Mueller 2), S.C. Jardin 2), T.R. Jarboe 1), M.G. Bell 2), H.W. Kugel 2), B. LeBlanc 2), R. Maqueda 3), J. Menard 2), M. Nagata 4), M. Ono 2) and The NSTX Research Team 1) University of Washington, Seattle, WA, USA 2) Princeton Plasma Physics Laboratory, Princeton, NJ, USA 3) Nova Photonics, Princeton, NJ, USA 4) University of Hyogo, Himeji, Japan Email address of main author: [email protected] Abstract Experiments in NSTX have now demonstrated the coupling of toroidal plasmas produced by the technique of Coaxial Helicity Injection (CHI) to inductive sustainment and ramp-up of the toroidal plasma current. This is an important step because an alternate method for plasma startup is essential for developing a fusion reactor based on the spherical torus concept. Elimination of the central solenoid would also allow greater flexibility in the choice of the aspect ratio in tokamak designs now being considered. In these new results from NSTX, the central Ohmic transformer was used to apply an inductive loop voltage to discharges with a toroidal current of about 100 kA created by CHI. The coupled discharges have ramped up to >700 kA and transitioned into an Hmode demonstrating compatibility of this startup method with conventional inductive operation. The electron temperature in the coupled discharges reached nearly 1 keV and the resulting plasma had low inductance, which is preferred for long-pulse high performance discharges. These results from NSTX in combination with the previously obtained record 160 kA non-inductively-generated closed-flux startup currents in an ST or tokamak in NSTX demonstrate that CHI is a viable solenoid-free plasma startup method for future STs and Tokamaks. 1. Introduction A method to start a tokamak plasma without reliance on the central solenoid would provide greater access to lower aspect ratio configurations and reduce reactor cost as a result of a simpler design. The benefits on plasma performance improvement as a result of operating at lower aspect ratio are now well known [1,2]. At extreme low values of the aspect ratio, such as in a spherical tokamak (ST) based reactor, due to the very restricted space for the central solenoid, elimination of the central solenoid is necessary [3]. Coaxial Helicity Injection (CHI), a method originally developed for spheromak formation is a promising candidate for solenoid-free plasma startup and for edge current drive during the sustained operating phase. The possibility of using CHI in an ST was first proposed in the late 1980’s [4]. At that time, it was generally believed that the development of non-axisymmetric perturbations of the plasma equilibrium was needed for plasma startup using the CHI process in STs. This approach was initially investigated in NSTX and in several other STs. However, in a significant development during the past few years, it was shown that for the purpose of plasma startup, axisymmetric reconnection can produce a high quality startup equilibrium. This method referred to as transient CHI was first demonstrated on the HIT-II experiment at the University of Washington [5]. As demonstrated on the HIT-II machine the method is compatible with tokamak and ST designs that use superconducting poloidal field coils, as rapidly changing coil currents are not required for generating the initial closed flux equilibrium. Using this method, solenoid-free plasma startup and subsequent coupling to 2 conventional inductive drive has now been successfully demonstrated on the NSTX device as well. 2. Transient CHI Startup NSTX has a major/minor radius of 0.86/0.68 m and a toroidal magnetic field at the nominal major radius up to 0.55 T. It is equipped with a central solenoid providing up to 0.7 Wb of inductive flux (double swung) which can generate plasma currents up to 1.5 MA. The outer poloidal field coils needed for equilibrium control are located about 0.5 m away from the plasma boundary. The entire plasma facing boundary of NSTX is composed of graphite tiles. NSTX routinely applies conventional helium and deuterium glow discharge cleaning as wall conditioning techniques. In the past, boronization of the plasma facing surfaces has been used but in the last two years, lithium coating of the lower divertor plates has been investigated and developed as an alternate wall conditioning method. On both the HIT-II and NSTX devices, CHI is implemented by driving current along externally produced field lines that connect the inner and outer vacuum vessel components in the presence of externally generated toroidal and poloidal magnetic fields. In both machines, the inner vessel components are electrically insulated from the outer vessel components using two toroidal ceramic breaks, one at the top and the second at the bottom of the machine. During CHI operation, the lower divertor plate region is referred to as the injector and the upper divertor plate region is referred to as the absorber region because the direction of the E x B drift is away from the injector and into the absorber region. As shown in Figure 1 a CHI discharge is initiated by first energizing the toroidal field coil and then the poloidal field coils located near the injector region to generate poloidal flux that connects the injector electrodes. On NSTX, two main coils near the injector region provide the required injector flux that connects the inner and outer divertor plates. Gas is then injected into a 100 Liter cavity below the lower divertor plates. The injected gas emerges through the gap between the inner and outer divertor plates. This is followed by discharging a small capacitor bank across the injector electrodes, which are the lower divertor plates. NSTX uses a 5 to 45 mF (typically 10 – 15 mF) capacitor bank charged up to 1.7 kV. At applied voltages of 1.6 kV or higher, the applied voltage is sufficient to cause the gas to breakdown and to produce a plasma discharge. At lower voltages, NSTX also benefitted from the injection of 10 kW of ECH at 18 GHz, which was also injected into the cavity below the lower divertor plates. This results in an injector current, which is the current provided by the discharging capacitor that flows through the plasma load. Typical injector currents in NSTX, during the absence of a condition known as an absorber arc, vary from 1.5 to 5 kA. The absorber arc is a condition during which part of the injected current flows along the surface of the absorber insulator. These currents are in the range required to meet a condition known as the ‘bubble burst’ condition. This condition requires that the injector current exceed a threshold value needed to overcome the poloidal field line tension of the injector flux. As this condition is satisfied the injector flux expands into the vessel and fills the vessel as shown by the fish-eye camera images in Figure 1. The size of the capacitor bank is chosen so as to provide adequate energy for the injected current to be maintained for the duration needed for the growing CHI discharge to fill the vessel. At about the time the CHI plasma fully fills the vessel, most of the capacitor bank energy is depleted, which causes both the voltage appearing across the injector and the injector current to rapidly diminish. This reduction in the injector current is further improved by using an actively switched ignitron that short-circuits the capacitor through a small resistive load. If the injector flux footprints are sufficiently close together in the injector region, then because the injector current is no loner able to drive the CHI plasma load, the 3 expanded CHI plasma detaches from the injector region through a process of axisymmetric reconnection near the injector region producing a closed flux equilibrium that now decays on a resistive L/R time scale. Fig.1: On the left, is a line drawing showing the main components in NSTX required for plasma startup using the CHI method. Top-right: CHI injector current and plasma current for a CHI-only started discharge and bottom-right: fast camera fish-eye images of the plasma at 7.2, 8 and 11 ms during the discharge. Note that after 10 ms the injector current is zero, so the CHI power supply is no longer driving the plasma load. The remaining toroidal plasma current flows in closed flux equilibrium, as seen in the fast camera image at 11 ms, which then decays resistively. A feature of CHI plasma generation using this method is that flux closure can be demonstrated unambiguously by the persistence of plasma current after the injector current has been reduced to zero [6]. This can be seen in the injector and plasma current traces shown in Figure 1. During recent experiments in NSTX, induction was applied to this closed-flux plasma equilibrium by ramping the current in the central solenoid to demonstrate coupling of the CHI produced current to conventional inductive operation. 3. Experimental Results In Figure 2, we show traces for the injector current, the plasma current, and the applied inductive loop voltage for two CHI started discharges that were coupled to induction. In these discharges approximately 3 kA of injector current produces about 100 kA of toroidal current initially. During the decay phase of this current, induction is applied from the central solenoid by ramping its current from zero with a rectifier power supply with an open-circuit voltage of 4 kV. The decrease in the applied loop voltage over time is due to resistance in the central solenoid circuit. The external poloidal field coil currents needed for equilibrium are preprogrammed during the first 40 ms of the discharge. After that the standard NSTX plasma control system [7] is used to control the plasma radial and vertical position, using real-time data from external flux loops and magnetic probes. This causes the decaying plasma current to ramp-up, reaching a peak value of 700 kA for shot 128401. In these discharges, starting at about 40 ms, neutral beams are also injected at a power of 3 - 4 MW to heat the plasma. During the neutral beam heating phase, discharge 128406 transitions into an H-mode at 175 ms. Evidence of the H-mode is the characteristic drop in divertor Dα emission and the development of a broad electron density (Ne) profile measured by the Thomson scattering 4 diagnostic (Figure 3). Figure 3 also shows the electron temperature (Te) and electron density from Thomson scattering for discharge 128401 that did not transition to an H-mode. The fact that a CHI initiated discharge is able to transition to a discharge that has features suitable for Figure 2: The CHI injector current, plasma current, applied loop voltage, Dα signal and the neutral beam power for an L-mode discharge (128401) and for a discharge that transitioned to an H-mode at 175 ms (128406). Both discharges have different gas programming. Figure 3: Te and density profiles from Thomson scattering diagnostic for the discharges shown in Fig. 2. The discharge that transitioned in to a Hmode reached central Te of 1 keV. a high-performance long-pulse operation bodes well for the application of this startup method to future machines. Initial attempts to add inductive drive to CHI initiated discharges resulted in no increase in the plasma current. However, in these discharges there was a significant increase in the O-II emission when the loop voltage was applied. It was not until extensive conditioning was performed in the form of D2 glow discharge cleaning (GDC) and electrode discharge conditioning that the plasma current could be increased by induction. Observation in NSTX is that as the size of the capacitor bank is increased above 10 mF, the radiated power signal approaches the input Ohmic power. Increase in the radiated power signal is correlated with large increases in the oxygen and carbon line radiation signals. These signals increase because with a larger power supply the magnitude of the injector current increases. At a higher level of injector current more of the electrode surface contaminants are liberated into the plasma and contribute to increased energy losses during startup. As demonstrated in experiments on HIT-II [5], a CHI discharge cannot couple to induction if the radiated power approaches the input Ohmic power. For these plasmas with about 3 – 4 V of applied loop voltage and at a plasma current of 100 kA, the input Ohmic power is less than 300 – 400 kW during the inductive coupling phase. Thus it is essential that during the startup phase that either (a) the radiated power be kept low through injector conditioning discharges or (b) some form of auxiliary heating system such as Electron Cyclotron Resonance Heating be used to heat the target plasma. Recently the possible benefits for CHI discharges of lithium coating applied to the lower divertor plates were also tested. During these experiments, lithium was evaporated from two ovens mounted at the top of the vacuum vessel onto the lower divertor region at a rate of 10 mg/minute for 10 minutes prior to the initiation of a CHI discharge. The evaporator system is described in Reference [8]. The immediate benefit of lithium was to make the CHI started discharges more reproducible. The application of lithium reduces recycling of deuterium at the electrode surfaces and it was anticipated that this would lower the density of the CHI produced plasma. At lower density the plasma should heat up to a higher 5 temperature, which should reduce the L/R decay rate and thereby increase the current at the time that induction was applied. A similar effect was seen on the HIT-II experiments when titanium gettering was employed as a wall conditioning technique. However, despite the benefit of lithium conditioning to the reproducibility of CHI startup in NSTX, the magnitude of the initial plasma current produced by CHI did not increase with the use of lithium. The reason may be that the amounts of lithium used in present experiments were too small to observe the pumping effect on the fast time scale of the plasma initiation. However, on a longer time scale wall pumping was observed as discharges after Li conditioning reached higher plasma currents after coupling to induction. Simulations using the TSC code Figure 4: Simulations using the TSC code (a) Shown are the poloidal flux contours at three times during the discharge evolution, (b) the attained plasma current and (c) the injector current. To understand and assess the full potential of the transient CHI method to NSTX and to future machines we have used the Tokamak Simulation Code (TSC) [9] to simulate NSTX CHI discharges. TSC, is a time-dependent, freeboundary, predictive equilibrium and transport code. It uses as input the NSTX vessel geometry and external circuit parameters. Generation of closed flux in TSC is as a result of an effective toroidal loop voltage induced by the CHI ejected poloidal flux that decreases as the injector current is reduced to zero. The code has been able to show consistency with earlier theory [4] for the scaling of CHI produced current with the injector and toroidal fluxes. These results in conjunction with experimental work on HIT-II and NSTX, two machines with different sizes, suggest that the amount of injector current required to pull the injector flux into the vessel increases with the injector flux but decreases with the toroidal field indicating that the scaling to future machines with stronger toroidal field is favorable. In Figure 4, we show results from such a simulation for a low current CHI produced discharge. The plasma evolves in much the same way as observed experimentally for a similar discharge shown in Figure 1. As in the experiment, the voltage to the injector is applied at 5 ms. To simulate the conditions in the experiment, the applied injector voltage is programmed to reduce to zero at about 10 ms. This causes the injector current, shown in Figure 4c, to reduce. The plasma current (b) increases to about 70 kA, it then decays over a time longer than that experimentally observed. The decay rate of the current could be increased by specifying a larger resistivity for the CHI plasma. The resulting poloidal flux contours are shown in Figure 4a. These show an initial open flux discharge expanding away from the injector and into the vessel and after about 9.5 ms the presence of a closed flux region is evident. Because TSC is an axisymmetric code and does not include reconnection physics, it was speculated that that there must be a toroidal loop voltage generated during the transient CHI process. To verify this conjecture, toroidal loop 6 voltage sensors were placed at different locations within the vessel boundary in the simulations. These show that during the time of the CHI plasma growth into the vessel a large negative loop voltage is induced as a result of the injected flux. However, as this flux begins to decay and reduce in magnitude, a modest positive loop voltage of about 2-3 Volts is generated, which is responsible for the generation of the closed flux surfaces. It is useful to note that in the actual experiment, the presence of oppositely directed field lines in close proximity to each other, in the presence if finite resistivity should naturally reconnect. The presence of a positive loop voltage would further facilitate the reconnection process, and make the generation of closed flux easier. Figure 5: Shown are results from a TSC simulation in which the toroidal field and the injector voltage was varied. For case (a) BT = 0.3 T, E-field=18 V/m, the maximum attained injector current was 6 kA and Ip was 90 kA. As BT was increased to 0.5 T (case b) at the same voltage the injector current reduced to 3 kA and Ip reduced to 60 kA. In case c, the E-field was increased to 30 V/m, this caused the injector current to increase to 5 kA and Ip to increase to 120 kA. Similar scans have been conducted on the HIT-II experiment. To test previous predictions of the role of toroidal field for CHI discharges, starting from a discharge such as the one shown in Figure 4, the toroidal field was reduced from 0.4 T to 0.3 T and in a subsequent simulation increased to 0.5 T. These are shown in Figure 5. Simulations show that as the toroidal field is reduced at fixed injector voltage, the injected flux more easily expands into the vessel. This is because of a larger increase in the injector current. The discharge at 0.3 T has a peak injector current at 6 kA, whereas at 0.5 T it decreases to 3 kA. Because of this, the discharge at 0.5 T at 9.5 ms has still not pulled into the vessel as much as the discharge at 0.3 T does at 8.8 ms. At higher toroidal field the injector current decreases because of an increase in the impedance of the plasma load. The higher toroidal field effectively increases the length of the field lines so that for a fixed applied loop voltage, the amount of current that can be driven is reduced. Again to test this hypothesis, starting from the discharge shown in Figure 5b, the applied voltage across the injector was increased. Now, at this higher value of the toroidal field and higher voltage the injector current increases to 5 kA, still 1 kA less than at 0.3 T, but this current is now sufficient to more easily pull the injector flux into the vessel. While more detailed simulations are required to precisely establish the relationship between the applied voltage, the injector flux and the toroidal field, the TSC results are consistent with the earlier predictions, from simple physics arguments [4]. Because the injected flux now more fully fills the vessel, the resulting CHI produced toroidal current increases above what was achieved at 0.3. The plasma and injector currents at 0.5 T and at the higher injector voltage are shown in Figure 5c. c) 7 It is this type of experimental optimization that allowed HIT-II to maximize the CHI produced plasma current. On HIT-II as the injector flux was increased, both the toroidal field and the CHI capacitor bank charging voltage both had to be increased to increase the magnitude of closed flux current. 4. Summary and Conclusions Using the method of transient CHI in NSTX, 160 kA of closed-flux toroidal current has been produced. Now, for the first time in NSTX, CHI started discharges have been successfully coupled to induction to show compatibility between CHI started discharges and the conventional inductive approach. While results similar to this have previously been demonstrated on the smaller HIT-II experiment, this is the first such demonstration on a large ST with a poloidal field configuration more prototypical of a compact ST reactor such as the Component Test Facility (CTF). Another significant new result is achieving a current multiplication factor up to 70, which is an order of magnitude larger than achieved in HIT-II and suggests a favorable scaling of the technique with machine size. The transitioning of the CHI-produced discharges in NSTX to a high confinement H-mode with low plasma inductance demonstrates the potential for the application of this method to future machines. Initial simulations using the TSC code are helping to understand the relationship between the injector current, injector voltage and the injector and toroidal fluxes, which is necessary for the extrapolation of CHI to larger machines. Acknowledgments We acknowledge the support of the NSTX team for operation of the machine systems and diagnostics. Special thanks are due to E. Fredd, R. Hatcher, S. Ramakrishnan, C. Neumeyer for support with CHI related systems, and to Dr. N. Nishino of Hiroshima University for providing a fast camera that was used in some of the experiments. The fast camera images in this paper are from a fast camera provided by Nova Photonics, Inc.. This work is supported by US DOE contract numbers FG03-96ER5436, DE-FG03-99ER54519 and DE-AC0276CH03073. References [1] PENG, Y-K M, et al., Plas. Phys. Cont. Fus., 47 B263 (2005) [2] BELL, M.G., BELL, R.E., GATES, D.A., et al., Nuclear Fusion 46, S565-S572 (2006) [3] NAJMABADI, F. and the ARIES Team, Fusion Eng. Design, 41, 365 (1998) [4] JARBOE, T.R., Fusion Technol. 15, 7 (1989) [5] RAMAN, R., JARBOE, T.R., HAMP, W.T., et al., Physics Plasmas 14, 022504 (2007) [6] RAMAN, R., NELSON, B.A., BELL, M.G., JARBOE, T.R., MUELLER, D., et al., Phys. Rev. Let., 97, 175002 (2006). [7] GATES, D., et al., Nuclear Fusion 46, 17 (2006) [8] KAITA, R., et al., “Plasma performance improvement with Lithium coated plasma-facing components in NSTX,” Proceedings of the 22nd IAEA Fusion Energy Conference, Geneva, EX/P4-9 (2008 [9] JARDIN, S.C., DeLUCIA, J.L., POMPHREY, N.J., Compt. Phys. 66, 481 (1986)