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Nucl. Fusion 59 (2019) 112009
In order to further improve FRC performance as well as to overcome engineering constraints mentioned above, the C-2U device has been again upgraded to C-2W (also called ‘Norman’, shown in figure 1). Substantial facility changes have taken place, in which the C-2U device was completely dismantled and the brand-new experimental device C-2W was constructed within one year. The C-2W device has the following key subsystem upgrades from C-2U: (i) higher injected power (up to ~21 MW), optimum and adjustable ener- gies (15–40 keV), and extended pulse duration (up to ~30 ms) of the NBI system; (ii) installation of inner divertors with upgraded edge-biasing electrode systems, which allow for higher biasing voltage and longer pulse operation (30 + ms), and in-vacuum fast-switching magnet coils (current up and down in a few milliseconds) inside the inner divertors that allow optimization of the magnetic field profile for effective FRC translation as well as increased thermal insulation of the peripheral plasma; (iii) increased overall stored energy in the FRC formation pulsed-power system to produce better target FRCs for effective NBI heating and current drive; (iv) fast external equilibrium/mirror-coil current ramp-up capability for plasma ramp-up and control; (v) installation of trim/saddle coils for active feedback control of the FRC plasma as well as for error field correction; and (vi) enhanced overall diagnostic suite to investigate and characterize both core FRC and open- field-line plasma performances.
The main goals of the C-2W experimental program are as follows: (i) demonstrate plasma ramp-up by NB heating and current drive; (ii) improve edge/divertor plasma per- formance to achieve high electron temperature both at the plasma edge and inside the core; (iii) develop plasma control on the time scale significantly longer than L/R vessel-wall time and plasma confinement times, and demonstrate con- trollable plasma ramp-up; and (iv) explore a wide range of plasma parameters such as plasma temperature, magnetic field and plasma size to confirm the previously emerged/obtained energy confinement scaling [6, 16]. There are also several key intermediate milestones to accomplish in scientific and engi- neering aspects on the C-2W experimental program in order to ensure that each subsystem of the machine operates within its designed parameters as well as to accelerate the program towards the main goals: producing robust FRC formation and translation through inner divertors; establishing adequately controlled magnetic-field structures in the inner divertor area to change it from the initial guiding straight magnetic field for FRC translation (in operations phase 1.1: OP1.1) to flared magnetic field structure (in operations phase 1.2: OP1.2), as can be seen in figures 1(b) and (c); transferring edge-biasing regions from outer to inner divertors as inner-divertor magn- etic field gets flared/expanded; demonstrating first-of-the-kind active-feedback magnetic flux and plasma control on FRC experiments; demonstrating sufficient particle refueling for plasma ramp-up; demonstrating effective external magnetic field ramp-up while increasing NB energy from 15keV to 40 keV (in operations phase 2: OP2); and, establishing effec- tive wall conditioning in the CV and high vacuum/pumping capability in all four divertors to reduce outgassing/secondary electron emissions from vessel-wall surfaces, thus improving
H. Gota et al
open-field-line plasmas. Table 1 lists key machine settings/ parameters as well as functionalities in those various operating phases to briefly summarize which subsystems are required or can be used for experiments.
In the paper, the C-2W experimental apparatus and plasma diagnostic suite including newly-upgraded/developed subsys- tems are described in section 2. Key highlights and accom- plishments of early C-2W experimental campaigns as well as characteristics of newly-obtained FRCs under different oper- ating regimes such as in OP1.1 and OP1.2 are described in section 3, where we present a few examples of different edge- biasing schemes and its effect on FRC performance. Lastly, a summary is provided in section 4.
2. C-2W experimental device, Norman
2.1. Experimental apparatus overview
The C-2W experimental device, shown in figure 1(a), is the world’s largest theta-pinch, CT collisional-merging system, newly built at TAE Technologies to form high flux, high temperature, stable and long-lived FRC plasmas. The C-2W device was constructed in the same place as the preceding C-2U device was located; the previous C-2U experimental program operated for about a year and then the machine was dismantled completely for C-2W. Figures 1(b) and (c) illus- trate typical FRC magnetic flux lines with density contours in the C-2W device under two different operating conditions with and without magnetic field flaring in the inner diver- tors; these density contours are obtained from a 2D multifluid force-balanced equilibrium calculation performed with the LReqMI equilibrium code [20].
The C-2W device has ~30 m in overall length and con- sists of the central confinement section surrounded by two newly-installed inner divertors, two field-reversed theta-pinch (FRTP) formation sections, and two outer divertors. These seven sections can be independently isolated by large-bore vacuum gate valves. The CV is made of Inconel with an inner radius rw ~ 0.8 m and thin wall thickness whose resistive wall time is about 2–3ms; this allows for magnetic-field ramp up/down as required during a plasma discharge. Because of the relatively short wall time of the CV, adequately controlled external magnetic field is critical to FRC plasma confinement and plasma ramp-up. The divertor vessels are made of stain- less steel and have a large internal volume (~15 m3 each) to create high volumetric pumping during a plasma discharge; furthermore, each divertor has its own internal cryogenic pumping system with titanium gettering and LN2 cooling to enhance its pumping capacity. The formation tubes are made of quartz, which are approximately ~3.5 m in length and ~0.6 m in diameter. The overall C-2W device accommodates an ultra-high vacuum (typical vacuum level is at around low 10−9 Torr range, achieved by a number of high-speed turbo pumps in the CV, divertor and formation sections) with adequately set up wall conditioning systems such as titanium (sublimation/ cathodic arc) gettering and LN2 cooling systems. Using metal gaskets for vacuum seals/boundaries also contributes to this excellent vacuum level.
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