EQUILIBRIUM FEATURES OF BEAN- SHAPED SPHERICAL TOKAMAK PLASMAS WITH AN ERGODIC LIMITER C. Ribeiro 1 , V. I. Vargas 2 , J. J. E. Herrera 3 , E. Chávez 4 1 School of Electronic Engineering, Instituto Tecnológico de Costa Rica, Cartago, Costa Rica 2 School of Physics, Instituto Tecnológico de Costa Rica, Cartago, Costa Rica 3 Instituto de Ciencias Nucleares, UNAM, Mexico City, Mexico 4 Instituto Nacional de Investigaciones Nucleares, Salazar, Mexico Corresponding author e-mail: celso_ribeiro@hotmail.com Abstract— Equilibrium simulations of spherical tokamak bean-shaped plasmas in natural divertor geometry with an Ohmic regime are presented for the first time. Preliminary results in a non-self-consistent scenario suggest higher (factor up to 2) beta values can be attained for these plasmas compared to the more usual spherical tokamak natural divertor D-shaped geometry. This can be attained simultaneously (but independently) with setting an edge localised ergodization as observed via a Poincaré mapping, using a low level of perturbation to the plasma current (~2 percent). In combination with the closer proximity of the bean-shaped plasmas to the in- vessel limiters (also observed here) this leads to a higher beta limit which extrapolates favorably for a more compact reactor based in the spherical tokamak concept which is thus more economical and has a more even thermal power load. Keywords—Tokamaks; Spherical Tokamaks; Bean-shaped plasmas; Beta limit; Second-stability; Equilibrium reconstruction; Ergodic limiter; Poincaré Mapping I. INTRODUCTION A toroidal plasma moderately indented on the high field (inboard) side is expected to lead to additional stability to ballooning modes[1]. This configuration provides a theoretical higher beta limit by accessing the second-stability region. Such a scheme has been previosuly tried in the high aspect ratio tokamak PBX and beta has increased even without reaching the second stability regime[2]. The modified version of this device (PBX-M) showed a further increase in beta closer but below the second stability region via a closer passive limiters and strong auxiliary heating from neutral beams[3] Previous Poincaré map simulations for natural divertor Ohmic regime based on the spherical tokamak (ST) MEDUSA[4] equilibrium reconstruction have been performed with the setting of inboard tilted poloidal field coils which lead to a unique ST bean-shaped plasma while creating a small ergodic region[5,6,7]. The ST MEDUSA (Madison Educational Small Aspect ratio tokamak), was originally designed, built and operated at the University of Wisconsin at Madison [4]. This device has been donated to the Technological Institute of Costa Rica ("ITCR") and was renamed as MEDUSA-CR. We aim here to explore such bean-shaped ST equilibria and present a preliminary indication that the beta parameter can also increase in STs in Ohmic regime, while an ergodic limiter in natural diverted plasmas is set. These are highly desirable features for testing a more compact thus economical, and feasible design (no intolerable power load in the X-point divertor) towards a ST reactor in which H-mode regime can still be envisaged as initial results from the Ohmic naturally diverted H-mode plasmas achieved in ST PEGASUS device suggests[8]. II. MEDUSA-CR DEVICE The device's main characteristics are[4]: plasma major radius R o = 9-14cm, plasma minor radius a = 4-10cm, aspect ratio A ≡ R o /a = 1.5 (1.35 min.), toroidal field at plasma major radius B T = 0.3T (0.5T max.), plasma current I p = 20kA (40kA max.), 1ms (3ms max.) pulse. The plasma is limited top/bottom at one toroidal location by movable stainless rail limiters. The vessel is made mainly of glass which allows real time field penetration. Titanium gettering is planned for conditioning, and lithium can also potentially be used[9]. Two unique planned systems for MEDUSA-CR are the external Alfvén Wave antennas and an ergodic limiter, both placed externally to the vessel, and aimed for studying RF heating/current-drive and plasma-wall interaction with simultaneous creation of bean- shaped plasma, respectively. “ITCR”, IAEA, and National Instruments of Costa Rica 978-1-4799-0171-5/13/$31.00 ©2013 IEEE