Evidence for a Pressure-Driven Instability in the CTX Spheromak

Abstract

Recent improvements in the operation of the CTX spheromak device have produced discharges containing evidence for a pressure-driven instability. The instability leads to a distinct event in the discharge, which can be studied in detail. Data are presented which reasonably discount Taylor relaxation of the current profile as the cause of the event. The critical value of the normalized pressure gradient has been measured and is compared with the Mercier limit.PACS numbers: 52.55.Hc, 52.35.Py A spheromak is a toroidal magnetic configuration with large toroidal and poloidal plasma currents which generate most of the internal magnetic field. This configuration is attractive for a fusion reactor because it is compact, has a high level of Ohmic heating power, and thus, may not require auxiliary heating (such as neutral beam or rf injection), and has a high engineering p (pcng&P/Blaw). However, the theoretically predicted internal p (p yo \ocP/(B 2 \ 0 \) calculated with the Mercier criterion is small, ranging from « 0.2% for the "classical" spheromak 1 to (1-2)% for nonspherical cross sections, 1 " 3 to ?£7% for the spheromaks with current holes. 2 " 4 In fact, many other toroidal magnetic configurations have low predicted p limits. Tokamaks are theoretically limited by the ballooning criterion 5 to ;S(5-10)%, and reversed field pinch equilibria stable to current-driven resistive tearing modes have a predicted p limit 6 of « 20%. The experimental significance of predicted p limits is often unclear, since other effects might limit plasma performance. If the p limits could be reached, the expected plasma behavior is often unknown. Tokamaks historically have had difficulty reaching the predicted p limit, 5 but recently large tokamaks have been used to explore plasma conditions at reactor relevant p values. The highest values obtained are empirically found to follow a scaling law of the form Pmax^3I/aB On %, MA, m, and T) 7,8 ; however, this can be consistent with either ballooning mode or kink mode limits. 7 In addition, operation at this p limit is found to be disruptive 7 in some cases and in other cases a degradation of energy confinement without disruption is observed. 8 Reversed field pinches normally operate 6 with roughly constant p («10%), indicating an empirical limit, but the value is less than the predicted value, which does not include the localized resistive interchange mode (sometimes called the "g mode"). In contrast, spheromaks often have p values above those predicted. 9 ' 10 In this Letter, strong evidence for a pressure-driven instability in CTX 11 spheromaks is presented. The instability leads to a distinct event in the discharge which can be analyzed in detail. It is found that when a particular threshold value in the pressure gradient is exceeded, internal plasma is expelled toward the wall in 10-20 jus. The resulting temperature and density profiles are both hollow (about the magnetic axis), strongly indicating a magnetic flux interchange. These hollow profiles are short lived, and evolve back to mod...

Suppression of MHD Fluctuations Leading to Improved Confinement in a Gun-Driven Spheromak

“…Assuming T e T i would double this value to give a total volume average ͗b͘ 8%. This is the highest value reported for a driven spheromak and comparable to the highest values reported for a decaying spheromak [20] (transient values of b e,local ഠ 20% have been reported prior to pressure driven instability [21]). …”

Observation of a pressure limit in a gun-driven spheromak

“…In toroidal magnetic fusion devices, transport can be enhanced by current and pressure driven magnetic instabilities, 2 and particularly by chains of magnetic islands. 7 In most magnetic fusion devices pressure limits are usually expressed in terms of the obtainable plasma ␤ ͑=nT/ B 2 ͒ for which a range of scalings have been obtained by considering the deleterious effect of both current and pressure-driven modes ͑see, e.g., Troyon in Ref. The ideal shift and tilt modes are stabilized by a closefitting copper shell, 4 and more recently, current-profile control has allowed the fluctuations from ideal current-driven modes to be suppressed, 5 but islands sometimes remain with finite amplitude, in agreement with fieldline calculations for equilibria in the presence of field errors.…”

Temperature and Lifetime Measurements in the SSX Wind Tunnel

“…The ion temperature dropped below the electron temperature after ∼65 µs, but this does not preclude ion-electron collisions as the primary cooling channel for ions. The ohmic power dissipated during the decay of the magnetic fields preferentially heats electrons [24,25]. It is possible that the electron transport rate controls the evolution of the plasma late in time.…”