Neutral-beam injection of up to 2.5 MW into plasmas in the ISX-B tokamak (R0 = 0.93 m, a = 0.27 m, BT = 0.9–1.5 T, Ip = 70–210 kA, n̄e = 2.5–10×1013 cm−3) has created plasmas with volume-averaged beta of up to ∼ 2.5%, peak beta values of up to ∼ 9%, and root-mean-square beta values of up to ∼ 3.5%. Energy confinement time is observed to decrease by about a factor of two as beam power goes from 0 to 2.5 MW; the decrease is caused predominantly by the electron confinement time falling below the predictions of ‘Alcator scaling’ by a factor of 3–4 at high beam power. An empirical relationship of the form fits our measurements over a wide range of plasma parameters. The function f(Pb), where Pb is the beam power, is linear for Pb ≤ 1.2 MW but tends to saturate for 1.2 MW ≤ Pb ≤ 2.5 MW. Although the equilibria attained in ISX-B are predicted to be above the threshold for the ideal magnetohydrodynamic (MHD) ballooning instability, no evidence of these modes is observed.

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The ISX-A (Impurity Study Experiment) tokamak operated with major radius R =92 cm, minor radius a =26 cm, and relatively low toroidal magnetic field B T < 15 kG. 1 * 2 Only Ohmic heating was appliedo Studies of plasma confinement in this device yielded unusually favorable results in comparison with empirical scaling formulas., For example, the gross-energy-confinement times, r E = !&[/(n e T e +W|Ti)dv]/Po m E. 9 exceeded the values expected from the scaling of Jassby et at? by factors of 1-3 (lo6 average) and were larger than the values predicted by the Hugill-Sheffield formula 4 [with scaling l-l] by factors of 1.5-4.5 (3.1 average). At line-average densities (n e ) above 10 13 cm" 3 , the ISX-.A data are closest to the scaling proposed by Mirnov, 5 r E = (3 x 1(T 9 )a(cm) x/(A)« e l72 sec (n e is given in units of 10 13 cm" 3 ), although they still exceed the expectations by an average value of 1.2. Also, the maximum value of n e achieved before a major disruption occurred was 7xl0 13 cm" 3 , a factor almost 4.5 times larger than that anticipated by B T /R 0 scaling. 6 The largest values of toroidal beta, P T (0) equal to No. GA-A14133, 1976 (to be published); see also Ref" 5, above. 7 G. R. Hopkins and John M. Rawls, Nucl. Technol. 36, 171 (1977), and references contained therein. 8 P

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A new method of reactivity determination is proposed which avoids some of the difficulties of the present methods, such as dependence on detection efficiency and inherent source strength, and the requirement for a calibration near delayed criticality.