Absence of tilt and shift modes in a reversed field pinch with no shell

Summary form only given. It has been proposed that the RFP (reversed field pinch) is subject to tilt and shift mode instabilities due to the similarity of its field configuration to that of the spheromak. The Reversatron II RFP has been operated with N=0.5 (the usual configuration); with N=-0.5, which should increase the growth rates of the tilt and vertical shift; and with N=+1.35, which should increase the growth rate of the horizontal shift. Stability was investigated by Fourier analysis of fluctuations in B₀ measured by 23 sine coils and 23 cosine coils distributed toroidally around the vacuum chamber. The discharge duration of 600 μs is significantly longer than the 4-μs penetration time of the vertical field into the vacuum chamber. There is no evidence of growing tilts or shifts for any of the three field indices. The only difference noted is that for N=+1.35, rigid vertical motion has, as expected, a lower fluctuation level than for N=+0.5. The data indicate that the large RFP experiments with thin shells which are under construction should not have problems with tilt or shift stability