Confinement of high density electron plasmas in a strong uniform magnetic field was experimentally studied in a multi-ring trap (MRT). The trap was housed inside a bore tube and surrounded by a superconducting solenoid. A 5 T magnetic field was used to provide radial confinement while an electrostatic harmonic or rectangular potential well was used for axial confinement. For trapped electrons of N = 1.2 × 1010 in a harmonic potential well (HPW) with the trap length of LT = 320 mm, the plasma lifetime was about 104 s and it became much longer at lower N = 4.5 × 109. Such long holding times were achieved without application of rotating electric fields. Contrastingly, in a rectangular potential well (RPW), the plasma of N = 1.2 × 1010 under the same trap length expanded to cover the whole Faraday Cup within 200 s, where its radial expansion rate was η = 3.2 × 10−2 mm/s, which was 20 times faster than HPW. The lifetime for RPW became shorter with increasing LT and scaled as 1/[LT]2. This scaling found for high density plasmas is similar to the obtained one with different Penning–Malmberg traps at UC San Diego (USCD).