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[en] Bose–Einstein condensation (BEC) at normal temperature (T=343K) has been observed because an electric field was first applied. There are two ways to achieve phase transition: lower the temperature of Bose gas or increase its density. This article provides more appropriate method: increase the voltage. In theory, 3s and 3p states of sodium are not degenerate, but Na may be polar atom doesnot conflict with quantum mechanics because it is hydrogen-like atom. Our innovation lies in we applied an electric field used for the orientation polarization. Na vapor was filled in a cylindrical capacitor. In order to determine the polarity of sodium, we measured the capacitance at different temperatures. If Na is non-polar atom, its capacitance should be independent of temperature because the nucleus of atom is located at the center of the electron cloud. But our experiment shows that its capacitance is related to temperature, so Na is polar atom. In order to achieve Na vapor phase transition, we measured the capacitance at different voltages. From the entropy of Na vapor S=0, the critical voltage V_c=68volts. When V< V_c, many atoms are in random orientation S>0; when V>V_c, the atoms become aligned with the field S<0, phase transition occurred. When V=390 volts »V_c, the capacitance decreased from C=1.9C_0 to C≈C_0 (C_0 is the vacuum capacitance), this result implies that almost all the Na atoms (more than 98%) are aligned with the field, Na vapor entered quasi-vacuum state. We create a BEC with 2.506×10"1"7 atoms, condensate fraction reached 98.9%. This is BEC in momentum space. Our experiment shows that if a Bose gas enters quasi-vacuum state, this also means that it underwent phase transition and generates BEC. Therefore, quasi-vacuum state of alkali gas is essentially large-scale BEC. This is an unexpected discovery. BEC and vacuum theory are two unrelated research areas, but now they are closely linked together. The maximum induced dipole moment d_i_n_d≤7.8×10"−"1"5 e cm can be neglected. Ultra-low temperature is in order to make Bose gas phase transition, we achieve the phase transition by the critical voltage, so the ultra-low temperature is not necessary. According to the standard proposed by Ketterle, although we didn’t use laser cooling atoms, our experiment is real ideal BEC. Na material with purity 99.95% was supplied by Strem Chemicals Co., USA. Our experiments are easily repeated because low temperature is not necessary.