  A sealed high gas pressure detector working in pure argon is assembled. It
consists of a 5 cm $\times$ 5 cm PCB THGEM (THick Gaseous Electron
Multipliers). The detector structure and experimental setup are described. The
performances under high pressure of 2 atm mainly consist in selecting optimal
voltages for ionization region and induction region. The dependence of the
shape of Alpha particle spectra measured with relative gas gain on gas pressure
(1.3 $\sim$ 2.0 atm) has been studied. The 8 groups of relative gas gain versus
working voltage of THGEM expressed by weighting filed $E/P$ are normalized,
being consistent with theory. The results show that the air tightness of the
chamber is good measured by a sensitive barometer and checked with gas gain.
The experimental results are compared with Monte Carlo simulation on energy
deposition without gas gain involved.
