We　measured　the　dependence　of　the　intensity　of　high　harmonics　on　the　gas　pressure　in　slab　waveguide.　The　phase　matching　of　61st　high　harmonic　was　obtained　according　to　the　quadratic　dependence　of　the　increase　in　efficiency　with　pres4ure.　For　higher　order　harmonics　such　as　H91,　periodic　oscillations　of　the　harmonic　intensity　with　gas　pressure　was　observed.　We　also　studied　the　effect　of　the　slit　width　of　the　slab　waveguide　on　the　high　harmonics　generation.　The　influence　of　the　slit　width　on　laser　transverse　mode　and　phase　mismatching　conditions　can　be　used　to　explain　this　effect.　(C)　2016　Elsevier　GmbH.　All　rights　reserved.