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We present a new scheme to produce preformed plasma waveguides to extend the acceleration length of laser-plasma accelerators. The plasma is produced in a hydrogen background by a high-voltage discharge between two hollow conic electrodes through a sequence of thin dielectric plates with apertures that fix the initial plasma diameter and position. The thickness of the dielectric plates is close to their aperture diameter and negligible when compared with the distance between them. This results in a fully ionized plasma cylinder that expands almost freely, evolving into a radial parabolic density profile with a minimum on axis as required for high-intensity guiding. In this paper, besides the description of the guiding device and its operation, we report the plasma density measurements of the first prototype with 16.6-mm length obtained by Mach-Zehnder interferometry as well as laser guiding results.