Particles and lasers | Citadel docs

fx.particle(name, pos, { count = 1, spread = 0, velocity = nil, speed = 0, color = nil, size = 1 })

name is the particle: dust, end_rod, explosion, firework, flame, note or totem_of_undying. Any other name sends nothing.
pos is a vec3, or a list of vec3s, with the same particle at every point. Each point is sent only to players in the same room within 64 blocks of its origin in 3D, including the boundary, before spread or velocity.
count particles appear at each point, in a random box spread blocks out from it. spread is a number or a vec3 (one size per axis).
velocity (a vec3) sends one particle that moves along it. It cannot go with count above 1 or with spread. speed scales it and is 1 with a velocity.
Without velocity, speed is how fast the particles scatter. 0 is still.
color and size (0.5 to 4) are for dust only. Dust is white and size 1 unless they say. On any other particle they are an error.
A client whose version lacks a particle does not get it. Before 1.13, dust is one particle a point, in its colour, with no count, spread or size. 1.7 shows dust only.

-- a beam: dust at points along a line, in one call
local points = {}
for i = 0, 7 do points[i + 1] = from:lerp(to, i / 7) end
fx.particle("dust", points, { color = colors.red, size = 1.5 })

fx.particle("flame", pos, { velocity = vec3(0, 1, 0), speed = 0.5 })        -- a jet
fx.particle("firework", pos, { count = 40, spread = 2, speed = 0.05 })       -- a burst

Particles fade on their own. A beam that stays is drawn again every other tick:

local beam = { from = vec3(0.5, 76, 0.5), to = vec3(0.5, 66, 0.5), color = colors.cyan }
local conn = time.on_tick(function(tick)
    if tick % 2 ~= 0 then return end
    local points = {}
    for i = 0, 7 do points[i + 1] = beam.from:lerp(beam.to, i / 7) end
    fx.particle("dust", points, { color = beam.color, size = 2 })
end)
tween(beam, { to = beam.to + vec3(4, 0, 0) }, { duration = 1 })   -- its fields are plain; set or tween them
conn:cancel()                                                     -- and it is gone