Is This F# Recursive Example Tail Recursive?

Question

I have a payment/lockbox file archiving utility that is called several times per day and with different parameters depending on the files that need archiving.

I know I can flatten argv into a string using a simple method like the following:

let r = Array.toList(argv)

let a = 
    r
    |> String.concat " "

so the utility can send me email with the parameters used to call it without all the brackets from a string [].

However, I wanted to see if I could flatten argv using tail recursion, so is the following example tail-recursive?

let squash_str_array (str_arg : string[]) =
    let array_len = str_arg.Length
    let rec inner_squash (inner_str_arg : string []) (build_up_list : string) idx =
        if idx = array_len then
            build_up_list 
        else
            let build_up_list = inner_str_arg.[idx] + " " + build_up_list
            inner_squash inner_str_arg build_up_list (idx + 1)

    inner_squash str_arg "" 0

[<EntryPoint>]
let main argv =
    let test_array = [| "/mu"; "/al"|]
    let new_list = squash_str_array test_array
    .
    .
    .