File download does not start with JQuery - php

Question

I'm trying to start the download of a file using JQuery. The file is created by a .php function.
The download DOES NOT start. It starts only if i open the .php file MANUALLY (commenting the isset and writing the data manually into the file).
This is the JQuery code:
$("#experimenter_dataset").click(function() {
var imei = $("#select_download_experimenters option:selected").text();
$.post("download_imei_activities.php", { imei:imei }).done(function( data ) {
});
});
This is the php code
if(isset($_POST["imei"])) {
$imei = $_POST["imei"];
$stmt = $pdo->prepare("SELECT * FROM performedactivity WHERE experimenter=:imei");
$stmt->bindParam(":imei", $imei, PDO::PARAM_STR);
//$output = $imei . ".txt";
$output = "";
if ($stmt->execute()) {
while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
$activity = $row["activity"];
$timestamp_start = $row["timestampstart"];
$timestamp_stop = $row["timestampstop"];
$date_hour_start = explode(" ", $timestamp_start);
$date_hour_stop = explode(" ", $timestamp_stop);
//$current = "";
$output .= $date_hour_start[0] . "\t" . $date_hour_start[1] . "\t" . $activity . "\t" . "START" . "\r\n";
$output .= $date_hour_stop[0] . "\t" . $date_hour_stop[1] . "\t" . $activity . "\t" . "END" . "\r\n";
//file_put_contents($output, $current, FILE_APPEND | LOCK_EX);
}
} else {
//echo json_encode($pdo->errorInfo());
}
// We'll be outputting a text file
header("Cache-Control: public");
header("Content-Description: File Transfer");
header('Content-type: text/plain');
header('Content-Disposition: attachment; filename='.$imei . ".txt");
header("Content-Transfer-Encoding: binary");
print($output);
//echo $output;
/*header("Cache-Control: public");
header("Content-Description: File Transfer");
header("Content-Disposition: attachment; filename= " . $output);
header("Content-Transfer-Encoding: binary");
readfile($output);*/
} else {
//echo "imei parameter is missing";
}