In addition to what merlyn had said, you may use

$kernel->refcount_increment( $session_id, $refcount_name );
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to increase the reference count on you session; this will prevent the Kernel to garbage collect your session;

this means also that you need to

$kernel->refcount_decrement($session_id, $refcount_name );
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at some point, when you know the session is not needed anymore; this doesn't mean it will go away immediately, but it will be garbage collected when it doesn't have anything to do