function [z,d] = gyrotronRadiusRT(R,theta)
% Based on the Radius and taper angle, the distance 
% between changes in angle is calculated.  This is 
% sort of the inverse of gyrotronRadiusTD, which 
% calculates radius based on distance and angle.
% The inputs are 'Radius' and 'Theta' (angle),
% hence the 'RT' in the function name.
%   Note: for the case of indeterminate angle (theta=0)
% INF is returned as the distance.
%
% Inputs:
%   'theta': [radians] a vector of theta angles 
%   that describe the increase in radius with increasing 
%   axial distance.
%   'R': [distance] the corresponding radius vector 
%   defining the points where theta changes.
% 
% Output:
%   'z': [distance] the cumulative axial distance vector.
%   'd': [distance] the axial spacing distance vector.
%
% Example:
%   R     = [1   2   3   2    1   1    2   ];
%   theta = [0.7 0.6 0.9 -0.5 0.6 -0.6 -0.8];
%   [z,d] = gyrotronRadiusRT(R,theta);
%   [X,Y,Z] = cylinder(R,50);
%   surf(X,Y,z'*ones(1,length(Z)) ); 
%   colormap autumn; axis equal;
%
% Last edit by Eunmi Choi and Colin Joye 7/15/03
A = R-[0 R(1:(end-1))];
B = tan(theta);
if( any( B./A <= 0 ) ),
    disp(['gyrotronRadiusRT.m: Radius and Theta values are inconsistent' ...
        'or theta is zero.']);
    d=[]; z=[];
    break;
end
d = R./abs(B);
z = cumsum( d );
% ------------ End gyrotronRadiusRT.m -------------