I need to plot V_a=q*V_m* cos(w_ot+) in matlab with a 20% step change in voltage gain at t=0.1s like below

where

q (voltage gain)=0.5 ;V_m=720v; f_o=50 Hz;=0

what is the change in this code to the voltage will continue to rise (with a 20% step change in voltage gain) after t=0.1s

Code:

clc; clear all; close all;

fs = 1000; t = 0:1/fs:0.2;

Vm = 720; % Volt f0 = 50 ;% Hz q = 0.5 ; % Voltage Gain alpha = 0;

Va = Vm*q*cos(2*pi*f0*t + alpha);

Vb = Vm*(q + 0.2*q*(t == 0.1)).*cos(2*pi*f0*t + alpha);

figure, plot(t,Va,'r',t,Vb,'b'); xlabel('Time'); ylabel('Va and Vb')