Before getting an answer for it first we have to understand nuclear fusion.
Nuclear fusion is a thermo-nuclear reaction in which two light unstable nuclei will form a heavy stable nuclei. In this process there will be some mass defect which will be converted into energy as per Einstein's mass energy equivalence theorem.
The theorem is stated as [tex]E = mc^{2}[/tex] where c is the velocity of light and m is the mass converted into energy.
One take an example of fusion in sun where 4 hydrogen atoms combine to form a helium nucleus which are explained below-
[tex]H_{1}^{1}+H_{1} ^{1} =H_{1} ^{2} +e_{+1} ^{0} +energy[/tex]
[tex]H_{1} ^{2} +H_{1} ^{1} =He_{2} ^{3} +energy[/tex]
[tex]He_{2} ^{3}+H_{1} ^{1} =He_{2} ^{4} +e_{+1} ^{0} +energy[/tex]
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[tex]4H_{1} ^{1} = He_{2} ^{4} +2e_{+1} ^{0} +energy[/tex]
Here [tex]e_{+1} ^{0}[/tex] is the positron.
In this process very high temperature is needed which is approximately equal to the temperature of the sun i.e [tex]10^{6} K[/tex]
Such temperature is very difficult to initiate the reaction on the earth surface. It should be carried out in an sustainable way also .Otherwise It will cause nuclear hazards.
