Сложение строк

{assign var=field_name value=$field.name|cat:'-main'}

The way you are doing it is call the "short form" of assign, you just need to use the correct quoting mechanism:

 {$yes_src="`$const1`yes`$const2`"}

Use assign:

{assign var="yes_src" val="`$const1`yes`$const2`"}

Use cat:

{$const1|cat:"yes"}{$const2}

You can also simply put the variables next to one another without assigning it to a variable:

{$const1}yes{$const2}

... no variable needed.

{capture assign=yes_src}{$const1}.'yes'.{$const2}{/capture}

{$yes_src}