While thinkorswim doesn't provide charts under 1 minute, there is a way to view sub-minute charts, but only for non-tradable indices, such as the SPX.
For non-tradable indices like the SPX, tick charts are set to have approximately one tick per second. Therefore, if you want to see a 30-second chart of the SPX, use a 30-tick chart.
While this may not be an exact match, it should be relatively close, according to TOS Support. One way to check how close they are is to bring up a 1-minute chart for the SPX, which TOS naturally offers. Then bring up a tick chart with a tick size of 60. If the SPX tick chart generates one tick per second, then the 60-second tick chart should appear very close to the 1-minute chart. Below is a side-by-side comparison of a 60-tick and 1-minute chart in TOS, which appear virtually identical.
With this realization, those looking to create a 30-sec chart should probably use a 30-tick chart rather than 33-tick chart as some have used, for maximum compatibility. Here's a side-by-side comparison of SPX 30-second in TradeStation (left) and 30-tick in thinkorswim (right).
Again, this will not work for all 30-tick charts - just the non-tradeable ones. As an example of it not working, compare a 30-sec SPY chart in TS vs. a 30-tick SPY chart in TOS, as shown in the following image. SPY is a tradeable ETF, and therefore the tick chart shows actual trade activity and is not synthetic. Just a quick check of the time axis at the bottom shows the entire chart on the right is encompassed by just the last few bars on the left (the right chart shows just the last 4 or 5 minutes, while the left chart shows over an hour of price history). So only use non-tradeable indices when trying to view sub-minute charts in TOS.