Why couldn't Triana have her halo orbit, and therefore a much longer continuous-coverage life? Why put DSCOVR in a Lissajous orbit instead of a halo orbit (to stay out of Sun exclusion zone)?
This is mostly a "guess" based upon what we experienced while inserting Wind into a halo orbit from a Lissajous orbit. A Lissajous orbit is cheaper, fuel-wise, because the out-of-plane component of the orbit is disconnected from the two in-plane components. To change from a Lissajous to a halo orbit, three big maneuvers were required on June 26, 2020 (HT-1), August 31, 2020 (HT-2), and November 9, 2020 (HT-3). There was a small trim maneuver on September 24, 2020 (HT-2 Trim) too. The stats on the manuevers are as follows:
HT-1
- Fuel used: ~5.518 kg
- $\Delta v$ (radial): < 1.0 m/s (i.e., Sun-Earth direction)
- $\Delta v$ (axial): >11 m/s (i.e., out of ecliptic plane)
- $\Delta v$ (total): ~11.667 m/s
- Fuel Remaining After: ~46.648 kg
HT-2
- Fuel used: ~9.243 kg
- $\Delta v$ (radial): ~0.923 m/s
- $\Delta v$ (axial): ~17.794 m/s
- $\Delta v$ (total): ~17.818 m/s
- Fuel Remaining After: ~37.405 kg
HT-2 Trim
- Fuel used: ~0.017 kg
- $\Delta v$ (radial): ~0.036 m/s
- $\Delta v$ (axial): ~0.0 m/s
- $\Delta v$ (total): ~0.036 m/s
- Fuel Remaining After: ~37.388 kg
HT-3
- Fuel used: ~0.732 kg
- $\Delta v$ (radial): ~1.265 m/s
- $\Delta v$ (axial): ~0.273 m/s
- $\Delta v$ (total): ~1.294 m/s
- Fuel Remaining After: ~36.656 kg
Normal Station Keeping Maneuver (both orbit types)
- Fuel used: ~0.100-0.150 kg
- $\Delta v$ (radial): ~0.20-0.30 m/s
- $\Delta v$ (axial): ~0.0 m/s
- $\Delta v$ (total): ~0.20-0.30 m/s
You'll notice the biggest burns for Wind were an axial burns that used >14 kg of fuel or $>$28 m/s $\Delta v$, compared to the normal station keeping maneuvers of which are ~100 times smaller.
So I think the critical aspect here is that a halo orbit requires a large $\Delta v$ directed out of the ecliptic plane whereas with the Lissajous one can insert for less $\Delta v$. Given that DSCOVR was going to need to do a lot of station keeping and pointing-requirement maneuvers, fuel consumption was a big concern when it launched. That is, fuel was the primary mission lifetime constraint for the DSCOVR mission, so reducing insertion fuel costs was a big priority. Unfortunately, one of the laser gyros had an anomaly in June 2019 but was brought back to normal operations by March 2020.