Trying to capture a Joni Mitchell appearance is like trying to capture an elusive shaft of sunlight filtering through breaking clouds. Sweet and pale, the mystique surrounding this beautiful performer makes her a choice character for a Thomas Mann story. Her fascination beckons through the clarity of her voice, the melancholy of her music and the poetic images of her songs. Hard indeed is the heart without a place for her.

Joni Mitchell went before the Assembly Hall Saturday night as a woman determined to shake off at least some of her goddess image. Although her first set with the L.A. Express fell flat on its face, and she didn't click until she came out by herself to do old favorites, she seemed to accomplish her aim. There was no doubt by the end of the concert that this was a different woman than that on the pedestal of her earlier albums.

Tom Scott and the L.A. Express began the concert with a generally solid half-hour of jazz-rock. They displayed themselves as very capable, but not exceptional, musicians.

Then, out came Joni, beginning with "This Flight Tonight" and "I'm a Radio." For the next half-hour, she performed mostly new material with the band as backup. Unfortunately, for most of the songs this arrangement just didn't work. The only thing the band really succeeded in doing was drowning out her voice and guitar. The tone was slickly commercial, rather than warmly personal.

An exception to this was their version of "Woodstock." While she managed to keep the bittersweet quality of her recorded version, the backup provided some of the energy for which the festival has been mythicized.

After the following intermission, she came out to do a memorable set primarily by herself on guitar and piano, with the occasional sensitive saxophone and flute of Tom Scott. She led off with an excellent version of "Cactus Tree," and moved into "Big Yellow Taxi." During this song, she asked the audience, which was by now eating out of her hand, to stop clapping along because it threw her off. Her attempts at perfection were not maintained throughout the performance though her voice was off several times, particularly on an otherwise-excellent "Both Sides Now."

She also revealed herself as the proud owner of a saucy tongue if need be. She told a photographer to "please put a silencer on your trigger," and replied to the traditional campus request for "White Rabbit," by saying, "Sweetheart, I'm slick but I'm not that slick."

She moved onto dulcimer for a bluesy-jazzy "All I Want," and a sprightly "Case of You." Kathi Bennett fans were treated to the original - Joni Mitchell playing piano "For Free."

The L.A. Express then came out to noticeably more restrained applause, but kept in the background for "Both Sides Now." Finishing with a well-done, Bette Midler-style boogie - she came back to do two encores, after one of the most sustained rounds of applause I have witnessed at the Assembly Hall.

Joni Mitchell showed herself to be a sensitive performer of her haunting songs that night. But she raised a host of new questions, too. Is she really cut out to be a club act and leader of a rock band? Does money speak louder than artistic integrity? Put more grossly, is she selling out?

Undoubtedly, she would say no. But I can't help feeling that the changes she seems so determined to make will end up like so much of the music she made during the first set - directionless.