Federal prosecutors recommended Friday that Robert Walker, the discredited former head of Bixby Energy Systems, be sentenced to 30 years in prison for his decadelong fraud that bilked nearly 2,000 investors out of $57 million.

In an eight-page legal filing, the U.S. attorney's office called Walker "absolutely incorrigible" and said, "No sentence is too long to stop the juggernaut of Walker's deceit."

The government acknowledged that 30 years would be a life sentence for the 71-year-old inventor of the Sleep Number mattress.

"Sometimes a just sentence is a sad one," said the filing by assistant U.S. attorneys David MacLaughlin and Benjamin Langner, who prosecuted the case.

Walker was convicted last March by a jury on a 17-count indictment of defrauding investors, tax evasion, witness tampering and conspiracy. He is scheduled for sentencing Sept. 25 before U.S. District Judge Susan Richard Nelson in St. Paul.

In a separate filing Friday, attorneys for Walker argued that he is more deserving of a sentence just shy of six years, based on the conviction of another Minnesotan, Patrick Kiley, who helped raise funds from investors for the $194 million Ponzi scheme operated by Trevor Cook. Kiley was sentenced to 20 years in prison.

"Kiley's fraud was far larger and far more nefarious than anything Mr. Walker did," wrote defense attorneys Peter Wold and Aaron Morrison.

"Mr. Walker led a good and decent life. He supported his family through ups and downs. He donated a kidney to his son. He has paid the bills of his children. Now in their 70s, he and his wife are destitute," Walker's attorneys argued. "On sentencing day, Mr. Walker will stand before this court a broken man."

Walker was convicted of misleading investors for years about the prospect of developing technology for converting coal into natural gas, even producing demonstration videos about the process and issuing newsletters that stated success was just around the corner. Walker also told investors they would be rich once Bixby went public, which he repeatedly assured them was imminent.

During the seven-week trial, Walker was described by his defense team as a visionary and a dreamer who in the 1980s founded Select Comfort, maker of the Sleep Number bed. He later left the company and launched Bixby Energy Systems, which successfully produced corn-burning stoves, only to see sales falter with rising corn prices.

"The blindly optimistic Mr. Walker is not capable of leading a criminal activity," his attorneys wrote.

The government, however, said greed motivated Walker to promote his failed company and to hire convicted felons to help him find investors.

"Walker lied about everything … so he could live in a big house that he could not afford, so his daughter could do the same and so that he could pretend to be a world-saving visionary, serial entrepreneur," the government said.

Walker's daughter, Melanie Bonine, earlier pleaded guilty to one count of evading taxes on proceeds from selling Bixby securities, but she is appealing the sentence. Bonine entered into a plea agreement with the government for a sentence of 18 to 24 months, but Judge Nelson sentenced her in May to three years based on evidence introduced in her father's trial.

Two other participants in the fraud, Gary Collyard and Dennis Desender, have already been sentenced to prison.

Under federal sentencing guidelines, Walker faces a prison sentence of 295 years. But the government says it is seeking a 30-year sentence to put Walker more in line with similar fraud convictions in Minnesota, including the cases of former Wayzata businessman Tom Petters and Cook where more money was lost but sentences were 50 years or less.

David Phelps • 612-673-7269