The search for a diplomatic solution to the crisis in Chad intensified today as French President Francois Mitterrand's chief adviser on African affairs arrived in Ndjamena, the Chadian capital, and held talks with Chadian President Hissene Habre.

At the same time, French Defense Minister Charles Hernu announced that he had ordered a unit of military instructors to Salal, where Habre's forces have set up a new protective perimeter 220 miles north of Ndjamena.

The contingent of French paratroopers in Chad, which will be increased from 500 to 700 men, now holds forward positions on the two main roads leading to the capital and their base of operations. Yesterday 150 to 200 French troops were dispatched to Abeche, 400 miles east of Ndjamena.

French officials admitted that the troop deployments did not necessarily portend imminent new battles. Rather, the moves were designed to discourage the Libyan-backed rebel forces of former president Goukouni Oueddei from advancing on the capital now that they control the northern part of Chad.

The rebels and their chief patron, Libya's Col. Muammar Qaddafi, are thought to be wary of risking a clash with the French forces. Since the Chadian government's new defense line is beyond the range of Libyan bombers, French officials are hoping for a military stalemate that will improve chances for a negotiated solution.

The diplomatic mission of Guy Penne, Mitterrand's top aide for African affairs, is expected to provide the first soundings for a potential compromise that might resolve the longstanding conflict between the two rival warlords, Goukouni and Habre.

While France says it supports the "legitimate government" of Habre, Paris has maintained contacts with Tripoli during the current crisis and may be prepared to probe prospects for negotiations in the wake of conciliatory comments that Qaddafi made in recent interviews.

A key problem, however, remains Habre's suspicious attitude toward the intentions of the French government.

Last week Habre derided Penne as "that poor idiot" and accused him of serving in the vanguard of a "pro-Libyan lobby" inside the Mitterrand government.

Penne wields extraordinary power as the chief presidential counselor on African affairs because France's policy toward former African colonies bypasses the Foreign Ministry and is conducted solely out of the Elysee Palace, the presidential office.

Penne's main assistant is Jean Christophe Mitterrand, 37, the president's son. He formerly worked in Africa as a reporter for the French news agency, Agence France-Presse, but was compelled to resign his post after his father's election, ostensibly because of the kidnaping risks involved if he continued to serve as a correspondent in Africa.

Jean Christophe Mitterrand's prestige among African leaders is substantial because, as one former colleague put it, he is "perceived on the continent as the son of the great chief."

Despite their stature, Penne and Jean Christophe Mitterrand reportedly were criticized by the president for failing to grasp the gravity of the crisis when Libyan troops, armor and aircraft intervened in the conflict and enabled rebel forces to capture the strategic oasis town of Faya Largeau in northern Chad last week.

Faya Largeau fell only hours after the first French paratroopers were sent to Chad following Habre's appeals for combat forces and aircraft, not just military equipment.

Habre and other African leaders, who were urging a more assertive French response, complained that the dispatch of paratrooper trainers was too little and too late to stop the rebel advance in the north, where Libyan aircraft operated with impunity.

French military officials said that four Jaguar jet fighters are now poised at a base in neighboring Central African Republic to be sent into action "in case they are needed."

The French government insists that its troops are conducting only training exercises with Habre's forces and will not carry out any aggressive actions in the civil war, but says they have permission to respond to any attack.